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Welcome to Physics 100:Physics for Society

Course website: http://faculty.wiu.edu/p-wang

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•Introduction to the syllabus

•Word puzzle:

Physics 100 is a GPA accelerator if and only if you P_ _ _ _ _ _ _ _ _ _.

Answer: P A R T I C I P A T E.

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Chapter 1 The Laws of Motion, Part 1

August 24: Skating−Newton’s first law of motion

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• When you’re at rest on a level surface, without a push, you remain stationary. with a push, you start moving that

direction.

• When you’re moving on a level surface, without a push, you coast steady and

straight. with a push, you change direction or

speed.

Movie: Ice skating −“Push … hold.”

Observations about the facts on skating:

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Question 1:

Why does a stationary skater remain stationary?

Demo: Tablecloth

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Question 2:

Why does a moving skater continue moving?

Thought experiment: Galileo’s inclined planes

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Physics concept: Inertia

• A body at rest tends to remain at rest• A body in motion tends to remain in motion

Newton’s first law of motion: (Start version)

An object that is free of external influences moves in a straight line and covers equal distances in equal times.A motionless object also obeys this law.

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Question:

Why is Newton’s first law of motion not so apparent to us in our everyday life?

•Real-world complications mask simple physics•Solution: minimize or overwhelm complications•To demonstrate inertia:

Work on level ground (minimize gravity influence)Use wheels, ice, or air support (minimize friction)Work fast (overwhelm friction)Use vacuum (remove air resistance)

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Read: Ch1: 1Homework: Ch1: E3,6.Due: September 4

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August 26: Skating − Newton’s second law of motion

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Physical quantities:

• Position – an object’s location A vector quantity has magnitude

and direction. The magnitude of position is called distance.

• Velocity – the change in position with time The magnitude of velocity is called speed.

• Force – a push or a pull

time

distancespeed

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Newton’s first law of motion

An object that is not subject to any outside forces moves at a constant velocity.

Newton’s first law of motion: (Start version review)

An object that is free of external influences moves in a straight line and covers equal distances in equal times.

Now let us use physical terms:

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Questions:

How does a skater start or stop moving?•He needs a push or pull to start or stop.

How does a skater respond to a push?•He changes his velocity.

Do all skaters respond equally to equal pushes?•Kids respond more quickly than adults do.

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More physical quantities:

• Mass – measure of an object’s inertiaEverything has a mass.

• Acceleration – change in velocity with time.

Acceleration is a vector which has the same direction as the force causing it.Deceleration is actually a type of acceleration.

2ndmeter/seco :onaccelerati ofunit

time

velocityinitial velocityfinalonaccelerati

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Examples of acceleration:

• A car is getting into the highway.• A car is going to a stop sign.• A car is shifting to the left to pass another car.• An elevator is leaving the first floor.• A ball is dropping from a window.• A cart is running down an incline.• The moon is circling the earth.

However, an object that is stationary or has a constant velocity is not accelerating.

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Net force – sum of all the individual forces exerted on an object.

Relation between net force, mass and acceleration:

mass

forcenet onaccelerati

onacceleratimassforcenet or

in algebra

aF

Fa

m m

net

net

Cause

EffectResistance

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Newton’s second law of motion

An object’s acceleration is equal to the net force exerted on it divided by its mass. That acceleration is in the same direction as the net force.

Demo: Hammer, blocks, and hand

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Question:

You are given two black bottles, one empty and the other with water in it. How do you distinguish them without lifting them?

Question:

A 5-Newton (N) force is applied on a cart which has a 10-kilogram (kg) mass. How much is the acceleration of the cart?

Answer:

.m/s 5.0kg 10

N 5 2m

Fa

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Read: Ch1: 1Homework: Ch1: E8;P1Due: September 4

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August 28: Skating − Measurement and units

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Newton’s second law of motion

An object’s acceleration is equal to the net force exerted on it divided by its mass. That acceleration is in the same direction as the net force.

Question:

How do we calculate the net force if an object receives more than one forces?

Answer:

Because forces are vectors, we must find the net force using the rule of addition of vectors.

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Vector addition:

Vector subtraction:

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Measurement and units:

The SI unit (Systéme Internationale d’Unités) has been adopted as the standard unit system in physics.

SI unitAbbrevia-tion English unit

Abbrevia-tion Relation

length meter m foot ft 1 m = 3.28 ft

time second s second s 1 s = 1 s

mass kilogram kg pound-mass lbm 1 kg = 2.20 lbm

The three basic SI units in mechanics:

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Merits of the SI units:

1)Different units for the same quantity are related by factors of 10, 100, 1000, …2)There are only a few basic units (meter, kilogram, second for mechanics).

Examples:

SI units1 kilometer = 1000 meter1 meter = 100 centimeter = 1000 millimeter1 kilogram = 1000 gram

English units1 mile = 5280 feet = 1760 yard1 foot = 12 inch

1 pound = 16 ounce

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Derived units:

Equation Units New name Abbreviation

velocity = position/time meter/second m/s

acceleration

= (change in velocity)/time

meter/second2 m/s2

force = mass × acceleration

kilogram·meter/second2

newton N

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The SI units and the English units:

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How to change a unit:

Example: 65 mile/hour = ? meter/second

Answer:

ndmeter/seco 29.1

ndmeter/seco 3600

160965

second 3600

meter 160965

hour

mile65mile/hour 65

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Question:

Your mass is 80 kilogram and you are standing on ice. Your friend starts to push you with a force of 10 Newton. How much is your acceleration?

Answer:

2

net

m/s 0.125kilogram 80

Newton 10

m

Fa

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Read: Ch1: 1Homework:

1.Dr. Wang’s height is 1.74 meters. His mass is 76.5 kilograms. Please express his height in foot+inch, and his mass in pound.

2.Change the units:i) 60 mile/hour = ? foot/second = ? kilometer/dayii) 170 pound force = ? Newton

Due: September 4

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August 31: Falling Balls − Gravity

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Absence policies

1) Students are expected to attend all classes. Absences are not permitted unless preapproved, except in emergency. Students are responsible for materials presented in class and for changes to the schedule or plans which are announced in class.

2) For absences due to prearranged university business, such as travel of athlete teams and military activities, appropriate document should be submitted in the beginning of the semester.

3) In case of emergency you can leave at any time, however an appropriate document for the nature of the emergency is required afterward.

4) Email the instructor before other planned absences. Your email should describe the event that prevents you from coming to the class. Usually you will get a quick reply from the instructor if the absence is excused. If otherwise it is not approved, a reason will be given in the reply.

5) Filing WIU OARS (Online Absence Reporting System) is not automatically treated as an approved absence.

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Observations about the facts on falling balls:

• When you drop a ball, itbegins at rest, but acquires downward speed.covers more and more distance each second.

• When you toss a ball straight up, itrises to a certain height and comes briefly to a stop.begins to descend, like a dropped ball.

•A thrown ball travels in an arc.

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Question 1:

Why does a dropped ball fall downward?

Gravity and weight

• Gravity is a physical phenomenon that exerts a force on the ball. This force is the ball’s weight.

• The earth’s gravity produces the ball’s weight. The weight points toward the earth’s center.

• The ball’s weight causes it to accelerate downward.

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Question 2:

Do different balls fall at different acceleration?

Weight and mass

• An object’s weight is always proportional to its mass.• Near the surface of the earth,

which is called the “acceleration due to gravity”.

in algebra (Demo using weights and a spring scale)gw mweight = mass · acceleration due to gravity

ogramnewton/kil 9.8constantmass

weight

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Acceleration due to gravity

• Why is the name?

weight/mass = force/mass = acceleration

Acceleration due to gravity is indeed an acceleration.

9.8 Newton/kilogram = 9.8 meter/second2

• On the surface of the earth, all falling balls accelerate downward at

• Therefore different balls fall at the same acceleration.

gravity todue onacceleratimass

gravity todue onacceleratimass

mass

weightonaccelerati

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Question:

Dr. Wang’s mass is 76.5 kilogram.

1)How much is his weight on the earth?

2)How much is his weight in the far space?

3)If he falls down from a cliff, how much is his acceleration?

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More about gravity: the law of universal gravitation

221

r

mmGF

Near the surface of the earth,

223

2423-11

2earth

earth m/s 8.9 m10 6371

kg 10 5.97s kg /m 10 6.67

r

mGg

gravitational

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Acceleration due to gravity (g) varies:

•At mountains•Shape of the earth: oblate spheroidAt the equator (r = 6378 km), at the poles (r = 6357 km)•At the moon•Dig a hole?

2earth

r

mGg

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Read: Ch1: 2Homework: Ch1: P9,10.Due: September 11

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September 2: Falling Balls − Projectile motion

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2:00-7:00 pm 2:00-7:00 pm

2:00-7:00 pm 2:00-7:00 pm

PHYSICS TUTORING CENTER(Especially for your homework)

Fall 2015, Currens Hall 515

They are prepared to help you. ALL FREE!

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The velocity of a falling ball

Observation: A falling ball accelerates downward. Its acceleration is a constant. Its velocity increases in the downward direction.

Question: How do we calculate the velocity of an object which has a constant acceleration?

time

velocityinitial- velocityfinaltime

yin velocit changeonaccelerati

in algebra, tif avv

timeonaccelerati velocityinitial velocityfinal

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The position of a falling ball

Observation: A falling ball accelerates downward steadily. Its altitude decreases ever faster.Question: How do we calculate the position of an object which is accelerating constantly?

timeonaccelerati2

1 velocityinitial

2

velocityfinal velocityinitial velocityaverage

2timeonaccelerati2

1time velocityinitialposition initial position final

in algebra, 2

2

1ttiif avxx

If the acceleration is constant, we can use“average velocity time” to find the change of position.

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The position and velocity of a ball falling from rest

t

ttif

g

gavv 0

2

2

2

2

12

100

2

1

t

tt

ttiif

g

g

avxx

Velocity:

Position:

“ J ” means downward.

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The position and velocity of a ball thrown upward

tt iif gvavv

2

2

2

2

12

10

2

1

tt

tt

tt

i

i

iif

gv

gv

avxx

Velocity:

Position:

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Throwing a ball at an angle

Simplification of physics:A vector can be separated into its horizontal and vertical components. The two components follow Newton’s laws of motion independently.

• In the vertical direction, the ball is falling. It goes up initially at viy.

• In the horizontal direction, the ball coasts at vix.

Gravity only affects the ball’s vertical motion:

iyixi vvv

v

xv

yv

Movie: Projectile motion

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Question:

A hunter is shooting at a monkey far away on a tree with a gun. If the monkey knows a little physics, when he sees the flash from the gun, should he

1) stay still on the tree, or

2) jump down from the tree?

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Read: Ch1: 2Homework: Ch1: P11,12,13.Due: September 11

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September 4: Ramps − Newton’s third law of motion

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Bonus on Answering In-class Questions

• This form is downloadable from our course website.• It is suggested that you raise your hand before answering an

in-calss question.• Please briefly record your oral answer when being called by

the instructor.• Please submit the form before each exam to get a bonus.• You get a bonus even when your answer is incorrect.

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Question:

A ball is resting on a table. The ball has weight, why doesn’t it fall into the table?

Answer:

The ball is pushing downward on the table, but the table is also pushing upward on the ball.

The upward force exerted by the table on the ball is a support force, which equals the ball’s downward weight in magnitude. The ball is stationary because there is no net force on it.

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Examples of pairs of forces between two objects:

•A ball is pushing on a table. The table is also pushing on the ball.

•If you push on a friend, that friend always push back on you.

•A hammer hits on a nail. The nail stops the hammer.

•You push on the ice surface. The ice pushes back on you, so you begin to slide.

•The earth attracts you (so you have a weight). You also attract the earth.

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Discovery 1: The forces between two objects are always in opposite directions

•A ball is pushing on a table. The table is also pushing on the ball. (One downward, the other upward)

•If you push on a friend, that friend always push back on you. (One toward your friend, one toward you)

•A hammer hits on a nail. The nail stops the hammer. (One downward, the other upward)

•You push on the ice surface. The ice pushes back on you, so you begin to slide. (One backward, the other forward)

•The earth attracts you (so you have a weight). You also attract the earth. (One toward the earth, the other toward you)

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Discovery 2: The forces between two objects are always equal in magnitude

Demo: Two persons pull each other with two different spring scales.

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Newton’s third law of motion

For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.

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More about Newton’s third law of motion

Other forms:

“For every action, there is always an equal but opposite reaction.”

“You can’t touch without being touched.”

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Additional notes:

•Newton’s third law is universal, it works whether the object is stationary or moving.

•The two forces are exerted on two different objects. They do not cancel directly.(cf. Two forces exerted on the same object may cancel each other.)

•The two forces exist at the same time.

•The two forces are always in opposite directions.In mechanics they are also along one line.

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Question:

Will the earth move down when I jump up?

Answer:

The acceleration on me:

The acceleration on the earth:

am

F

amF

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More examples on action and reaction:

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Read: Ch1: 3Homework: Ch1: E26.Due: September 11

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September 9: Ramps − Energy and work

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Physical quantity: Work

The work you do on an object is the product of the force you exert on it times the distance it travels along the direction of your force.

distanceforcework

in algebra dFW

Unit of work: newton · meter = joule (abbreviated as J)

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Question:

Our textbook weighs about 10 newton. You lift it slowly from ground to a table of 0.75 meter high. How much work have you done on the textbook?

Answer:

joule 7.5

meter 0.75newton 10

dFW

You need a force of about 10 newton to lift the textbook. The work you have done is

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More about work

•Force is exerted over the whole distance.

•If the object does not move exactly in the direction of the force, then

work = component of the force along the direction of motion distance

= force component of the motion along the direction of the force

•If the angle between the motion and the force is

1)less than 90°, the work done is positive.

2)equal to 90°, the work done is 0.

3)larger than 90°, the work done is negative.

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Physical quantity: Energy

Energy is the capacity of an object to do work.

The unit of energy is joule.

Examples of objects with energies:

•A running car

•A lifted rock

•A compressed spring

•A firework

•A pancake

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Two principle forms of energy

Kinetic energy: Energy contained in the motion of an object.

Potential energy: Energy stored in a certain shape or structure of an object.

Examples of kinetic and potential energies:

•A running car

•A lifted rock

•A compressed spring

•A firework

•A pancake

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Gravitational potential energy

When you lift an object, its gravitational potential energy increases by the amount of work you have done on it.

heightgravity todueon accelerati mass

energy potential nalgravitatio

in algebra hgmU

by choosing the ground level for zero potential energy.

The amount of energy are always relative.

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Conservation of Energy

Energy cannot be created or destroyed. It may be transformed from one form into another, but the total amount of energy never changes.

Relations between work and energy:

•Energy is the capacity to do work.

•Work is the mechanical means of transferring energy.

Newton’s third law and the conservation of energy.

Movie: Energy transfer − Fly without a wing: Rope riding

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Read: Ch1: 3Homework: Ch1: E31,34.Due: September 18

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September 11: Ramps − Mechanical advantage

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More about support force

•Originated from the microscopic structure of the materials.

•Always in the direction perpendicular to the surface (while friction force is along the surface).

•Can adjust itself to balance the force received (like a pressed spring).

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How to lift a cart on a ramp

weight

support force

ramp force (sum)

You need to balance the ramp force so that the cart begins to move uphill.

Your push force is needed.

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Question:

How much uphill force is needed to push the cart up the ramp?

Answer:

ramp theoflength

ramp theofheight cart theofweight forcepush your

ramp theoflength

ramp theofheight

cart theofweight

forcepush your

Using the property of similar triangles,

heig

ht

length

weight

support

force

ramp force

(sum)

Your push force

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Question:

The builders of the pyramids used a long ramp to lift a 20,000-kg block. If the block rose 1 m in height while traveling 20 m along the surface of the ramp, how much force was needed to push the block up the ramp?

Answer:

N 9800

m 20

m 1N/kg 9.8kg 20000

ramp theoflength

ramp theofheight cart theofweight forcepush

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Work done on lifting the cart on the ramp

The more gradual the ramp, the easier to push:

• Going up a steep ramp: Large force, short distance: W = F · d

• Going up a gradual ramp: Small force, long distance: W = F · d

ramp theoflength

ramp theofheight cart theofweight forcepush your

ramp theofheight cart theofweight

ramp theoflength forcepush your youby donework

The work done by you is the same in either way. It is also independent of the steepness of the ramp:

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Mechanical advantage:

A mechanical device does a specific amount of work by altering the balance between force and distance.

A ramp provides mechanical advantage. It allows you to push less hard, but you must push for a longer distance so that the work you do is always the same.

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Various devices with mechanical advantages

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Read: Ch1: 3Homework: Ch1: E39; P18,21.Due: September 18