Download force A - Mr. Deakin

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Newton’s Laws of Motion
There are many different kinds of
motion. We experience some kind of
motion every minute of every day. As
complicated as motion is, there are only
three laws that describe all motion!
These laws were discovered by a man
name Sir Isaac Newton. They are called
Newton’s 1st Law, Newton’s Second
Law, and Newton’s 3rd Law.
Newton’s 1st Law
(Also called the Law of Inertia)
Before we learn about Newton’s first law, we need to
understand inertia, matter and mass.
Mass is the amount of ___________
an object has.
matter
Examples: Underline the thing with more mass, and circle
the thing with more matter.
1)
Elephant
Mouse
2)
Bicycle
Car
3)
Beach ball
Bowling ball
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4)
100kg Lead ball
70kg Aluminum ball
5)
Empty balloon
Filled balloon
6)
Full bucket
Empty bucket
Explanation of number 5:
Two identical empty balloons have the same amount
of matter, so they have the same mass. When one of
the balloons is filled with air, the filled balloon has
more matter. (Air is a kind of matter.) So the filled
balloon has a greater mass.
Students may argue that air doesn’t have mass or even that a balloon filled with helium (a type of
matter) floats and an empty balloon doesn’t float. The balloon filled with helium floats in earth’s
atmosphere because of the buoyant force (upward buoyant force = weight of air displaced by the
object). Helium gas is less dense than air, so it floats. If the masses of an empty balloon and a
balloon filled with helium gas are measure in a vacuum (where there is no buoyant force), the filled
balloon will register a greater mass.
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The more mass something has, the harder it is to change
the way it is moving.
Examples:
1) It is easy to change the motion of a balloon moving
at 10km/h, but it is difficult to change the motion of
car (for example)
a _______________
traveling at 10km/h.
2) It is easy to change the motion of a rolling beach ball,
but it is difficult to change the motion of a rolling
bowling ball (for example)
__________________.
Remember
that no
motion is
also a type
of motion.
3) It is easy to change the motion of a pencil sitting on a
desk, but it is harder to change the motion of a
book (for example)
_____________
sitting on a desk.
more
Objects with ____________
mass resist change in their
less
motion more than objects with ___________
mass.
Inertia = Resistance an object has to a change in its motion.
Answer the following questions. Use the word “inertia” in
your answer:
1) Why it is easy to change the motion of a balloon
moving at 10km/h, but it is difficult to change the
motion of a car traveling at 10km/h?
The car has more inertia, so it is more difficult to change its motion.
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2) Why it is easy to change the motion of a rolling
beach ball, but it is difficult to change the motion of
a rolling bowling ball?
The bowling ball has more inertia, so it is more difficult to change its motion.
3) Why it is easy to change the motion of a pencil
sitting on a desk, but it is harder to change the
motion of a book sitting on a desk?
The book has more inertia, so it is more difficult to change its motion.
4) Why it is easy to change the motion of a __________
but difficult to change the motion of a ___________?
5) Explain why the items in the following picture do
not move when the table is pulled away very quickly.
(Remember to use the word inertia.)
The items on the table have inertia.
_____________________
So when the table is pulled away
_____________________
quickly, the items resist a change
_____________________
in their motion, so they do not
_____________________
move to the left with the table.
_____________________
_____________________________________________
_____________________________________________
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Newton’s 1st Law
=
Every object stays at rest or moves
in a straight line at a constant speed
unless a net force changes its motion.
Use Newton’s 1st Law to answer the following question:
1)
2)
3)
The coin is at rest and remains at rest when the card is hit out from underneath it.
1) ____________________________________________
There is not enough force to make with the card.
____________________________________________
The hammerhead is moving downward at a constant speed. When the hammer stops
2) ____________________________________________
suddenly, the hammerhead continues its motion downward until it is forced to stop.
____________________________________________
3) ____________________________________________
When the force is applied slowly, the force on the upper string is greater than the
force on the lower string (Fupper = Fpull + Fball’s gravity), so the string breaks above
the ball.
____________________________________________
When the force is applied quickly, the ball continues to stay at rest because it is
initially at rest. There is no additional force on the upper string, so the lower
____________________________________________
string breaks.
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4) You roll a golf ball flat on the ground.
On the ground is a curved wall. The ball
follows the path in the figure. When it
gets to the end of the wall, which way
will it go: path 1, path 2, or path 3? Use
Newton’s 1st Law to explain why.
The golf ball will follow path 2, because it must move in a straight line at
____________________________________________
st
constant speed since there are no other forces acting on it. This is N’s 1 law.
____________________________________________
____________________________________________
5) A car is driving along a flat, straight road at
50km/h. What net force is necessary to keep this
motion the same?
None! Any net force will cause the motion of the car to change. There must be
____________________________________________
no net force if the car is to continue with the same motion.
____________________________________________
____________________________________________
6) A car is driving along a flat, straight road at
50km/h. The tires are pushing the car forward
(force A), and the wind and road are pushing back
on the car (force B). If the motion of the car does
not change, which of the following is true?
a) Force A is bigger than force B
b) Force A is equal to force B.
c) Force A is smaller than force B.
__________________________________________
b) A = B, because there must be no net force for motion to remain unchanged.
__________________________________________
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7) Imagine that you are floating in space. You have
two identical boxes. One is empty, and one is filled
with sand. How can you tell which one has the sand
if you can not open either of the boxes?
Shake the boxes. The box with the greater mass offers greater resistance to a
____________________________________________
change in its motion.
____________________________________________
____________________________________________
8) Why is an axe with more mass better at chopping
wood that an axe with less mass?
____________________________________________
When the more massive axe hits the wood, it is more resistive to changing its
motion than the less massive axe would be. It will hence go deeper into the wood
____________________________________________
and chop the wood more effectively.
____________________________________________
9) When you use one hand to pull a piece of toilet
paper from a roll, why is it better to pull quickly
than to pull slowly?
____________________________________________
The inertia of the toilet paper roll resists a rapid change in motion more than it
resists a slow change in motion. Pulling slowly will unwind the roll. Pulling
____________________________________________
quickly will rip off a piece of toilet paper.
____________________________________________
10) Can the motion of an object change if there is no
net force acting on it?
____________________________________________
No! Newton’s 1st Law states that a force is necessary to change the motion of an
object.
____________________________________________
____________________________________________
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Newton’s 2nd Law
(Also called the Law of Acceleration)
Newton’s 1st Law tells us how an object behaves when
there is no net force acting on it.
Newton’s 2nd Law tells us how an object behaves when
there is a net force acting on it.
Before we can learn about Newton’s 2nd Law, we must
understand acceleration.
Acceleration = Change is motion over time.
In each of the following situations, is there acceleration?
Circle “YES” or “NO”:
1) A car changes speed from 100km/hr to 35km/h.
YES or NO
2) A car speeds up by 20km/h.
YES or NO
3) A car traveling at 60km/h turns to the left.
YES or NO
4) A skydiver jumps out of a plane and falls towards
the ground faster and faster.
YES or NO
5) A skydiver reaches terminal velocity and falls
towards the ground at a constant speed.
YES or NO
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6) A car driving on a straight road at constant speed.
YES or NO
7) A person sitting on the grass.
YES or NO
Newton’s 2 Law
nd
Newton’s 2nd Law
=
=
A net force on an abject causes the
object to accelerate in the same
direction as the net force.
F = ma
Examples:
1) A car drives at 50km/h east along a flat highway.
The car feels a net force to the east. How does the
motion of the car change?
The car starts to speed up but continues to travel east.
____________________________________________
2) A car drives at 50km/h east along a flat highway.
The car feels a net force to the west. How does the
motion of the car change?
The car starts to slow down but continues to travel east.
____________________________________________
3) A car drives at 50km/h east along a flat highway.
The car feels a net force to the north. How does the
motion of the car change?
The car starts to turn towards the north.
____________________________________________
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4) A car drives at 50km/h east along a flat highway.
The car feels a net force to the south. How does the
motion of the car change?
The car starts to turn towards the south.
____________________________________________
5) A skydiver falls straight towards the earth at a
terminal velocity of 120km/h. She feels a net force
downward. A) How does her motion change?
B) What situation could make this happen?
starts speeding up, but continues to fall straight downwards.
A) She
_________________________________________
She could be falling through an area of less wind resistance or she could
B)__________________________________________
have tucked herself into a ball.
6) A skydiver falls straight towards the earth at a
terminal velocity of 120km/h. She feels a net force
upward. A) How does her motion change?
B) What situation could make this happen?
starts to slow down, but continues to fall straight downwards.
A) She
_________________________________________
She could be falling through an area of more wind resistance or have just
B)__________________________________________
deployed her parashute.
7) A skydiver falls straight towards the earth at a
terminal velocity of 120km/h. She feels a net force
to the left. A) How does her motion change?
B) What situation could make this happen?
starts to fall diagonally – mostly downward, but partially to the left.
A) She
_________________________________________
She could be falling through an area of a left wind.
B)__________________________________________
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8) A skydiver falls straight towards the earth at a
terminal velocity of 120km/h. She feels a net force
to the right. A) How does her motion change?
B) What situation could make this happen?
starts to fall diagonally – mostly downward, but partially to the right.
A) She
_________________________________________
She could be falling through an area of a right wind.
B)__________________________________________
Newton’s 2nd Law tells use that F = ma .
kilograms
The units of mass are _______________(kg)
meters per second squared
The units of acceleration are ___________________
(m/s2)
newtons
The units of force are ________________
(N)
So 1N = (1kg)x(1m/s2)
Answer the following questions:
1) A 1000kg car accelerates at 2.0m/s2. What force is
being made on the car?
F = ma, so F = (1000kg)(2.0m/s2) = 2000N
2) A 2000kg car accelerates at 1.0m/s2. What force is
being made on the car?
2
F = (2000kg)(1.0m/s ) = 2000N
____________________________________________
3) A 500kg car accelerates at 4.0m/s2. What force is
being made on the car?
2
F = (500kg)(4.0m/s ) = 2000N
____________________________________________
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4) What do you notice about the answers to questions
1, 2, and 3?
They are the same.
____________________________________________
We see then that there are two things that control how big
a force is: the acceleration and the mass.
So a big acceleration on a small mass and a small
acceleration on a big mass could both be caused by the
same force. That is,
F=
m
a and
F=m
a
Answer the following questions:
1) A 5000kg car accelerates at 2.4m/s2. If an equal
force is made on a 10000kg truck, what will the
truck’s acceleration be?
(5000kg)(2.4m/s2) = (10000kg)(?)
So, the truck’s acceleration is 1.2m/s2.
If the mass is twice as big, then the acceleration must
be half as big for the same force.
2) A 1000kg car accelerates at 4.0m/s2. If an equal
force is made on a 4000kg car, what will the
acceleration be?
1.0m/s2
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3) A 1000kg car accelerates at 4.0m/s2. If an equal
force is made on a 500kg car, what will the
acceleration be?
8.0m/s2
4) A 900kg car accelerates at 2.5m/s2. If an equal
force causes a second car to accelerate at 7.5m/s2,
what is the mass of the second car?
(900kg)(2.5m/s2) = (?)(7.5m/s2)
So, the mass of the second car is 300kg.
5) A 1000kg car accelerates at 2.0m/s2. If an equal
force causes a second car to accelerate at 8.0m/s2,
what is the mass of the second car?
250kg
6) A 1000kg car accelerates at 2.0m/s2. If an equal
force causes a second car to accelerate at 0.5m/s2,
what is the mass of the second car?
4000kg
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Since the force equation has three quantities, (force, mass,
and acceleration), we can really make three equations:
1) F = ma
F
m
=
2)
a
F
a
=
3)
m
=
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We can use these three equations to answer different types
of questions about Newton’s 2nd Law.
Answer the following questions:
1) A 550kg car accelerates at 2.0m/s2. What is the
force the engine makes on the car?
____________________________________________
1100N
2) A 5000kg truck accelerates at 1.0m/s2. What is the
force the engine makes on the truck?
5000N
____________________________________________
3) A 500000kg plane accelerates at 8m/s2. What is the
force the engines make on the plane?
4000000N
____________________________________________
4) A force of 10000N accelerates a 2000kg car. What
is the acceleration of the car?
0.5m/s2
____________________________________________
5) A force of 1000000N accelerates a 50000kg plane.
What is the acceleration of the plane?
20m/s2
____________________________________________
6) A force of 10000N accelerates a 100000kg boat.
What is the acceleration of the boat?
0.1m/s2
____________________________________________
7) A force of 1000N accelerates a car at 10m/s2. What
is the mass of the car?
100kg
____________________________________________
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8) A force of 10000N accelerates a car at 5m/s2. What
is the mass of the car?
2000kg
____________________________________________
9) A force of 50N accelerates a bicycle at 1.0m/s2.
What is the mass of the bicycle?
50kg
____________________________________________
10) A 980kg car accelerates at 2.45m/s2. What is the
force the engine makes on the car?
2401N
____________________________________________
11) A force of 850N accelerates a 42.5kg box. What is
the acceleration of the box?
20m/s2
_____________________________________________
12) A force of 7654N accelerates a car at 3.147 m/s2.
What is the mass of the car?
2432kg
____________________________________________
13) A rocket becomes easier and easier to accelerate as
it travels through space. Why is this? (Hint: About
90% of the mass of a rocket is fuel)
As mass decreases (because the fuel is burned), the force necessary to make
____________________________________________
the same acceleration gets smaller and smaller, since F = ma.
____________________________________________
14) If an object has no acceleration, can we conclude
that no forces are acting on the object? Explain.
No! We can only conclude that there is not net force acting on it. There may
____________________________________________
be several forces acting on it but they all must cancel each other out.
____________________________________________
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Newton’s 3rd Law
(Also called the Law of Interaction)
Newton’s 1st Law tells us how an object behaves when
there is no net force acting on it.
Newton’s 2nd Law tells us how an object behaves when
there is a net force acting on it.
Newton’s 3rd Law tells us how a force on an object affects
other objects.
Newton’s 3rd Law
=
When one object exerts a force on
another object, the second object
exerts a force of equal size on the
first object in the opposite direction.
Examples:
1) A book pushes down on a
table (see diagram) with a
force of 12N. The table
upward on the
pushes _______
of 12N
book with a force _______.
2) A hand pushes on a spring
with a downward 40N
spring
force. The __________
pushes on the __________
book
upward
with an _____________
40N
force
______
_________.
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pulls
downward
3) A girl’s weight _________
___________
force
with a _________of
600N. The rings
__________
____________
with a
pull
upward
force
600N
__________
of _________.
4) See the figure. The action force is the car
tires pushing on the ground. The
the ground pushing on the car tires.
reaction force is ____________________
__________________________________.
5) See the figure. The action force is the
rocket pushing on the gas. The reaction
the gas pushing on the rocket.
force is ________________
______________________________.
6) See the figure. The action force is
the earth pulling down on the ball.
The reaction force is
______________________________
the ball pulling up on the earth.
______________________________.
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7) As you stand on a floor, the floor exerts an
upward force (normal force) on your feet.
Why don’t you accelerate upward from this
force? See the figure.
Your weight and the normal force of the floor pushing up on you
____________________________________
are equal but in opposite direction. They cancel out to zero, so you don’t
accelerate. (Careful: these are NOT action-reaction forces. The reaction force of
____________________________________________
the earth pulling you down [weight] is you pulling up on the earth.)
8) A girl hangs from the ends of the rope as
shown in the figure. If gravity pulls her
downward with a force of 500N, what is the
reading on the force scale near her left hand?
250N. The sum of the upward forces (250N + 250N) must equal the
________________________________________
sum of the downward forces (500N).
________________________________________
9) For each of the following forces, what is the
opposite force from Newton’s 3rd Law?
a) The downward force of a hammer on a nail.
b) The force of gravity pulling down on a book.
c) The force of helicopter blades pushing air down.
d) The force of air resistance on a thrown baseball.
The upward force of the nail on the hammer.
a) __________________________________________
The force of gravity pulling up on the earth.
b) __________________________________________
The force of air pushing helicopter blades up.
c) __________________________________________
The force of the baseball pushing forwards on the air.
d) __________________________________________
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10) In each of the situations shown in the figures
below, does the force scale read 0N, 100N, 200N,
or something else? Why?
100N. The total force to the right of the spring scale is 100N
a) __________________________________________
100N. These two situations are identical as far as the spring scale is concerned.
b) __________________________________________
a)
b)
Now that we have studied Newton’s three laws, it is time
to put them all together and answer questions that use
more than one of Newton’s laws. First, complete Newton’s
Three Laws: (Try to fill in the blanks without reviewing)
Newton’s 1st Law
Newton’s 2nd Law
Newton’s 3rd Law
=
rest
Every object stays at _____
or moves
speed
in a straight line at a constant _____
net
force
unless a ___
____
changes its motion.
=
net
force
A ____
_____
on an abject causes the
object to _____________
in the same
accelerate
net _____.
force
F = ma
direction as the ____
=
force
When one object exerts a _______
on
another object, the second object
exerts a ______
of _____
on the
force
size
equal ____
opposite
first object in the _______
direction.
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1) For this question, the two rocks in space
(shown in the figure to the right) are
made of the same material, so a larger
rock means a larger mass. The only
force is gravity.
i) How does the gravitational force
of rock B on rock A compare to
the gravitational force of rock A
on rock B in each situation?
Which Law explains this?
It is equal in size and opposite in direction.
_____________________________
Newton’s 3rd law.
_____________________________
_____________________________
_____________________________
ii)
Which is bigger: the acceleration
of rock A or the acceleration of
rock B in each situation. Why
a) _____________________________________
Rock B, because it has less mass.
Rock B, because it has less mass.
b) _____________________________________
The have the same acceleration, because they have the same mass.
c) _____________________________________
Rock A, because it has less mass.
d) _____________________________________
Rock A, because it has less mass.
e) _____________________________________
iii) Which law did you use to answer part ii)?
Newton’s 2nd Law.
______________________________________
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iv) The earth and the moon are like two rocks in
space. Why does the moon orbit the earth
and not the earth orbit the moon?
The force on each body is the same - but in opposite directions.
_______________________________________
(Newton’s 3rd Law) The moon is accelerated much more than the
earth, because it has so much less mass than the earth. (Newton’s
_______________________________________
2nd Law)
v)
Which two laws did you use to answer part
iv)?
See above.
______________________________________
vi) If the earth suddenly disappeared, how
would the motion of the moon be different?
The moon would travel in a straight line at constant speed because
_______________________________________
there would be no more forces acting on it.
vii) Which law did you use to answer part vi)?
Newton’s 1st Law.
_______________________________________
2) Your empty hand is not hurt when it bangs lightly
against a wall. But when you bang it against a wall
while carrying a heavy load, it hurts. Why? Which
of Newton’s law’s explains this?
Your empty hand is not hurt because there is little mass, so there is little
____________________________________________
inertia - little resistance to a change in motion (i.e. stopping). With a heavy
load, there is much mass and much inertia, so your hand has more of a
____________________________________________
tendency to remain moving. Your hand gets squashed a bit and it hurts.
Newton’s 1st Law.
____________________________________________
____________________________________________
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3a.
3) Before Galileo and Newton, people
thought that if you dropped a stone from
the mast of a ship that is moving at a
constant speed in a straight line, the
stone would fall to the ground behind
the mast as in figure 3a. But we know
3b.
that it will fall as in figure 3b. Can you
explain this using Newton’s laws?
There is no net force in the horizontal direction, so the
____________________________________
horizontal motion of the rock will not change – it will continue
moving forward at the same speed as the ship. It will then move
______________________________________
as far forward horizontally as the ship does and falls as is shown
in figure 3b.
________________________________________
4) Bob the painter swings from his bosun’s
chair to paint buildings. His weight is 500N,
and the rope can hold a maximum of 300N.
Why does the rope not break?
Bob’s weight (500N) is supported by two ropes (which together
_____________________________________
can support a maximum of 600N), so the rope doesn’t break.
_____________________________________
_____________________________________
5) Bob the painter decides to change
things a little. He ties his bosun’s chair
as in the figure to the right. Why does
the rope break in this case?
Bob’s weight (500N) is supported by one rope (which
__________________________________
can support a maximum of 300N), so the rope breaks. See ya
__________________________________
Bob!
__________________________________
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6) You hold an apple over your head. a) Write down
all the forces on the apple and their reaction forces.
b) Then you drop the apple. Identify all the forces
on the falling apple and their reaction forces.
Action: earth pulling down on apple – Reaction: apple pulling up on earth.
a) __________________________________________
____________________________________________
Action: hand pushing up on apple – Reaction: apple pushing down on hand
Action: earth pulling down on apple – Reaction: apple pulling up on earth.
b) __________________________________________
____________________________________________
Action: air pushed up on apple – Reaction: apple pushing down on air
7) If we see an object that is not moving (or moving at
constant speed in a straight line) but we know there
is force acting on it, what conclusion can we make?
No motion, or motion in a straight line at constant speed, means there is no net
____________________________________________
force acting on the object. So there must be at least one other force acting on the
object to counteract the force we know is there.
____________________________________________
____________________________________________
8) When your car moves along the highway at a
constant speed in a straight line, there is
(obviously) no net force acting on the car. Then
why do you have to keep pushing in the gas pedal?
You must push on the gas pedal to provide a forward force for the car to equal
____________________________________________
the forces of friction (air and ground) but in the opposite direction so that the net
force is zero.
____________________________________________
____________________________________________
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9) If you push with a force of 200N to slide a box
across a floor at a constant speed in a straight line,
how much friction is exerted by the floor on the
box?
200N in the opposite direction to motion. There must be zero net force in order
____________________________________________
____________________________________________
that the box moves in a straight line at constant speed.
____________________________________________
10) If a big truck and a tiny car have a head-on
collision, a) which vehicle experiences the greater
force? b) Which vehicle experiences the greater
acceleration? Explain both of your answers.
They each experience the same size force (in opposite directions of course).
a) __________________________________________
Newton’s 3rd Law.
____________________________________________
The tiny car will experience a larger acceleration, because its mass is less.
b) __________________________________________
Newton’s 2nd Law.
____________________________________________
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11) Sua and Mark are astronauts floating in space?
They are some distance apart and connected by a
long rope. If Mark starts pulling on the rope, will
he pull Sua towards him, will he pull himself
toward Sua, or will they both move towards each
other? Explain.
They will both move towards each other. Mark’s pull on the rope is transmitted
____________________________________________
to Sua causing her to accelerate towards Mark. By Newton’s 3rd Law, the rope
____________________________________________
pulls back on Mark with equal force causing him to accelerate towards Sua.
____________________________________________
12) Look at the figure to the right where
someone drives a nail into a piece of
wood that is set on top of a stack of
books on top of a girl’s head. Why
doesn’t this hurt her?
The stack of books consists of many pages that are at rest.
__________________________________
They have inertia (because they have mass) so they resist
__________________________________
accelerating (they resist a change in state of rest). Each page is
__________________________________
like a different body, so when the nail is hit, there is a chain of
__________________________________
collisions taking place on the pages all the way down to her
____________________________________________
head. There is little change in motion of the last page on top
of her head, so it doesn’t hurt.
____________________________________________