What is a force?A force is a push or

pull, or an action that has the ability to change motion.

Forces can increase or decrease the speed of a moving object.

Forces can also change the direction in which an object is moving.

5.1 How are forces created? Forces are created in many ways.

For example, your muscles create force when you swing a baseball bat.

Two Kinds of Forces

1.Contact-actual touching

2. Long-Range—forces acting at a distance

Frictional ForceTensional ForceNormal ForceAir Resistance ForceApplied ForceSpring Force

A GravitationalB. ElectromagneticC. Nuclear

5.1 How forces actOne way forces act is the result of direct contact.

A contact force is transmitted by matter directly touching other matter such as wind acting to slow a parachute.

5.1 How forces actThe force of gravity between Earth and Moon appears to be what people once called “action-at-a-distance”.

Today we know that the gravitational force is carried from the Earth to the Moon by a force field.

Four Elemental Forces

All forces in the universe come from only four basic forces.

Electromagnetic forces are important to technology.

Gravity is a universal force.

Classify these forces as contact forces or the result

of force fields.

Effects of ForcesForces can cause objects toChange speedChange velocityChange shape

Measuring ForcesTo measure a force, we can use

Spring scales Force gauges

What are the units for FORCE?

Newtons-abbreviated (N)

-metric unit for force

Pounds-abbreviated (lb)

-English unit for force

5.1 Units of force

The pound is a unit of force commonly used in the United States.

For smaller amounts, pounds are divided into ounces (oz.).

There are 16 ounces in 1 pound.

5.1 Newtons

Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons.

The newton (N) is a metric unit of force.

5.1 Unit conversions

The newton (N) is a smaller unit of force than the pound (lb).

If one pound of force equals 4.448 newtons, then a 100 lb person weighs 444.8 newtons.

5.1 Contact forces from ropes and springs

Ropes and springs are often used to make and apply forces.

Ropes are used to transfer forces or change their direction.

The pulling force carried by a rope is called tension.

Tension always acts along the direction of the rope.

5.1 Spring forcesThe force created by a

spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length.

If you stretch a spring twice as much, it makes a force that is twice as strong.

Vectors (Representing forces)Vectors or scalars are used to represent represent

quantitiesquantities in physics. We will use vectors to represent forces.

A scalar is a line segment, whose length represents only magnitude (amount)

A vector is an “arrow” whose length represents magnitude and the direction it points represents the direction the force acts.

5.1 The force vector

The direction of a force makes a big difference in what the force does.

That means force is a vector, like velocity or position.

Arrows are often used to show the direction of forces in diagrams.

Drawing VectorsLength represents magnitude, or amount

Direction is direction of force

Draw a vector representing a force of 25 Newtons acting to the right.

5.1 Drawing a force vector

The arrow points in the direction of the force.

Drawing VectorsLength represents magnitude, or amount

Direction is direction of force

Draw a vector representing a force of 25 Newtons acting to the right.

Draw a vector representing a force of 50 N acting to the left

Resultant Force

A NET FORCE is the force that results from the addition of two or more forces.

VA VB

VR

Sometimes, forces act together. When they do, it is as if they were equal to just one force

Tip to Tail MethodPlace the tail of one vector at the tip of the other.

The vector sum (also called the resultant) is shown in red. Draw resultant vectors from beginning (tail) first vector to end (tip) of last vector.

The resultant is also called the “NET FORCE”

16 N

20 N

4 N

20 N16 N

12 N

9 N12 N

21 N

9 N

Draw the Vectors….. Two workers are moving

a refrigerator. One pushes forward with a force of 200 newtons, the other pushes forward with a force of 300 newtons.. The net force they (together) exert on the refrigerator is:?

Draw the Vectors…..

Two students are moving a refrigerator. One pushes forward with a force of 200 newtons, the other pushes backward with a force of 300 newtons. Force is a vector. The total force they (together) exert on the refrigerator is:

Forces in Combination

Forces acting in same direction are added together to find “Net Force”

Forces acting in opposite directions are subtracted to find “Net Force”

Equilibrium

If the net force acting on an object is equal to zero, the object is said to be in equilibrium, and the forces are “balanced”

Equilibrium

Unbalanced ForcesWhen net force acting on an object, the forces are “Unbalanced”Unbalanced forces cause objects to change their motion, either by changing their speed or velocity

What will happen to the load?

Gravitational Force

The attractive force acting between all masses

This is a major force but it is the weakest of all the long range forces!

Factors effecting the Stength of a gravitational attraction

Masses of two masses

Distance between the two masses

Larger masses = greater attractionLarger distances between masses = less attraction

5.1 Gravity The force of gravity on an object is called weight.

At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

5.1 Weight vs. massWeight and mass are not the same.

Mass is a fundamental property of matter measured in kilograms (kg).

Weight is a force measured in newtons (N).

Weight depends on mass and gravity.

Weight depends on mass and gravity

A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

5.1 Calculating weight

Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars.

1. Looking for: …weight of person in newtons on both planets Given: …mass = 60 kg; g = 3.7 N/kg on Mars; …implied g = 9.8 N/kg on Earth

2. Relationships: W = m x g

3. Solution: 60 kg x 9.8 N/kg = 588 N 60 kg x 3.7 N/kg = 222 N

Sig. fig. = 600 N

Solving Problems

Sig. fig. = 200 N

Would you weigh more or less on a mountain compared to a plain?

Ocean TidesThe Moon is primarily responsible for ocean tides on Earth.

The Sun contributes to tides also.

What are spring tides and neap tides?

SunEarth New MoonFull Moon

Spring Tides

SunEarth

First Quarter

Last Quarter

Neap Tides