Lesson Plan 1The Four Fundamental Forces

Author: Brittany Krutty

Date lesson will be taught: Friday, October 21st, 2011, and Monday, October 24th, 2011

Grade level: 11

Knowledge Package:

Lesson Source: http://www.learner.org/resources/series42.html;

http://science.howstuffworks.com/environmental/earth/geophysics/question2322.htm;

http://csep10.phys.utk.edu/astr162/lect/cosmology/forces.html;

http://library.thinkquest.org/27930/forces.htm; http://science.howstuffworks.com/magnet.htm;

http://science.howstuffworks.com/magnet4.htm;

http://aether.lbl.gov/elements/stellar/strong/strong.html; http://www.physorg.com/news/2011-01-

weak-nuclear.html

Concepts: The four fundamental forces of nature are the gravitational force, the electromagnetic

force, the strong nuclear force, and the weak nuclear force. All other forces can be attributed to

these four fundamental forces, or interactions. The gravitational force is responsible for the

attraction between any two objects with mass. The electromagnetic force acts between

electrically charged particles. The strong force binds quarks together and binds protons and

neutrons together into nuclei. The weak force plays a role in the radioactive disintegration of

certain nuclei and also in particle decay.

Objectives:

Students will be able to:

∙ Describe the four fundamental forces.

∙ Identify which fundamental forces are responsible for a given common phenomenon.

∙ Compare and contrast the fundamental forces

Kansas Science and/or Mathematics Standards, Benchmarks, and Indicators:

Science Standards Grades 8-12:

12.1.1.1. Actively engages in asking and evaluating research questions.12.2.3.1. There are four fundamental forces in nature: strong nuclear force, weak nuclear force, electromagnetic force, and gravitational force.12.4.3.1. Understands gravitational attraction of objects in the solar system keeps solar system objects in orbit12.7.1.3. Recognizes the universality of basic science concepts and the influence of personal and cultural beliefs that embed science in society.

12.7.2.1. Understands scientific knowledge describes and explains the physical world in terms of matter, energy, and forces. Scientific knowledge is provisional and is subject to change as new evidence becomes available.

Materials list, advance preparation, and handouts:

For each group of 3 students (there will be 9 groups):1 whiteboard1 whiteboard marker

Advance Preparation1. 1st day: The teacher’s name and particleadventure.org will already be written on the

board, along with the words FUNDAMENTAL and FORCE. Students will be sitting with their groups. Have pre-tests printed and upside-down on students’ desks.

2. 2nd day: The video will already be loaded and the projector will be set up. Have post-tests printed and ready to hand out.

Accommodations: All students should be able to participate unless there is a particular physical disability.

Any student unable to physically complete any portion of the lesson may require additional assistance from a Para-professional, a classroom assistant, or a student assistant. ELL students and students who are hard-of-hearing will benefit from the “big ideas” presented in this lesson and the visuals drawn on the board or presented in the video. The teacher should ask appropriate questions for the stage of language acquisition and use nonverbal signals to emphasize the meaning of instructions and subject matter. The teacher should write down key terms and concepts on the board and perhaps even provide additional written materials on the fundamental forces to students who are hard-of-hearing.

Safety:

Be sure that students are using the materials appropriately. Do not allow students to continue the experiment if they are mistreating the materials. Ensure that students feel the classroom is a safe environment in which to ask questions, make mistakes, and learn.

Five-E Plan – Day 1

Teacher Does Probing Questions Student responses

Engage:

Learning Experience(s)

Time: ___15___minutes

Critical questions that will connect to prior knowledge and create a need to know.

Expected Student

Responses/Misconceptions

Ask for two volunteers to write on the board. The

1. What is a force? 1. A push or pull. The effect on a particle due to

rest of the class will tell them what they already know about the words fundamental and force. This strategy is used by the teacher to gauge prior understanding. If no one volunteers an answer to the questions, treat the questions as “Think-Pair-Share” questions, where students think about the answer, talk with their partner/group about the answer, and then share their answer with the class.

2. Do all forces have to be contact forces?

3. What are some forces you already know about?

4. What does a force do?

5. What does fundamental mean?

6. What makes a force fundamental?

the presence of other particles. A thing that is passed between two objects.

2. No.

3. Gravity, normal force, friction, torque, magnetic force, electricity, centrifugal/centripetal force, etc.

4. Changes an object’s motion, deforms an object in some way, etc.

5. Central, important, not made of anything smaller or more basic.

6. A force is fundamental if it can’t be attributed to any other force. All “forces” in our world can be attributed to the fundamental forces.

Ask the volunteers to return to their seats. Discuss the difference between a force and an interaction, and write the four fundamental forces on the board.

Give students the pre-test (at the end of this document).

1. A lot of times physicists use the words force and interaction interchangeably. What does interaction mean?

2. What do you think interaction means in physics?

1. When two things act upon each other or affect each other.

2. The effect on a particle due to the presence of another particle. A thing that is passed between two objects.

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand what fundamental force means? If so, then move on to the Explore section. If not, then revisit this concept.

Teacher Does Probing Questions Student responses

Explore:Learning Experience(s)

Time: __10____minutes

Critical questions that will guide students to a common set of experiences.

Expected Student Responses/Misconceptions

Have five of the groups write everything they know about gravity on their whiteboards, and the other four groups write everything they know about electromagnetism on their whiteboards. Use questions to lead them in the right direction. Have the groups put the whiteboards up on the whiteboard marker rails around the classroom when they are done.

1. What does the force do?

2. How strong is the force?

3. What are some examples of this force in action?

5. Is it weaker or stronger than other forces you have heard about?

6. What types of objects experience this force?

1. Gravity: attracts objects, makes stuff fall.Electromagnetism: attracts or repels charged objects.

2. Gravity: VERY strong, 9.8 m/s/s (expected answer). Electromagnetism: Stronger when you are closer to the magnet (expected answer).

3. Gravity: object falling, planetary orbits, etc. Electromagnetism: lightning, magnets, electric circuits, etc.

5. Gravity: weaker. Electromagnetism: stronger.

6. Gravity: all objects that have mass. Electromagnetism: magnets, protons and electrons, etc.

Teacher Decision Point Assessment: When the students are done writing on the boards, move on

to the Explain section.

Teacher Does Probing Questions Student responses Explain:Learning Experience(s)

Critical questions that will help students clarify their understanding and

Expected Student Responses/Misconceptions

Time: ___20___minutes

introduce information related to the lesson concepts/skills.

Discuss and explain gravity and electromagnetism, starting with the groups’ whiteboards in front of the class. Draw a chart on the board (pictured below) to help organize the four forces.

Gravity: Students should understand that gravity is the weakest interaction, and every object in the universe exerts a gravitational force on everything else. It acts on all particles having mass, and the gravitational force is equal between the two masses. It has an infinite range, and is always attractive. It affects everything from the structure of galaxies to planetary orbits to everyday objects on Earth.

Electromagnetism: Students should understand that the electromagnetic force acts between electrically charged particles. The electrostatic force acts on charged particles at rest, and the electric and magnetic forces act on charged particles moving relative to each other. Electromagnetism also has infinite range. The residual electromagnetic force (draw a picture on the board) explains why I don’t fall through the floor and

even explains friction. The electromagnetic force causes oppositely charged objects to attract and like-charged objects to repel each other. Electromagnetism and gravitation are both described by similar equations (the inverse square law). Discuss and explain the strong nuclear force and weak nuclear force. Use the chart on the board (pictured below) to help organize the four forces.

Strong nuclear force: Students should understand that this force holds quarks together through gluons, and that the residual strong force holds the nucleus together. It has a very short range.

Weak nuclear force: Students should understand that it is responsible for some nuclear phenomena like beta decay. It can change one type of quark or other fundamental particle into another. It is a fundamental force, even though it does not have to do with things pushing or pulling each other. Above certain energies, the electromagnetic force and the weak force merge into the electroweak force.

Force/Interaction: Strong Electromagnetic Weak GravitationCurrent theory: Quantum Quantum Electro- Electroweak General Relativity

Chromo- dynamicsdynamics

Relative strength: 1038 1036 1025 1Long-distancebehavior: 1/r2 1/r2

Range (meters): 10-15 infinity 10-18 infinity

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand the difference between the fundamental forces? If so, then move on to the Extend/Elaborate section. If not, then revisit this concept.

Teacher Does Probing Questions Student responses

Extend / Elaborate:Learning Experience(s)

Time: ___5___minutes

Critical questions that will help students extend or apply their newly acquired concepts/skills in new situations.

Expected Student Responses/Misconceptions

Ask questions that involve all the fundamental forces. Compare and contrast the fundamental forces, and discuss unification of the forces if time permits.

1. Why doesn’t the electromagnetic force dominate gravity on the scale of our solar system, since it’s so much stronger and has the same range?

2. Why doesn’t the weak nuclear force or strong nuclear force dominate gravity on the scale of our solar system?

3. What force holds you and me together? What force holds all matter together?

4. Scientists have concluded that the moon is held in its orbit about the earth by gravity and not by the electrical force. Why would they think that?

5. Many physicists strive to

1. There is no huge concentration of positive and negative charge, since they “cancel each other out.” However, there are huge concentrations of mass.

2. They do not have as large a range as gravity.

3. Electromagnetic force (technically, the residual electromagnetic force).

4. The earth and the moon are essentially neutral and, therefore, have no electrical force between them.

5. Other forces have been united in the past:

unite the four fundamental forces of nature. What evidence would cause them to feel this may be possible?

6. How are the electromagnetic and gravitational forces similar? How are they different? When is each most dominant?

“chemical” force and electric force (battery), electricity and magnetism, electromagnetism and weak, etc. At high energies, the equations describing the forces look similar. The inverse square law applies to gravity and electromagnetism.

6. They both have infinite range and follow the inverse square law, but gravity is only an attractive force while electromagnetism can be attractive or repulsive. The electromagnetic force is dominant on small scales, and the gravitational force is dominant on large scales.

Have students write one question they still have about the fundamental forces. This is their “exit slip.”

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand why gravity dominates on a large scale? If not, then revisit this concept.

Teacher Does Probing Questions Student responses Evaluate:Include formative and summative evaluation below.

Time: _______minutes

Critical questions that ask students to demonstrate their understanding of the concepts and process skills. These questions must directly relate to the lesson’s performance objectives.

Expected Student Outcomes

Formative evaluation, day 1: Determine a reasonable point to move on after each concept based on student responses and participation. During group work in the Explore section, walk around the classroom and ask students leading questions to help them complete the task.

Five-E Plan – Day 2

Teacher Does Probing Questions Student responses

Engage:

Learning Experience(s)

Time: ___2___minutes

Critical questions that will connect to prior knowledge and create a need to know.

Expected Student

Responses/Misconceptions

Conduct a review of gravity using Barbie and Ken dolls (this should be funny!).

1. Does Mr. Strawderman have mass?

2. Does Barbie have mass?

3. Is Mr. Strawderman attracted to Barbie?

4. And is Barbie attracted to Mr. Strawderman?

5. So it’s a mutual attraction! Who is attracted to whom more?

6. Is Mr. Strawderman attracted to other things, too?

7. Uh-oh, Barbie’s going to be jealous! Are they attracted to him?

8. Show of hands: who is attracted to Ken?

1. Yes.

2. Yes.

3. Yes.

4. Yes.

5. Neither. The force of gravity is equal for both of them.6. Yes.

7. Yes.

8. (Everyone raises his/her hand)

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand that the gravitational force is equal and attractive between two objects of different mass? If so, then move on to the Explain section. If not, then revisit this concept.

Teacher Does Probing Questions Student responses

Explore:Learning Experience(s)

Time: __38____minutes

Critical questions that will guide students to a common set of experiences.

Expected Student Responses/Misconceptions

Watch the video found at http://www.learner.org/resources/series42.html, stopping frequently to ask questions and check for understanding.

1. The strong nuclear force “binds protons and neutrons into nuclei.” – How?

2. On the range of the strong force: “12 zeros follow the decimal point to express it numerically as a fraction of a centimeter.” The range of the strong force is 10-15 m (about the diameter of a medium-sized nucleus). The weak force “has effects that are by no means weak” – then why is it called the weak force?

3. Gravity: it “makes the moon fall, … it is a fundamental force of the Universe.” – What do they mean the moon “falls”?

4. “Gravity reaches out and shakes hands with the entire Universe.” – What do they mean by this?

5. On electricity: “It binds together everything in the

1. Quarks in the protons and neutrons are attracted to each other due to the residual strong nuclear force by exchanging gluons.

2. Its range is 10-18 (about 1/1000 the diameter of a proton), and its strength is much less than the electromagnetic force and strong nuclear force.

3. It experiences the force of gravity due to the earth’s mass. Since the earth’s mass is much larger than the moon’s, we say the moon is “falling.” When you jump, you “fall” towards the earth and the earth “falls” towards you, but the earth is much more massive.

4. An object with mass is attracted to everything else in the Universe. (Some may refer to the Barbie example from the Engage section.)

5. The protons in one atom are attracted to the electrons

Universe. Everything from soup to nuts.” – How?

6. “When people shake hands, why don’t they fry each other like potatoes?” – Why don’t we feel the effects of electricity in our bodies?

7. When the video is going through the equations for gravity and electricity, ask: If the strength of both of these forces has the same relationship with distance (1/r2), then why does gravity dominate over large distances?

8. “Found in the debris of nuclear collisions, these fragments suggest a deep underlying connection between the electric force and the weak nuclear force.” – This is the basis for electroweak theory. Some textbooks even consider there to be 3 fundamental forces instead of 4. At very short distances or very high energies, the strengths of the electromagnetic force and the weak force are comparable.

9. “Gravity – it’s the force that makes some objects fall and keeps others from spinning off the face of the earth.” – What does this mean? Can you think of an example of each?

in surrounding atoms due to the residual electromagnetic force.

6. We have an about equal amount of positive and negative charges in our bodies, so they are canceled out and we are left with no electromagnetic force between us and other people.

7. There is no huge concentration of positive and negative charge, since they “cancel each other out.” However, there are huge concentrations of mass.

9. Objects with mass, such as balls, pencils, and people, fall when dropped from any height above the surface of the earth. Very massive objects, such as buildings, are kept from spinning off the face of the earth by gravity. (Think of a mouse trying to hang on to a

10. “Gravity – it keeps the moon orbiting the earth, and the planets orbiting the sun.” – How?

11. “When two objects come into contact, their positive and negative charges interact, and their surfaces are, at least temporarily, bonded together.” Can anyone visualize this? Draw what you think this looks like on your whiteboards.

12. “Seeing the positive charge just up ahead, the ion feels attracted and accelerates.” – What do they mean when they say the ion “sees” the positive charge?

13. “Carbon ions are directed through an evacuated beam pipe toward the target, helium.” – What do they mean by evacuated? Why is this important?

14. “And we’ll get on with those problems next time.” – These topics lead to the Standard Model of physics, the Grand Unified Theory, and the Theory of Everything.

spinning basketball, where the force of gravity between the objects is weak enough to be ignored.)

10. The moon experiences the force of gravity due to the earth’s mass, keeping it in orbit. Planets experience the force of gravity due to the sun’s mass, keeping them in orbit.

12. They mean that the electromagnetic force between the ion and the positive charge becomes stronger and stronger as the ion approaches the positive charge.

13. Evacuated means the beam pipe is a vacuum; there are very few particles in the beam pipe for the ions to interact with.

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand the difference

between the fundamental forces? If so, then move on to the Extend/Elaborate section. If not, then revisit this concept.

Teacher Does Probing Questions Student responses Explain:Learning Experience(s)

Time: ___2___minutes

Critical questions that will help students clarify their understanding and introduce information related to the lesson concepts/skills.

Expected Student Responses/Misconceptions

Explain the video, elaborate, and review from yesterday.

1. Which fundamental force is responsible for friction?

2. Nuclear bonding?

3. Planetary orbits?

4. Which interactions act on the protons in you? Why?

5. Which force is most fundamental? Why?

1. Electromagnetism

2. The strong force

3. Gravity.

4. All of them because they have mass, electric charge, and they are on the scale of the nucleus of an atom.

5. The four fundamental forces are all equally fundamental because they cannot be attributed to any other forces.

‍

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand where different forces apply? If so, then move on to the Extend/Elaborate section. If not, then revisit this concept.

Teacher Does Probing Questions Student responses

Extend / Elaborate: Critical questions that will help students extend or

Expected Student Responses/Misconceptions

Learning Experience(s)

Time: ___2___minutes

apply their newly acquired concepts/skills in new situations.

Ask review questions, based on the video, that tie concepts concerning the fundamental forces.

1. Which forces dominate over large distances? Small distances?

1. In the nucleus of the atom, the strong force dominates. On the atomic scale, electricity dominates. On very large scales, gravity dominates.

Go through the “exit slips” from the day before, answering the questions or even having the students answer the questions.

Ask for any other questions.

1. What questions do you still have about fundamental forces?

Teacher Decision Point Assessment: Based on class participation and the answers to the

questions, do the students understand the main concepts of the lesson? If so, then move on to the Evaluate section. If not, revisit the main points.

Teacher Does Probing Questions Student responses Evaluate:Include formative and summative evaluation below.

Time: __6____minutes

Critical questions that ask students to demonstrate their understanding of the concepts and process skills. These questions must directly relate to the lesson’s performance objectives.

Expected Student Outcomes

Summative assessment (below)

Name ______________________

Fundamental Forces Pre-Test

1. What are the four fundamental forces of nature?

Gravitational, electromagnetic, strong nuclear, weak nuclear

2. Which force holds all matter together?

Electromagnetic

3. In some models of the atom, electrons orbit the nucleus in much the same way that planets orbit the sun. Why can this analogy be made?

a. Both forces depend inversely on the square of the distance.b. Both forces are proportional to the masses of the attracting bodies.c. The electromagnetic force, like gravity, depends on mass.d. The electromagnetic force is really gravity acting at small distances.

4. Which one of the following is not a consequence of the gravitational force?a. The formation of galaxies.b. The organization of the solar system.c. A stone falling.d. The structure of the nucleus.

5. Particle decays are a manifestation of which force?

Weak nuclear

6. Scientists have concluded that the moon is held in its orbit about the earth by gravity and not by the electrical force. Which of the following statements gives the reason for that conclusion?

a. Only gravity depends inversely on the square of the distance between two bodies.

b. The electrical force is canceled out by the strong force between the earth and the moon.

c. The earth and the moon are essentially neutral and, therefore, have no electrical force between them.

d. The gravitational constant is larger than the electrical constant in the respective force laws.

Name ______________________

Fundamental Forces Post-Test

7. What are the four fundamental forces of nature?

Gravitational, electromagnetic, strong nuclear, weak nuclear

8. Which force holds all matter together?

Electromagnetic

9. In some models of the atom, electrons orbit the nucleus in much the same way that planets orbit the sun. Why can this analogy be made?

a. Both forces depend inversely on the square of the distance.b. Both forces are proportional to the masses of the attracting bodies.c. The electromagnetic force, like gravity, depends on mass.d. The electromagnetic force is really gravity acting at small distances.

10. Which one of the following is not a consequence of the gravitational force?a. The formation of galaxies.b. The organization of the solar system.c. A stone falling.d. The structure of the nucleus.

11. Particle decays are a manifestation of which force?

Weak nuclear

12. Scientists have concluded that the moon is held in its orbit about the earth by gravity and not by the electrical force. Which of the following statements gives the reason for that conclusion?

a. Only gravity depends inversely on the square of the distance between two bodies.

b. The electrical force is canceled out by the strong force between the earth and the moon.

c. The earth and the moon are essentially neutral and, therefore, have no electrical force between them.

d. The gravitational constant is larger than the electrical constant in the respective force laws.

Backup plan (mainly in case video doesn’t work)/ potential lesson for 3rd day:- Assign a topic related to the fundamental forces to each group and hand each

person in a group an information sheet (below) corresponding to their fundamental force.

- Have students write the main points on their group’s whiteboard.- Let each group present their fundamental force in front of the class.- Review the topics that would have been covered in the video

Unification of the ForcesProperties of the Fundamental Forces

• The strong interaction is very strong, but very short-ranged. It acts only over ranges of order 10-13

centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.

• The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.

• The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.

• The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

Unification of the Forces of NatureAlthough the above discussion indicates that the fundamental forces in our present Universe are distinct and have very different characteristics, the current thinking in theoretical physics is that this was not always so. There is a rather strong belief (although it is yet to be confirmed experimentally) that in the very early Universe when temperatures were very high compared with today, the weak, electromagnetic, and strong forces were unified into a single force. Only when the temperature dropped did these forces separate from each other, with the strong force separating first and then at a still lower temperature the electromagnetic and weak forces separating to leave us with the 4 distinct forces that we see in our present Universe. The process of the forces separating from each other is called spontaneous symmetry breaking.There is further speculation, which is even less firm than that above, that at even higher temperatures (the Planck Scale) all four forces were unified into a single force. Then, as the temperature dropped, gravitation separated first and then the other 3 forces separated as described above. The time and temperature scales for this proposed sequential loss of unification are illustrated in the following table.

Loss of Unity in the Forces of Nature

Characterization Forces Unified Time Since Beginning Temperature (GeV)*

All 4 forces unified Gravity, Strong, Electromagnetic, Weak ~0 ~infinite

Gravity separates (Planck Scale)

Strong, Electromagnetic, Weak 10-43 s 1019

Strong force separates (GUTs Scale) Electromagnetic, Weak 10-35 s 1014

Split of weak and electromagnetic forces None 10-11 s 100

Present Universe None 1010 y 10-12

*Temperature Conversion: 1 GeV = 1.2 x 1013 KTheories that postulate the unification of the strong, weak, and electromagnetic forces are called Grand Unified Theories (often known by the acronym GUTs). Theories that add gravity to the mix and try to unify all four fundamental forces into a single force are called Superunified Theories. The theory that describes the unified electromagnetic and weak interactions is called the Standard Electroweak Theory, or sometimes just the Standard Model.Grand Unified and Superunified Theories remain theoretical speculations that are as yet unproven, but there is strong experimental evidence for the unification of the electromagnetic and weak interactions in the Standard Electroweak Theory. Furthermore, although GUTs are not proven experimentally, there is strong circumstantial evidence to suggest that a theory at least like a Grand Unified Theory is required to make sense of the Universe.

Messenger Particles

There is a unique messenger particle associated with each of the four fundamental forces. These particles can be considered the "smallest amount" of each force than can exist in nature. Experiments have confirmed the existence of three of the four particles, but the graviton has yet to be discovered. Calculations show that it should be massless. The weak gauge bosons come in two separate varieties with different masses.

Force Particle MassGravitational Force Graviton 0Electromagnetic Force Photon 0

Weak Nuclear Force Weak Gauge Bosons 86, 97Strong Nuclear Force Gluon 0GravityThe messenger particle of gravity is the graviton. It has not been experimentally verified, mainly because it is extremely hard to find the smallest denomination of the weakest force. Recent calculations show that it will likely be massless. Interestingly, all versions of modern string theory incorporate gravity (unlike previous quantum theories) and not only allow but require a particle with the properties of the graviton. Its discovery will likely represent a major victory for string theory, since previous quantum theories based on the model of point particles give illogical, infinite answers when gravity is incorporated.

ElectromagnetismThe electromagnetic force is actually second in effective strength only to the strong force, but it is listed out of order here because it, like gravity, is more familiar to most people. Its strength is less than 1% of that of the strong force, but it, like gravity, has infinite range. However, unlike gravity, the electromagnetism has both attractive and repulsive properties that can combine or cancel each other out. Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two positive or two negative things will repel each other, but one positive and one negative attract each other. This can be neatly illustrated by magnets: two of the same "pole" will repel each other, but two opposite poles attract each other.This is the principle that keeps atoms together: the positively charged nucleus and the negatively charged electrons attract each other. This is also the principle of atomic size: more electrons have greater repulsive force, so atoms with more electrons are larger because of the electrons' mutual repulsion. Similarly, atoms with larger nuclei and the same number of electrons are smaller overall because they exert a greater attractive force on the electrons.The messenger particle of electromagnetism is the photon, a massless particle that logically (since light is a manifestation of electromagnetism) travels at the speed of light (299 792 458 m/s or 299 972 km/s).

The Weak Nuclear ForceThe weak nuclear force is one of the less familiar fundamental forces. It operates only on the extremely short distance scales found in an atomic nucleus. The weak force is responsible for radioactive decay. In actuality, it is stronger than electromagnetism, but its messenger particles (W and Z bosons) are so massive and sluggish that they do not faithfully transmit its intrinsic strength.

The Strong Nuclear ForceThe strong nuclear force is the other unfamiliar fundamental force. Like the weak force, its range is limited to subatomic distances. Its "duties" are keeping quarks together inside protons and neutrons, and keeping protons and neutrons inside atomic nuclei. Its messenger particle is the massless gluon, so named because it "glues" elementary particles together.

Einstein's Gravity

Albert Einstein, who won the Nobel Prize in Physics in 1921, contributed an alternate theory of gravity in the early 1900s. It was part of his famous General Theory of Relativity, and it offered a very different explanation from Newton's Law of Universal Gravitation. Einstein didn't believe gravity was a force at all; he said it was a distortion in the shape of space-time, otherwise known as "the fourth dimension" (see How Special Relativity Works to learn about space-time).

Basic physics states that if there are no external forces at work, an object will always travel in the straightest possible line. Accordingly, without an external force, two objects travelling along parallel paths will always remain parallel. They will never meet.

But the fact is, they do meet. Particles that start off on parallel paths sometimes end up colliding. Newton's theory says this can occur because of gravity, a force attracting those objects to one another or to a single, third object. Einstein also says this occurs due to gravity -- but in his theory, gravity is not a force. It's a curve in space-time.

According to Einstein, those objects are still travelling along the straightest possible line, but due to a distortion in space-time, the straightest possible line is now along a spherical path. So two objects that were moving along a flat plane are now moving along a spherical plane. And two straight paths along that sphere end in a single point.

Still more-recent theories of gravity express the phenomenon in terms of particles and waves. One view states that particles called gravitons cause objects to be attracted to one another. Gravitons have never actually been observed, though. And neither have gravitational waves, sometimes called gravitational radiation, which supposedly are generated when an object is accelerated by an external force [source: Scientific American].

Gravitons or no gravitons, we know that what goes up must come down. Perhaps someday, we'll know exactly why. But until then, we can be satisfied just knowing that planet Earth won't go hurdling into the sun anytime soon. Gravity is keeping it safely in orbit.

How Magnets Work

It all started when we went shopping for a magnet for a demonstration on liquid body armor. We wanted to show that a magnetic field could cause certain liquids to behave as solids. Along with the petri dishes and iron filings we needed, the Steve Spangler Science catalog had a neodymium magnet it described as "super strong." We ordered our supplies, hoping that the magnet would be powerful enough to create an effect we could capture on film.The magnet didn't just transform our iron-and-oil fluid into a solid -- sometimes, its pull on the fluid cracked the petri dish holding it. Once, the magnet unexpectedly flew out of a videographer's hand and into a dish full of dry filings, which required considerable ingenuity to remove. It also adhered itself so firmly to the underside of a metal table that we had to use a pair of locking pliers to retrieve it. When we decided it would be safer to keep the magnet in a pocket between takes, people wound up momentarily stuck to the table, a ladder and the studio door.Around the office, the magnet became an object of curiosity and the subject of impromptu experiments. Its uncanny strength and its tendency to suddenly and noisily jump from unwary grips to the nearest metal surface got us thinking. We all knew the basics of magnets and magnetism -- magnets attract specific metals, and they have north and south poles. Opposite poles attract each other while like poles repel. Magnetic and electrical fields are related, and magnetism, along with gravity and strong and weak atomic forces, is one of the four fundamental forces in the universe.But none of those facts led to an answer to our most basic question. What exactly makes a magnet stick to certain metals? By extension, why don't they stick to other metals? Why do they attract or repel each other, depending on their positioning? And what makes neodymium magnets so much stronger than the ceramic magnets we played with as children?

Iron filings (right) align along the magnetic field lines of a cylindrical neodymium magnet.To understand the answers to these questions, it helps to have a basic definition of a magnet. Magnets are objects that produce magnetic fields and attract metals like iron, nickel and cobalt. The magnetic field's lines of force exit the magnet from its north pole and enter its south pole. Permanent or hard magnets create their own magnetic field all the time. Temporary or soft magnets produce magnetic fields while in the presence of a magnetic field and for a short while after exiting the field. Electromagnets produce magnetic fields only when electricity travels through their wire coils.

Iron filings (right) align along the magnetic field lines of a cubical neodymium magnet.Until recently, all magnets were made from metal elements or alloys. These materials produced magnets of different strengths. For example:

• Ceramic magnets, like the ones used in refrigerator magnets and elementary-school science experiments, contain iron oxide in a ceramic composite. Most ceramic magnets, sometimes known as ferric magnets, aren't particularly strong.

• Alnico magnets are made from aluminum, nickel and cobalt. They're stronger than ceramic magnets, but not as strong as the ones that incorporate a class of elements known as rare-earth metals.

• Neodymium magnets contain iron, boron and the rare-earth element neodymium.Samarium cobalt magnets combine cobalt with the rare-earth element samarium. In the past few years, scientists have also discovered magnetic polymers, or plastic magnets. Some of these are flexible and moldable. However, some work only at extremely low temperatures, and others pick up only very lightweight materials, like iron filings.

Magnet Myths

Every time you use a computer, you're using magnets. A hard drive relies on magnets to store data, and some monitors use magnets to create images on the screen. If your home has a doorbell, it probably uses an electromagnet to drive a noisemaker. Magnets are also vital components in CRT televisions, speakers, microphones, generators, transformers, electric motors, burglar alarms, cassette tapes, compasses and car speedometers.In addition to their practical uses, magnets have numerous amazing properties. They can induce current in wire and supply torque for electric motors. A strong enough magnetic field can levitate small objects or even small animals. Maglev trains use magnetic propulsion to travel at high speeds, and magnetic fluids help fill rocket engines with fuel. The Earth's magnetic field, known as the magnetosphere, protects it from the solar wind. According to Wired magazine, some people even implant tiny neodymium magnets in their fingers, allowing them to detect electromagnetic fields [Source: Wired].Magnetic Resonance Imaging (MRI) machines use magnetic fields to allow doctors to examine patients' internal organs. Doctors also use pulsed electromagnetic fields to treat broken bones that have not healed correctly. This method, approved by the United States Food and Drug Administration in the 1970s, can mend bones that have not responded to other treatment. Similar pulses of electromagnetic energy may help prevent bone and muscle loss in astronauts who are in zero-gravity environments for extended periods.Magnets can also protect the health of animals. Cows are susceptible to a condition called traumatic reticulopericarditis, or hardware disease, which comes from swallowing metal objects. Swallowed objects can puncture a cow's stomach and damage its diaphragm or heart. Magnets are instrumental to preventing this condition. One practice involves passing a magnet over the cows' food to remove metal objects. Another is to feed magnets to the cows. Long, narrow alnico magnets, known as cow magnets, can attract pieces of metal and help prevent them from injuring the cow's stomach. The ingested magnets help protect the cows, but it's still a good idea to keep feeding areas free of metal debris. People, on the other hand, should never eat magnets, since they can stick together through a person's intestinal walls, blocking blood flow and killing tissue. In humans, swallowed magnets often require surgery to remove.

Cow magnetsPhoto courtesy AmazonSome people advocate the use of magnet therapy to treat a wide variety of diseases and conditions. According to practitioners, magnetic insoles, bracelets, necklaces, mattress pads and pillows can cure or alleviate everything from arthritis to cancer. Some advocates also suggest that consuming magnetized drinking water can treat or prevent various ailments. Americans spend an estimated $500 million per year on magnetic treatments, and people worldwide spend about $5 billion. [Source: Winemiller via NCCAM].Proponents offer several explanations for how this works. One is that the magnet attracts the iron found in hemoglobin in the blood, improving circulation to a specific area. Another is that the magnetic field somehow changes the structure of nearby cells. However, scientific studies have not confirmed that the use of static magnets has any effect on pain or illness. Clinical trials suggest that the positive benefits attributed to magnets may actually come from the passage of time, additional cushioning in magnetic insoles or the placebo effect. In addition, drinking water does not typically contain elements that can be magnetized, making the idea of magnetic drinking water questionable.Some proponents also suggest the use of magnets to reduce hard water in homes. According to product manufacturers, large magnets can reduce the level of hard water scale by eliminating ferromagnetic hard-water minerals. However, the minerals that generally cause hard water are not ferromagnetic. A two-year Consumer Reports study also suggests that treating incoming water with magnets does not change the amount of scale buildup in a household water heater.

The Strong Nuclear Force (also referred to as the strong force)…

is one of the four basic forces in nature (the others being gravity, the electromagnetic force, and the weak nuclear force). As its name implies, it is the strongest of the four. However, it also has the shortest range, meaning that particles must be extremely close before its effects are felt. Its main job is to hold together the subatomic particles of the nucleus (protons, which carry a positive charge, and neutrons, which carry no charge. These particles are collectively called nucleons). As most people learn in their science education, like charges repel (+ +, or - -), and unlike charges attract (+ -).

If you consider that the nucleus of all atoms except hydrogen contain more than one proton, and each proton carries a positive charge, then why would the nuclei of these atoms stay together? The protons must feel a repulsive force from the other neighboring protons. This is where the strong nuclear force comes in. The strong nuclear force is created between nucleons by the exchange of particles called mesons. This exchange can be likened to constantly hitting a ping-pong ball or a tennis ball back and forth between two people. As long as this meson exchange can happen, the strong force is able to hold the participating nucleons together. The nucleons must be extremely close together in order for this exchange to happen. The distance required is about the diameter of a proton or a neutron. If a proton or neutron can get closer than this distance to another nucleon, the exchange of mesons can occur, and the particles will stick to each other. If they can't get that close, the strong force is too weak to make them stick together, and other competing forces (usually the electromagnetic force) can influence the particles to move apart. This is represented in the following graphic. The dotted line surrounding the nucleon being approached represents any electrostatic repulsion that might be present due to the charges of the nucleons/particles that are involved. A particle must be able to cross this barrier in order for the strong force to "glue" the particles together.

In the case of approaching protons/nuclei, the closer they get, the more they feel the repulsion from the other proton/nucleus (the electromagnetic force). As a result, in order to get two protons/nuclei close enough to begin exchanging mesons, they must be moving extremely fast (which means the temperature must be really high), and/or they must be under immense pressure so that they are forced to get close enough to allow the exchange of meson to create the strong force. Now, back to the nucleus. One thing that helps reduce the repulsion between protons within a nucleus is the presence of any neutrons. Since they have no charge they don't add to the repulsion already present, and they help separate the protons from each other so they don't feel as strong a repulsive force from any other nearby protons. Also, the neutrons are a source of more strong force for the nucleus since they participate in the meson exchange. These factors, coupled with the tight packing of protons in the nucleus so that they can exchange mesons creates enough strong force to overcome their mutual repulsion and force the nucleons to stay bound together. The preceding explanation shows the reason why it is easier to bombard a nucleus with neutrons than with protons. Since the neutrons have no charge, as they approach a positively charged nucleus they will not feel any repulsion. They therefore can easily "break" the electrostatic repulsion barrier to being exchanging mesons with the nucleus, thus becoming incorporated into it.

Weak nuclear force is less weakJanuary 13, 2011 By Phillip F. Schewe

The force that governs some of the reactions that keep our sun shining is not quite as weak as scientists had previously thought. As a consequence, our estimation of how energetic the sun actually is just went up by a tiny amount.The evidence for this weak nuclear force comes from the decay of muons, essentially heavier cousins of the electron, one of the building blocks of atoms.Just as biologists sometimes study the tiniest and most ephemeral of organisms such as fruit flies, which live for barely a day, to learn things about human disease, so physicists often study the properties of particles that last a fraction of a second to learn about the universe.The muon lives only about 2 millionths of a second -- 2 microseconds -- far from the realm of human sensation but long enough for scientists to make detailed measurements. The state of digital electronics is so advanced that measurements far shorter than this, even down to trillionths of a second or less, can easily be made…Researchers then gathered a fine spray of muons, directed them and stopped them in their own metal target which was surrounded by a detector that could track the muons' demise. The decay of over 2 trillion muons provided the best yet value for the average muon lifetime. It comes out to 2.1969803 microseconds."This is the most precise lifetime determination of any state in the atomic or subatomic world," said David Hertzog, one of the leaders of the experiment and a professor at the University of Washington in Seattle.This lifetime, known to an uncertainty of one part per million, is so precise that it can be used to make a new determination of the intrinsic strength of the weak nuclear force, which operates over only a very short range inside the nucleus of atoms.Scientists know of four physical forces. Gravity, a form of mutual attraction, keeps the Earth going around the sun and keeps us from floating into space. The electromagnetic force is responsible for holding atoms together, for bonding atoms into molecules, for impelling the movement of electrons through wires in the form of electricity, and for light waves. The strong nuclear force holds nuclei together and is responsible for some kinds of radioactivity. The weak nuclear force, the fourth and last force to be discovered by physicists in the twentieth century, helps to turn protons into neutrons inside the sun, a necessary step in converting those protons into heavier elements like helium and releasing the radiant energy that makes its way to Earth. The weak force also acted billions of years ago inside exploding stars known as supernovas to make the elements such as oxygen and carbon found in our own bodies and other natural things on Earth.The strength of the weak force is encapsulated in a number called the Fermi constant, named for the Italian-American scientist Enrico Fermi. Hertzog said that the new value for the Fermi constant is about 0.00075 percent greater than the previous value. Thus the weak force is just a tiny bit stronger than we thought…Another expert on the weak force, University of Wisconsin professor Michael Ramsey-Musolf, considers the muon experiment to be a tour-de-force piece of work. The important thing for him is that the uncertainty of the muon lifetime has now dropped by a factor of ten. But he also said that a more precise lifetime and a more precise knowledge of the strength of the weak nuclear force tells us just a bit more about nature."This implies that the sun does indeed burn more brightly and that the decay of nuclei is somewhat faster," Ramsey-Musolf said.

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