Transcript Chapter 1

Welcome to Physics 100:
Physics for Society
Course website: http://faculty.wiu.edu/p-wang
1
• 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.
2
Chapter 1 The Laws of Motion, Part 1
August 24: Skating−Newton’s first law of motion
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Observations about the facts on skating:
Movie: Ice skating −“Push … hold.”
• 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.
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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: 1
Homework: 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.
distance
speed 
time
• Force – a push or a pull
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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:
Newton’s first law of motion
An object that is not subject to any outside
forces moves at a constant velocity.
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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 inertia
Everything has a mass.
• Acceleration – change in velocity with time.
final velocity  initial velocity
accelerati on 
time
unit of accelerati on : meter/seco nd 2
Acceleration is a vector which has the same
direction as the force causing it.
Deceleration is actually a type of acceleration.
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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:
Effect
or
in algebra
net force
accelerati on 
mass
Cause
Resistance
net force  mass  accelerati on
Fnet
a
m
Fnet  m  a
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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:
F
5N
a 
 0.5 m/s 2 .
m 10 kg
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Read: Ch1: 1
Homework: Ch1: E8;P1
Due: 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.
The three basic SI units in mechanics:
SI unit
Abbreviation
English unit
Abbreviation
Relation
length
meter
m
foot
ft
1 m = 3.28 ft
time
second
s
second
s
1s=1s
mass
kilogram kg
pound-mass
lbm
1 kg = 2.20 lbm
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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 units
1 kilometer = 1000 meter
1 meter = 100 centimeter
= 1000 millimeter
1 kilogram = 1000 gram
English units
1 mile = 5280 feet
= 1760 yard
1 foot = 12 inch
1 pound = 16 ounce
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Derived units:
Equation
Units
velocity
= position/time
meter/second
m/s
acceleration
= (change in
velocity)/time
meter/second2
m/s2
force
= mass ×
acceleration
kilogram·
meter/second2
New name
newton
Abbreviation
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:
mile
1609 meter
65 mile/hour  65
 65 
hour
3600 second
1609
 65 
meter/seco nd
3600
 29.1 meter/seco nd
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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:
Fnet
a
m
10 Newton

 0.125 m/s 2
80 kilogram
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Read: Ch1: 1
Homework:
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/day
ii) 170 pound force = ? Newton
Due: September 4
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August 31: Falling Balls − Gravity
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Absence policies
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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, it
begins at rest, but acquires downward
speed.
covers more and more distance each
second.
• When you toss a ball straight up, it
rises 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,
weight
 constant  9.8 newton/kil ogram
mass
which is called the “acceleration due to gravity”.
weight = mass · acceleration due to gravity
in algebra
w  mg
(Demo using weights and a spring scale)
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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
weight mass  accelerati on due to gravity
accelerati on 

mass
mass
 accelerati on due to gravity
• Therefore different balls fall at the same acceleration.
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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
gravitational
G  m1  m2
F
r2
Near the surface of the earth,


G  mearth 6.67  10-11 m3 / kg s 2  5.97  10 24 kg
2
g


9
.
8
m/s
2
rearth
6371 103 m 2


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Acceleration due to gravity (g) varies:
G  mearth
g
r2
•At mountains
•Shape of the earth: oblate spheroid
At the equator (r = 6378 km), at the poles (r = 6357 km)
•At the moon
•Dig a hole?
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Read: Ch1: 2
Homework: Ch1: P9,10.
Due: September 11
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September 2: Falling Balls − Projectile motion
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(Especially for your homework)
Fall 2015, Currens Hall 515
They are prepared to help you. ALL FREE!
Monday
2:00-7:00 pm
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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?
change in velocit y
accelerati on 
time
final velocity - initial velocity

time
final velocity  initial velocity  accelerati on  time
in algebra, v f  v i  a  t
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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?
If the acceleration is constant, we can use
“average velocity  time” to find the change of position.
initial velocity  final velocity
2
1
 initial velocity  accelerati on  time
2
average velocity 
1
final position  initial position  initial velocity  time  accelerati on  time 2
2
1
in algebra, x f  x i  v i  t  a  t 2
2
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The position and velocity of a ball falling from rest
Velocity:
v f  vi  a  t  0  g  t
 g  t
“ ̶ ” means downward.
Position:
1
x f  xi  v i  t  a  t 2
2
1
 0  0t  g t2
2
1
  g t2
2
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The position and velocity of a ball thrown upward
Velocity:
v f  vi  a  t  vi  g  t
Position:
1
x f  xi  v i  t  a  t 2
2
1
 0  vi  t  g  t 2
2
1
 vi  t  g  t 2
2
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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.
vy
v
vx
v i  v ix  v iy
• 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:
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: 2
Homework: 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.
50
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.
57
Question:
Will the earth move down when I jump up?
Answer:
F
The acceleration on me:
a
m
The acceleration on the earth:
F
m

a
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More examples on action and reaction:
59
Read: Ch1: 3
Homework: 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.
work  force  distance
in algebra
W  F d
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:
You need a force of about 10 newton to lift the
textbook. The work you have done is
W  F d
 10 newton  0.75 meter
 7.5 joule
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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.
64
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
65
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
66
Gravitational potential energy
When you lift an object, its gravitational potential energy
increases by the amount of work you have done on it.
gravitatio nal potential energy
 mass  accelerati on due to gravity  height
in algebra
U  m g h
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
68
Read: Ch1: 3
Homework: Ch1: E31,34.
Due: September 18
69
September 11: Ramps − Mechanical advantage
70
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).
71
How to lift a cart on a ramp
weight
You need to balance the ramp force so that the cart begins
to move uphill.
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How much uphill force is
needed to push the cart up
the ramp?
weight
height
Question:
Answer:
Using the property of similar triangles,
your push force
height of the ramp


weight of the cart length of the ramp
your push force  weight of the cart 
height of the ramp
length of the ramp
73
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:
push force  weight of the cart 
height of the ramp
length of the ramp
1m
 20000 kg  9.8 N/kg 
20 m
 9800 N
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Work done on lifting the cart on the ramp
The more gradual the ramp, the easier to push:
your push force  weight of the cart 
height of the ramp
length of the ramp
• Going up a steep ramp: Large force, short distance: W = F · d
• Going up a gradual ramp: Small force, long distance: W = F · d
The work done by you is the same in either way. It is also
independent of the steepness of the ramp:
work done by you  your push force  length of the ramp
 weight of the cart  height 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: 3
Homework: Ch1: E39; P18,21.
Due: September 18
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