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NEWTON’S LESSON 14: Newton's Third Law of Motion Continued
You are familiar with Newton's third law: For every action there is an equal
and opposite reaction.
The statement of this law is fairly easy, but the understanding is not. People
often say the words without truly understanding their meaning.
Action-Reaction Pairs
You probably know that Earth orbits the Sun because a gravitational force
acts between them, so try this question.
Which is greater the gravitational force of the Sun on Earth or the
gravitational force of Earth on the Sun?
ANSWER: If you thought they were exactly the same,
congratulations. If not, lets look at this question using Newton's third
law.
The action and reaction in Newton's third law are forces. For every force
there is an equal and opposite reaction force.
The Sun's gravitational force acting on Earth is an action.
The Newton's third law reaction is Earth's gravitational force acting
on the Sun.
Earth's gravitational force on the Sun and the Sun's gravitational force on
Earth are an action-reaction pair.
Pair means two, so an action-reaction pair includes only two objects. There
is never a third object in a Newton's third law action-reaction pair.
If the Sun acts on Earth, the reaction is Earth acting on the Sun with
an equal but opposite force.
For Newton's third law always remember action-reaction pairs, are
always two objects, never three.
This part often confuses people. Not all equal and opposite forces form a
Newton's third law action-reaction pair.
EXAMPLE: If you weigh 120 pounds that means that Earth's gravity
is pulling down on you with a force of 120 pounds. If you are standing
on a solid floor, you do not accelerate downward because the floor is
holding you up. The floor exerts an upward force of 120 pounds on
you.
These forces are equal and opposite, so you remain at rest on the
floor.
Are these two equal and opposite forces an action-reaction pair?
Let's look at them.
The forces are Earth on you and floor on you. There are three
objects involved, not two. So these forces are not a Newton's third law
action-reaction pair. They are JUST two different forces acting on you
that happen to be equal and opposite.
The Newton's third law reaction to Earth pulling you down with a 120
pound force is that you pull Earth up with a 120 pound force. The
forces, Earth on you and you on Earth, form an action-reaction pair.
Diagram of the action-reaction forces:
The reaction to earth's gravity pulling you down, is you pull the Earth up
with exactly the same force. So if the forces are the same, why do we fall
down to Earth rather than having Earth fall up towards us?
Earth is much more massive than we are.
By Newton's second law a 120 pound force will affect us, but it will
not significantly affect the much more massive Earth. So we fall
down; Earth does not fall up.
Or we can look at it as you and the Earth have the same force acting
on each of you. But your mass is much much smaller than the
Earth’s. Therefore, for you both to have the same force, the lighter
mass much have a much much larger acceleration.
a m
m
=F=
YOU
a
EARTH
Note that the larger the acceleration, the larger the velocity and the
larger the distance traveled over a specified period of time. That is
why it appears as if we are falling to Earth and not the other way
around.
!!!!!! Remember: Since action-reaction forces act on different objects
they cannot cancel each other out. Balanced and unbalanced forces act
on the same object so they can cancel each other out. This concept will
become key in further discussions. !!!!!!
ACTION REACTION FORCES DO NOT CANCEL because they
influence the motion of different objects.
INVESTIGATION:
Spring Scales
This scale reads force in newtons.
Have two people each hold one of the two spring scales.
Hook the two spring scales together. Notice how the scales are connected
together below, rather than attaching them hook-to-hook, they are aligned
hook to top. This makes it easier to compare the readings on both scales.
Two spring scales hooked together horizontally always read the same force.
Have the people pull back and forth.
Notice that the spring scales always read the same value.
Trick question, what is the acceleration of each spring scale? What is the net
force on the pair of spring scales?
The scales are always at rest and remain at rest, their acceleration is zero.
Thus, the net force on them is zero from Newton's Law F = m a. Where "a"
is the acceleration, which here is zero, and F is the net force on the object.
Here the force of you pulling on the scale balances the force of the other
spring pulling on the scale.
Notice that scales do not read the net force of zero.
What's Going On?
Scales read the magnitude of one of the two forces acting on them,
the force they exert on the thing they are attached to.
So one scale reads the magnitude of the action force the other scale exerts on
it.
The other scale reads the reaction force.
By Newton's third law the action and reaction forces are equal and opposite.
Thus, these forces always have the same magnitude and so the scales always
read the same value.
Tension in the rope = Fab = Fba
EXAMPLE: HORSE AND WAGON
DOES NEWTON’S THIRD LAW
PREVENT THE HORSE FROM
MOVING THE WAGON?
Noah Formula notes that Newton's Third
Law says that if I pull on the wagon, the
wagon exerts an equal and opposite force
on me. He believes that if these two forces are equal and opposite, they will
cancel, so that the net force is zero. If the net force is zero, then Newton's
Second Law (and Newton's First Law, too) says that the acceleration of the
wagon must be zero, so the wagon can never move.
Please help Noah. He know that Newton’s Laws are correct, and he knows
that horses really can pull wagons. There has to be an error in the argument,
but what is it? Can we explain why if the horse pulls the wagon, it will
move?
After some study and thought, I hope that you will find answers like "The
wagon moves because it's attached to the horse." or "If the horse pushes
harder on the ground than the wagon pulls on the horse, then the wagon
accelerates." as entertaining as I, your physics teacher does!
The Key
Even though a complete answer to the Horse and Wagon Question can get
rather involved, a clear explanation only hinges on a couple of simple points:

An object accelerates (or not) because of the forces that push or pull
on it. (Newton's 2nd Law)

Only the forces that act on an object can cancel. Forces that act on
different objects don't cancel - after all, they affect the motion of
different objects!
THE HORSE AND WAGON
WITHOUT FRICTION
What are the Newton's Third Law Force Pairs?
1. The two forces colored yellow in the diagram are a Newton's Third Law
force pair - "horse pulls wagon" and "wagon pulls horse". They are equal in
magnitude and opposite in direction.
2. The two forces colored blue in the diagram are a Newton's Third Law
force pair - "horse pushes ground" and "ground pushes horse". They are also
equal in magnitude and opposite in direction.
Why does the wagon accelerate?
Newton's 2nd Law says that an object accelerates if there is a net
(unbalanced) force on it.
Looking at the wagon in the diagram, you can see that there is just one force
exerted on the wagon - the force that the horse exerts on it.
The wagon accelerates because the horse pulls on it! The amount of
acceleration equals the net force on the wagon divided by its mass (Newton's
Second Law).
Why does the horse accelerate?
There are 2 forces that push or pull on the horse in the diagram above. The
wagon pulls the horse backwards, and the ground pushes the horse forward.
The net force is determined by the relative sizes of these two forces.
If the ground pushes harder on the horse than the
wagon pulls, there is a net force in the forward
direction, and the horse accelerates forward.
If the wagon pulls harder on the horse than the ground pushes, there is a net
force in the backward direction, and the horse accelerates backward. (This
wouldn't happen on level ground, but it could happen on a hill...)
If the force that the wagon exerts on the horse is the same
size as the force that the ground exerts, the net force on the
horse is zero, and the horse does not accelerate.
In any case, the acceleration of the horse equals the net force on the horse
divided by the horse's mass (Newton's Second Law).
Why does the ground push on the horse, anyway?
The force "ground pushes horse" is the Newton's Third Law reaction force to
"horse pushes ground". These 2 forces are exactly the same size. If the horse
wants the ground to push him forward, he just needs to push backwards on
the ground.
These two forces do not cancel because they act on different objects. The
force "ground pushes horse" tends to accelerate the horse, and the force
"horse pushes ground" tends to accelerate the ground.
What about the ground?
Looking at the force diagram at the top of the page, you see that there is one
horizontal force pushing on the ground - the horse pushes on the ground.
Therefore, there is a net force on the ground, so the ground should
accelerate. Does it?
Of course it does! However the amount of acceleration equals the size of the
net force divided by the mass of the Earth - and the mass of the earth is
about 6 x 1024 kg. This means that the acceleration of the ground is much,
much too small to notice.
Summary:
So, it is possible for horses to pull wagons! It is true that the force that the
horse exerts on the wagon is the same size as the force that the wagon exerts
on the horse, but these forces do not combine to produce a zero net force.
The force exerted on the wagon (by the horse) affects the motion of the
wagon, and the force exerted on the horse affects the motion of the horse.
HOW THE HORSE AND WAGON WORKS WITH FRICTION
The Forces:
Compared to the previous diagram,
you can see that two new forces have
been added to the diagram at the right.
The friction force acting on the wagon
(colored red) tries to oppose the motion
of the wagon. It is exerted by the
ground. Its Newton's Third Law force partner is the force "wagon pushes
ground". Note that the force pushing the wagon is drawn on the wagon, and
the force pushing the ground is drawn on the ground.
As always, these two forces don't cancel because they act on different
objects.
Why does the wagon accelerate?
Consulting the diagram, notice that there are now two forces acting on the
wagon. The net force on the wagon equals the force the horse exerts minus
the friction force the ground exerts. If the horse pulls harder on the wagon
than the friction force, there will be a forward-pointing net force, and the
wagon will accelerate forward. If the pull of the horse exactly balances the
friction force, then the net force on the wagon will be zero, and the wagon
will not accelerate. (This is the situation when the horse is pulling the wagon
at constant velocity.)
Why does the horse accelerate?
The situation for the horse is the same as in the previous (no friction)
situation.
Does the ground accelerate?
There are now 2 forces acting on the ground - the horse pushes it backwards
and the wagon pushes it forward. The net force on the ground equals the
force that the horse exerts on the ground minus the force that the ground
exerts on it. If the horse pushes harder, there will be a backward net force on
the ground. If the wagon pushes harder, there will be a forward net force on
the ground. If they push equally, the net force on the ground will be zero. In
any case, the acceleration of the ground will not be noticeable, due to the
enormous mass of the earth.
EXAMPLE: TUG OF WAR
An elephant and a teenager are having a tug of war. Does Newton’s third
law imply a draw?
Answer: No, unless the elephant is very weak, the teenager will definitely
lose. It is true that the force the elephant exerts on the teenager, Fet is equal
and opposite to the force the teenager exerts on the elephant, Fte, but the
motion of either “object” depends on the resultant of all the forces acting on
it. Both the teenager and the elephant are pushing the ground forward with
their feet and in each case the ground exerts an opposite reaction force.
Diagram shows where Fgt and Fge represent the horizontal forces exerted by
the ground on the teenager and on the elephant, respectively. Thus, for
example, suppose that Fet = Fte = 250 N. We might have Fgt = 100 N and Fge
= 650 N. Then a net force of 150 N acts on the teenager to the left and he
moves leftward. Similarly, a net force of 400 N acts on the elephant to the
left and the elephant moves leftward.
NEWTON’S LESSON 14 HOMEWORK
DO CONCEPTUAL PHYSICS WORKSHEET p 12.
1. In the top picture (below), Kent Budgett is pulling upon a rope which is attached to a
wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an
elephant. In each case, the force scale reads 500 Newtons. Kent is pulling ...
a. with more force when the rope is attached to the wall.
b. with more force when the rope is attached to the elephant.
c. the same force in each case.
2. Two people pull on a rope in a tug-of-war. Each pulls with 400 N of force. What is the
tension in the rope?
a) Zero
b) 400 N
c) 600 N
d) 800 N
e) none of the above
3. As a ball falls, the action force is the pull of the earth’s mass on the ball. What is the
reaction
to this force?
a) Air resistance acting against the ball
b) The acceleration of the ball
c) The pull of the ball’s mass on the earth
d) Nonexistent in this case
e) none of the above
4. A person is attracted towards the center of the earth by a 500-N gravitational force.
The force
with which the earth is attracted toward the person is
a) very very small.
b) very very large.
c) 500 N.
5. An unfortunate bug splatters against the windshield of a moving car. Compared to the
force
of the car on the bug, the force of the bug on the car is
a) larger.
b) smaller.
c) the same.
d) Need more information to say
6. An unfortunate bug splatters against the windshield of a moving car. Compared to the
deceleration of the car, the deceleration of the bug is
a) larger.
b) smaller.
c) the same.
7. If a horse pulls on a wagon at rest, the wagon pulls back equally as much on the horse.
Will
the wagon be set into motion?
a) No, because the forces cancel each other.
b) Yes, because there is a net force acting on the wagon.
c) Yes, because there is a time delay between action and reaction.
d) Yes - The horse’s pull on the wagon is larger than the wagon’s pull on the horse.
8. A Mack truck and a Volkswagen traveling at the same speed have a head-on collision.
The
vehicle to undergo the greater change in velocity will be the
a) Volkswagen.
b) Mack truck.
c) Both the same
9. According to Newton’s third law, if you push gently on something, it will push
a) gently on you.
b) gently on something else.
c) on something only under the right conditions.
d) on you only if you aren’t moving.
10. The earth pulls on the moon, and similarly the moon pulls on the earth. This is
evidence that
the
a) earth and moon are simply pulling on each other.
b) earth’s and moon’s pulls comprise an action-reaction pair.
c) both a and b
d) neither a or b
11. Nellie Newton holds an apple in her hand. If action is the earth pulling on the apple,
then the reaction is
a) her hand pushing up on the apple.
b) her hand providing a normal force on the apple.
c) both a and b
d) neither a or b
12. Bronco the skydiver falls toward the earth. The attraction of the earth on Bronco
pulls him down. The reaction to this force is
a) Bronco finally pushing against the earth’s surface.
b) the earth’s surface finally pushing against Bronco.
c) Bronco pulling up on the earth.
d) neither a, b or c
13. Arnold Strongman and Suzie Small each pull very hard on opposite ends of a
massless rope in a tug-of-war. The greater force on the rope is exerted by
a) Arnold, of course.
b) Suzie, surprisingly.
c) both the same, interestingly enough.
14. Two people, one having twice the mass of the other, play tug-of-war with a 12 meter
rope on frictionless ice. After a brief time, they meet. The heavier person slides a distance
of
a) 3 m.
b) 4 m.
c) 5 m.
d) 6 m.
Explain.
15. A piece of rope is pulled by two people in a tug-of-war. Each pulls with 400 N of
force. What is the tension in the rope?
a) zero
b) 400 N
c) 600 N
d) 800 N
e) none of these
16. A horse exerts 500 N of force on a heavy wagon. The wagon pulls back on the horse
with an equal force. The wagon still accelerates because
a) these forces are not an action-reaction pair.
b) nevertheless there is still an unbalanced force on the wagon.
c) the horse pulls on the wagon a brief time before the wagon reacts.
d) the wagon does not accelerate because these forces are equal and opposite.
e) unlike the horse, it is not alive.
17. According to Newton's third law, every force is accompanied by an equal and
opposite reaction force. The reason that these forces do not cancel each other is ____.
a. the action force acts for a longer time period
b. the two forces are not always in the same direction
c. one of the two forces is greater than the other
d. the two forces act upon different objects; only forces on the same object
can balance each other.
e. ... nonsense! They do cancel each other. Objects accelerate because of
the presence of a third force.
Explain.
18. A golf pro places a ball at rest on the tee, lines up his shot, draws back his club, and
lets one rip. During the contact of the golf club with the golf ball, the force of the club on
the ball is ____ the force of the ball on the club and the acceleration of the club is ____
than the acceleration of the ball.
a. greater than, greater than b. greater than, equal to
d. less than, less than
e. less than, equal to
g. equal to, equal to
h. equal to, greater than
c. greater than, less than
f. less than, greater than
i. equal to, less than
19. While driving down the road, a firefly strikes the windshield of a bus and makes a
quite obvious mess in front of the face of the driver. This is a clear case of Newton's third
law of motion. The firefly hit the bus and the bus hits the
firefly. Which of the two forces is greater: the force on the
firefly or the force on the bus?
20. Many people are familiar with the fact that a rifle recoils
when fired. This recoil is the result of action-reaction force
pairs. A gunpowder explosion creates hot gases which
expand outward allowing the rifle to push forward on the
bullet. Consistent with Newton's third law of motion, the bullet pushes
backwards upon the rifle. The acceleration of the recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
21. Consider the interaction depicted below between foot A, ball B, and foot C. The three
objects interact simultaneously (at the same time). Identify the two pairs of actionreaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements.
22. Identify at least six pairs of action-reaction force pairs in the following diagram.
23. When do action and reaction pairs in physics cancel eachother out?
24. If I push on a lawn mower, it pushes back on me with an equal, but opposite force.
Explain why we don’t both just stay still.
25. What, if anything is wrong with the following statement? When a book rests on a
table, the downward force of gravity on the book is equal and opposite to the upward
force of the table on the book. This is an example of Newton’s third law.
26.
27. When an object falls in a vacuum, what are the action-reaction forces, since there is
no air resistance?
HOMEWORK KEY NEWTON’S LESSON 14
1. c
2. b
3. c
4. c
5. c
6. a
7. b
8. a
9. a
10. c
11. d
12. c
13. c
14. b
15. b
16. b
17. d
18. i
19. no key
20. b
21. no key
22. no key
23. no key
24. no key
25. no key
26. no key
27. no key
NEWTON’S LESSON 14 HOMEWORK
DO CONCEPTUAL PHYSICS WORKSHEET p 12.
1. In the top picture (below), Kent Budgett is pulling upon a rope which is attached to a
wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an
elephant. In each case, the force scale reads 500 Newtons. Kent is pulling ...
a. with more force when the rope is attached to the wall.
b. with more force when the rope is attached to the elephant.
c. the same force in each case.
Answer: Kent is pulling with 500 N of force in each case. The rope transmits the force
from Kent to the wall (or to the elephant) and vice versa. Since the force of Kent pulling
on the wall and the wall pulling on Kent are action-reaction force pairs, they must have
equal magnitudes. Inanimate objects such as walls can push and pull.
2. Two people pull on a rope in a tug-of-war. Each pulls with 400 N of force. What is the
tension in the rope?
a) Zero
b) 400 N
c) 600 N
d) 800 N
e) none of the above
3. As a ball falls, the action force is the pull of the earth’s mass on the ball. What is the
reaction
to this force?
a) Air resistance acting against the ball
b) The acceleration of the ball
c) The pull of the ball’s mass on the earth
d) Nonexistent in this case
e) none of the above
4. A person is attracted towards the center of the earth by a 500-N gravitational force.
The force
with which the earth is attracted toward the person is
a) very very small.
b) very very large.
c) 500 N.
5. An unfortunate bug splatters against the windshield of a moving car. Compared to the
force
of the car on the bug, the force of the bug on the car is
a) larger.
b) smaller.
c) the same.
d) Need more information to say
6. An unfortunate bug splatters against the windshield of a moving car. Compared to the
deceleration of the car, the deceleration of the bug is
a) larger.
b) smaller.
c) the same.
7. If a horse pulls on a wagon at rest, the wagon pulls back equally as much on the horse.
Will
the wagon be set into motion?
a) No, because the forces cancel each other.
b) Yes, because there is a net force acting on the wagon.
c) Yes, because there is a time delay between action and reaction.
d) Yes - The horse’s pull on the wagon is larger than the wagon’s pull on the horse.
8. A Mack truck and a Volkswagen traveling at the same speed have a head-on collision.
The
vehicle to undergo the greater change in velocity will be the
a) Volkswagen.
b) Mack truck.
c) Both the same
9. According to Newton’s third law, if you push gently on something, it will push
a) gently on you.
b) gently on something else.
c) on something only under the right conditions.
d) on you only if you aren’t moving.
10. The earth pulls on the moon, and similarly the moon pulls on the earth. This is
evidence that
the
a) earth and moon are simply pulling on each other.
b) earth’s and moon’s pulls comprise an action-reaction pair.
c) both a and b
d) neither a or b
11. Nellie Newton holds an apple in her hand. If action is the earth pulling on the apple,
then
reaction is
a) her hand pushing up on the apple.
b) her hand providing a normal force on the apple.
c) both a and b
d) neither a or b
12. Bronco the skydiver falls toward the earth. The attraction of the earth on Bronco
pulls him
down. The reaction to this force is
a) Bronco finally pushing against the earth’s surface.
b) the earth’s surface finally pushing against Bronco.
c) Bronco pulling up on the earth.
d) neither a, b or c
13. Arnold Strongman and Suzie Small each pull very hard on opposite ends of a
massless rope in a tug-of-war. The greater force on the rope is exerted by
a) Arnold, of course.
b) Suzie, surprisingly.
c) both the same, interestingly enough.
14. Two people, one having twice the mass of the other, play tug-of-war with a 12 meter
rope on
frictionless ice. After a brief time, they meet. The heavier person slides a distance of
a) 3 m.
b) 4 m.
c) 5 m.
d) 6 m.
Explain.
Answer: b. The force exerted by each skater are equal and opposite as stated by
Newton’s third law, however, their masses are different and therefore they accelerate at
different rates. Therefore a person with twice the mass will accelerate at ½ the
acceleration. Therefore will travel at ½ the velocity (a = v/t, directly proportionate
relationship) and therefore will travel ½ the distance as the other skater. Therefore the
lighter skater travels 8 m and the heavier skater travels 4 m for a total of 12 m.
15. A piece of rope is pulled by two people in a tug-of-war. Each pulls with 400 N of
force. What is the tension in the rope?
a) zero
b) 400 N
c) 600 N
d) 800 N
e) none of these
16. A horse exerts 500 N of force on a heavy wagon. The wagon pulls back on the horse
with an equal force. The wagon still accelerates because
a) these forces are not an action-reaction pair.
b) nevertheless there is still an unbalanced force on the wagon.
c) the horse pulls on the wagon a brief time before the wagon reacts.
d) the wagon does not accelerate because these forces are equal and opposite.
e) unlike the horse, it is not alive.
17. According to Newton's third law, every force is accompanied by an equal and
opposite reaction force. The reason that these forces do not cancel each other is ____.
a. the action force acts for a longer time period
b. the two forces are not always in the same direction
c. one of the two forces is greater than the other
d. the two forces act upon different objects; only forces on the same object
can balance each other.
e. ... nonsense! They do cancel each other. Objects accelerate because of
the presence of a third force.
Explain.
Answer: d Action and reaction forces always act upon the interacting objects for the
same amount of time with the same magnitude. So if object A pushes on object B, then
object B simultaneously pushes on object A with the same amount of force. The force on
object B will be one of perhaps many forces which will govern is motion. But the
reaction force is on object A and cannot contribute to object B's motion since it is not
acting upon object B. Action-reaction forces can NEVER cancel each other.
18. A golf pro places a ball at rest on the tee, lines up his shot, draws back his club, and
lets one rip. During the contact of the golf club with the golf ball, the force of the club on
the ball is ____ the force of the ball on the club and the acceleration of the club is ____
than the acceleration of the ball.
a. greater than, greater than b. greater than, equal to
d. less than, less than
e. less than, equal to
g. equal to, equal to
h. equal to, greater than
c. greater than, less than
f. less than, greater than
i. equal to, less than
Answer: i. For every action, there is an equal and opposite reaction force. In this case,
the force on the club is equal to the force on the ball. The subsequent accelerations of the
interacting objects will be inversely dependent upon mass. The more massive club will
have less acceleration than the less massive ball.
19. While driving down the road, a firefly strikes the windshield of a bus and makes a
quite obvious mess in front of the face of the driver. This is a clear case of Newton's third
law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces
is greater: the force on the firefly or the force on the bus?
Answer: Trick Question! Each force is the same size. For
every action, there is an equal ... (equal!). The fact that the
firefly splatters only means that with its smaller mass, it is
less able to withstand the larger acceleration resulting from
the interaction. Besides, fireflies have guts and bug guts have
a tendency to be splatterable. Windshields don't have guts.
There you have it.
20. Many people are familiar with the fact that a rifle recoils when
fired. This recoil is the result of action-reaction force pairs. A
gunpowder explosion creates hot gases which expand outward
allowing the rifle to push forward on the bullet. Consistent with
Newton's third law of motion, the bullet pushes backwards upon
the rifle. The acceleration of the recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
Answer: b. The force on the rifle equals the force on the bullet. Yet, acceleration
depends on both force and mass. The bullet has a greater acceleration due to the fact that
it has a smaller mass. Remember: acceleration and mass are inversely proportional.
21. Consider the interaction depicted below between foot A, ball B, and foot C. The
three objects interact simultaneously (at the same time). Identify the two pairs of actionreaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements.
Answer: The first pair of action-reaction force pairs is: foot A pushes ball B to the right;
and ball B pushes foot A to the left. The second pair of action-reaction force pairs is: foot
C pushes ball B to the left; and ball B pushes foot C to the right.
22. Identify at least six pairs of action-reaction force pairs in the following diagram.
Answer: The elephant's feet push backward on the ground; the ground pushes forward
on its feet. The right end of the right rope pulls leftward on the elephant's body; its body
pulls rightward on the right end of the right rope. The left end of the right rope pulls
rightward on the man; the man pulls leftward on the left end of the right rope. The right
end of the left rope pulls leftward on the man; the man pulls rightward on the right end of
the left rope. The tractor pulls leftward on the right end of the left rope; the left end of the
left rope pulls rightward on the tractor. etc., etc.
23. When do action and reaction pairs in physics cancel each other out?
Answer: Well in a single action-reaction pair, they cannot cancel out! The action and
reaction forces act on different bodies. Lets say that we have a football. I kick it with 200
N of force. That is the action force, so the reaction force must be 200N(in the opposite
direction) as well. The key here is that the reaction force did not act on the football, but
on your foot! So the net force of the football is still 200 N in the direction I kicked it!
24. If I push on a lawn mower, it pushes back on me with an equal, but opposite force.
Explain why we don’t both just stay still.
Answer: The answer is that these forces are acting on different bodies (and there are
other forces to consider). It doesn’t matter to the lawn mower that there is a force on
me… all that matters to the lawn mower is that there is a force on it, so it starts to move!
Another action-reaction pair you need to consider is that I am pushing backwards on the
ground, and it pushes forwards on me.
25. What, if anything is wrong with the following statement? When a book rests on a
table, the downward force of gravity on the book is equal and opposite to the upward
force of the table on the book. This is an example of Newton’s third law.
Answer: It is true that if a book is at rest on a table then the force of the table up on the
book (the normal force) is equal and opposite and so the book is at rest. But the normal
force is not the “equal and opposite” force of Newton’s 3rd law. This law is referring to
the effect of the book on the origin of the ‘downward force of gravity’. That is if the
Earth exerts a force on the book (the weight) then the book exerts the same amount of
force on Earth. But since that is not a force that acts on the book (it is acting on the Earth)
it is not included in our sum of forces.
26.
Answer:
It is true that the force the mule exerts on the cart, Fmc is equal and opposite to the force
the cart exerts on the mule, Fcm, but the motion of either “object” depends on the
resultant of all the forces acting on it. Both the cart and the mule are pushing the
27. When an object falls in a vacuum, what are the action-reaction forces, since there is
no air resistance?
Answer: There is no air resistance, but Earth's gravity pulls down on the object as the
object's gravity pulls upward on Earth with EXACTLY the same magnitude of force.
More accurately, their mutual gravitational attraction pulls the object and Earth together,
toward each other.
Usually, when we say an object is falling, we are not paying much attention to the thing
that it is falling toward. Let us take the case of a ball falling to Earth. Earth is so much
more massive than the ball that is seems unaffected by it, and we rarely think of the Earth
as falling toward the ball, but actually, both objects are falling toward each other.