Sierzega: Dynamics 8
Newton’s Third Law
Newton’s Third Law of Motion: When two objects interact, object 1 exerts a force on object 2. Object 2 in
turn exerts an equal-magnitude, oppositely directed force on object 1:
⃗
⃗
Each force above is caused by one object and is exerted on another object. Since these two forces are exerted
on two different objects, they cannot be added to find a net force.
8.1 Represent and Reason
Fill in the empty spaces and draw pictures representing the situations. Place force arrows on the pictures;
remember to think about the lengths of the arrows.
a)
b)
c)
d)
When the Earth exerts a force on the book, the book exerts a force on __________
When a table exerts a force on the book, the book exerts a force on _________
When a tennis racket exerts a force on the ball, the ball exerts a force on_________
When car tires push back on the Earth’s surface, the Earth’s surface _________
8.2 Relate
List 5 everyday experiences that support the idea that when object B exerts a force on object A, object A will
simultaneously exert a force on object B. Discuss whether you can always observe the effects of these forces
that the interacting objects exert on each other. List possible reasons.
8.3 Reason
a) A book sits on the tabletop. What is the Newton’s Third Law pair for the force that the Earth exerts on
the book?
b) If the Earth exerts a 5 N force on the book, what is the force that the book exerts on the Earth?
c) What is the acceleration of the book if the Earth is the only object exerting a force on it?
d) Why does the book fall onto the Earth but the Earth does not fall onto the book? The mass of the Earth is
6.00 x 1024 kg.
Need Some Help?
Use the Earth’s mass to calculate the Earth’s acceleration. What does this tell you about the motion of the
Earth?
e) The Sun’s mass is 2.00 x 1030 kg. It pulls on the Earth, exerting a force of about 1020 N. What is the
force that the Earth exerts on the Sun?
8.4 Reason
Two students sit on office chairs with wheels. Student A pushes student B away from him. Student B does
nothing. Does student B exert the force on A? How do you know?
Lesson Activity modified from PUM and Active Learning Guide, Etkina and Van Heuvelen, 2010
1
Sierzega: Dynamics 8
8.5 Reason
a) You hit a stationary puck with a hockey stick. The stick exerts a 100 N horizontal force on the puck.
What is the force exerted by the puck on the stick? How do you know?
b) A truck rear ends a small sports car that is moving in the same direction as the truck. The collision
makes the truck slow down and the sports car is propelled forward. What object exerts a larger force on
the other object: the truck on the car or the car on the truck. Explain how your answer reconciles with
Newton’s third law and with the fact that the sports car is damaged more than the truck.
c) The Earth pulls on an apple exerting a 1.0 N force on it. What is the force that the apple exerts on the
Earth? Why does the apple fall towards the Earth but the Earth does not move towards the apple?
d) The tree branch exerts a 1.0 N force holding the apple. What is the force that the apple exerts on the tree
branch?
8.6 Represent and Reason
Your friend says that if Newton’s third law is correct, no object would ever start moving. Here is his argument:
“You pull a sled exerting a 50 N force on it. According to Newton’s third law the sled exerts the force of 50 N
on you in the opposite direction. The total force is zero, thus the sled should never start moving. But it does.
Thus Newton’s third law is wrong.”
What is your opinion about this answer? How can you convince your friend of your opinion?
8.7 Reason
The Moon orbits the Earth because the Earth exerts a force on it. The Moon, therefore, has to exert a force on
the Earth. What is the visible result of this force?
8.8 Reason
You stand on a bathroom scale in a moving elevator. What happens to the scale reading if all of a sudden the
cable holding the elevator breaks?
a) The reading will increase
b) The reading will not change
c) The reading will decrease a little
d) The reading will drop to 0 instantly
Reflect: What did you learn about Newton’s third law so far? How is it different from the second law?
Give 3 examples of Newton’s third law from everyday life.
Lesson Activity modified from PUM and Active Learning Guide, Etkina and Van Heuvelen, 2010
2