Download Newton`s 3rd Law Study Guide—Chapter 7 Name: Period:____

Newton’s 3rd Law Study Guide—Chapter 7
Name:________________________ Period:____
1.
The Big Idea is for every force there is an equal and opposite force
2.
If you lean over and push on a wall, why don’t you fall over? The wall pushes back on you
3.
When you paddle a kayak, your paddle pushes on the water. What makes the kayak move forward?
The water pushes forward on the paddle
Section 7.1 Forces and Interactions
A force is always part of a(n)_mutual action_____ that involves another force.
Describe the interaction forces between a nail and a hammer that hits it and explain why the hammer stops moving.
The nail exerts an equal and opposite force on the hammer. The hammer stops because it has an unbalanced force
4.
5.
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7.
Section 7.2 Newton’s Third Law
State Newton’s 3rd Law Whenever one object exerts a force on a second object, the second object exerts an equal
and opposite force on the first object
State the rule for deciding which force is the action force and which is the reaction force It makes no difference
which force is called the action force. The other force is called the reaction force.
8.
9.
Action and reaction forces are equal in __magintude___and opposite in ___direction______
Describe the reaction force for each of the following
To walk, you push on the floor
The floor pushes on you
A dog wags its tail
The tail wags the dog
A car’s tires push against the road
The road pushes on the car’s tires
You push on a door
The door pushes on you
Your right hand pushes on your left hand
Your left hand pushes on your right hand
A baseball bat exerts a force on the ball
The ball exerts a force on the bat
10. Use the words action force and reaction force to explain why a person walking on ice might not be able to move
forward For a person to walk, it must be possible to apply an action force on the ice. The ice is not able to supply
enough of a reaction force to push back on the person.
Section 7.3 Identifying Action and Reaction
11. State the two steps for identifying a pair of action-reaction forces
a. __Determine which two objects are interacting______________
b. __If the action force is A on B, then the reaction force is B on A
12. When you drop your smart phone from the top of a building and it falls to the ground,
a. The two interacting objects are:_A:__smart phone__ and _B:__earth________________
b. The action of A on B is ___earth pulls phone__________________________________
c. The action of B on A is ___phone pulls on earth_______________________________
13. Complete the following table and describe the reaction forces AND their direction
A car’s tires push backwards on the road
Road pushes forward on car’s tires
A rocket pushes gas out behind
Gas pushes forward on rocket
A pool ball rolls into another ball and exerts
Second ball exerts a force back on the first ball
a force on it
A swimmer pushes backwards on the water
Water pushes forward on swimmer
A helicopter’s blades push down on the air
Air pushes up on blades
The earth pulls downward on a falling piano Piano pulls upward on earth
14. Describe the force interaction pair when a ball is falling towards the earth
Earth pulls downward on the ball. The ball pulls upward on the earth.
Section 7.4 Action and Reaction o n Different Masses
15. When you drop a pencil from your desk, the earth exerts a force on the pencil. How big is the force that the pencil
exerts on the earth? The pencil exerts and equal and opposite force on the earth
Newton’s 3rd Law Study Guide—Chapter 7
Name:________________________ Period:____
16. State Newton’s Second Law Acceleration is directly proportional to the force applied to an object and inversely
proportional to its mass. The formula is a = F ÷ m
17. When a pencil falls towards the earth, the earth accelerates towards the pencil. Explain using Newton’s Second Law
why we don’t feel the acceleration of the earth towards the pencil (use the formula in your explanation)
Both the pencil and the earth feel the same force, but because the earth’s mass is very large, its acceleration is very
small. Using the formula for the pencil
and for the earth
, we see the earth accelerates less
18. When you throw a small ball, you exert a force on the ball and it accelerates. Newton’s Third Law says that the ball
exerts an equal force on you. Explain using Newton’s Second Law why you don’t go flying backwards as you throw.
You both feel the same force, but because you have more mass, you accelerate less than the ball.
19. When a cannonball is fired from a cannon, the force that the cannon exerts on the ball is _equal_______ and
____opposite_______ to the force the ball exerts on the cannon.
20. Explain why the change in velocity of the cannonball is much greater than the change in velocity of the cannon:
The cannon has MUCH more mass than the cannonball so it accelerates
MUCH less than the cannonball
21. People used to think that rockets could not accelerate in space because there is no air to push against. Explain why
this is not true (and compare it to the cannonball example). The rocket pushes on each molecule of gas, and just like
the cannonball example, each molecule of gas pushes back on the rocket. Because there are so many molecules of
gas, they exert a net force that moves the rocket forward. The air has no effect on the rocket.
22. What is lift? Lift is the upward force of air on an airplane’s wings or helicopter’s blades that allows it to fly.
23. If two different objects receive the same amount of force, how is it that they accelerate at different rates? If they
have different accelerations with the same force, then they must have different masses.
24. Match the condition on the left to the result on the right
__c__1. Lift equals the helicopter’s weight
__b_2. Lift is greater than the helicopter’s weight
__a__3. Lift is less than the helicopter’s weight
Section 7.5 Defining Systems
a.
b.
c.
Helicopter moves downward
Helicopter moves upward
Helicopter hovers in mid-air
25.
Explain why the force of the orange on the apple does NOT cancel the force of the orange on
the apple and the orange accelerates The force applied by the apple is external to the orange system
26.
Explain why the force of the orange on the apple DOES cancel the force of the apple on the
orange and the orange DOES NOT accelerate The force of the apple on the orange is internal to the system. Only
external forces can accelerate an object.
27. Inside a golf ball there are billions of atoms, all exerting action-reaction forces on each other. Explain why the ball
does not accelerate on its own All the forces are internal, and do nothing to make the object accelerate.
Newton’s 3rd Law Study Guide—Chapter 7
Section 7.6Horse and Cart Problem
Cart
Ffriction
Name:________________________ Period:____
Fhorse
Horse
Fcart
Froad
A horse is pulling on a cart, accelerating the cart forward.
1. a) Make a force diagram for the cart to the left of the picture.
b) Make a force diagram for the horse to the right of the picture.
2. Is the force exerted on the cart by the horse greater than the force exerted on the horse by the cart? How do you know?
The forces are equal and opposite due to Newton’s Third Law
3. Is the force exerted on the cart by the horse greater than the force of friction on the cart exerted by the ground?
How do you know?
Yes, since the cart is accelerating, then it must have an unbalanced force. The force by the horse on the cart must be
greater than the force of friction on the cart
4. Is it true to say that the net force exerted on the cart is in the forward direction? How do you know?
Yes, an object accelerates in the direction of the net force on it.
5. Is it true to say that the net force exerted on the cart is zero? How do you know?
No, because the cart is accelerating, the net force cannot be zero.
6. Is it true to say that the net force of each Newton’s third law pair is zero? How do you know?
Yes, because the forces are equal and opposite. The sum of two equal and opposite vectors is zero.
7. Is the force of friction on the horse less than the force of friction on the cart? How do you know?
Yes, because the horse is able to cause the cart to accelerate. The push by the road on the horse must be less than
the resistance by the road on the cart.
Newton’s 3rd Law Study Guide—Chapter 7
Name:________________________ Period:____
Concept Checks
1. Can an action force exist without a reaction force? No, according to Newton’s Third Law
2. When a hammer exerts a force on a nail, how does this amount of force compare with that of the nail on the
hammer? It is equal and opposite
3. When you walk on a floor, what pushes you along? The floor pushes back
4. State Newton’s third law of motion. For every force there is an equal and opposite force.
5. Consider hitting a baseball with a bat. If we call the force the bat exerts against the ball the action force,
identify the reaction force. The reaction force is the force of the ball on the bat
6. If a bat hits a ball with 1000 N of force, can the ball exert less than 1000 N of force on the bat? More than
1000 N? The ball exerts a force of 1000N on the bat.
7. If the world pulls you downward against your chair, what is the reaction force? The reaction force is the chair
pushing up on you
8. When a cannon is fired, are the forces on the cannonball and on the cannon equal in magnitude? Are the
accelerations of the two equal? No
9. When a cannon is fired, why do the cannonball and cannon have very different accelerations? The cannon
has much more mass than the cannonball and accelerates much less due to Newton’s second law
10. Identify the force that propels a rocket. The rocket pushes backwards on the gas and the gas pushes
forwards on the rocket
11. How does a helicopter get its lifting force? The rotors push down on the air and the air pushes back up on
the rotors
13. When can two kicks on a soccer ball produce a net force of zero on the ball? When they are on opposite
sides and in opposite directions AND they are equal in magnitude
14. Why don’t the enormous number of interatomic forces inside a baseball accelerate the baseball? Internal
forces do nothing to accelerate an object. An object requires and external force to accelerate
18. If you hit a wall with a force of 200 N, how much force does the wall exert on you? 200N
21. Three sets of double boxes rest on a table. Rank the following from greatest to least.
a. the normal force that the table exerts on the sets A = B = C
b. the normal force exerted by the bottom block on the top block B > C > A
22. A van exerts a force on trailers of different masses m. All velocities v are constant.
Compared with the force exerted on the trailer, rank the magnitude of force the trailer exerts on the van. Or are
all pairs of forces equal in magnitude? A = B = C
25. Your weight is the result of the gravitational force of Earth on your body. What is the corresponding reaction
force? Your body pulls up on the earth
28. If you walk on a log that is floating in the water, the log moves backward. Why? The backward force of your
push on the log is greater than the forward push of the water on the log
29. Why is it easier to walk on a carpeted floor than on a smooth, polished floor? (Explain in terms of
action/reaction forces) The carpet can produce a greater reaction force due to greater friction
36. A small car bumps into a van at rest in a parking lot. Upon which vehicle is the force of impact greater?
Same Which vehicle undergoes the greater change in acceleration? Defend your answer. The car accelerates
Newton’s 3rd Law Study Guide—Chapter 7
Name:________________________ Period:____
more because it has less mass. They both feel the same force.
45. Your teacher challenges you and your best friend to each pull on a pair of scales attached to the ends of a
horizontal rope, in tug-of-war fashion, so that the readings on the scales will differ. Can this be done? Explain.
The readings on the scale can never be different because Newton’s
Third Law says that every action force has an equal and opposite reaction force.
Extra Credit Problems. Do these on this page or on a separate sheet of paper. SHOW YOUR WORK TO GET CREDIT
1.
2.
3.
4.
What will be the acceleration of recoil when a 60-kg person on rollerskates pushes against a wall with a
force of 30 N? 0.5 m/s2
Two people attempt a tug-of-war on lowfriction ice. One person has four times the mass of the other.
Relative to the acceleration of the heavier person, what will be the acceleration of the lighter person?
4 times
What is the net force on a falling 100-N barrel hitting a pavement with 5000 N of force? 4900N
A 70-kg skydiver is falling at her terminal speed. Show that she exerts a 700-N downward force on the air
as she falls.