Name__________________________________
Per _________
NEWTON’S THIRD LAW: FORCE PAIR DIAGRAMS
I.
Review drawing a force diagram
Drawing A
A. Draw a force diagram for a book at rest on a level table.
1)
2)
book
A correct force diagram should not have anything on it except
a dot for book and the forces acting on the book
Make sure to label each force w/ the type and “on_by” objects
Force diagram for book
Answer the following questions:
1. How do you know each of the forces you labeled in your
diagram exists?
FN
On book
By Table
There is a gravitational force between Earth and all objects with mass, so
there is a gravitational force on the book by Earth drawn down in th e
diagram. Sin ce the book is in contact with the table, there must b e a force
by the table pushing on the book. This force is called the normal force,
because it is a contact fo rce perpendicula r to the surfa ce pushing.
FG
On book
By Earth
2. How do you know the different sizes of each force acting on
the book? How did you represent this in your diagram?
Since the book is not moving the forces on the book should sum to zero, o r
be balanced. The arrows were drawn the same size to show that th e forces
are balanced.
B. A second book with twice the mass is added on top of the
first book. Draw a force diagram for each of the books in
Drawing B. Label all the forces.
Answer the following questions:
1. Which forces are contact and which forces are non-contact?
Drawing B
Upper book-U
Lower book-L
Force Diagram for upper book
Contact: all 3 normal fo rces Non-contact: bo th gravita tional forces
FN
On book-U
By book-L
2. Compare the forces you just drew on the two force diagrams.
Can the same force appear on two different force diagrams?
Explain.
FG
On book-U
By Earth
No, different diagrams are fo r forces acting only on the one object
represented in the diagram.
3. What type of force does the upper book exert on the lower
book?
Force Diagram for lower book
Normal force- a pushing force perpendicula r to the surfa ce pushing, in this
case downward.
FN
On book-L
By table
4. What object causes the force due to gravity on another
object?
Earth causes most gravity fo rces that we talk about, and is th e object
exerting the gravity force on both the books.
5. If the upper book were taken off the lower book and set
beside it would the force due to gravity still act on the upper
book?
Yes. Earth would still be ex erting a non-contact pulling force on the book.
Teacher Check In _________________
FN
On book-L
By book-U
FG
On book-L
By Earth
Name__________________________________
Per _________
6. If the upper book were taken off the lower book and set beside it, would the upper book still push down on the
lower book? Why or why not?
No. Since the books would no longer be touching, the normal force by the upper on the lower, which is a contact forces,
would no longer be acting on the book.
7. Why is it incorrect to say that the force due to gravity on the upper book is pushing on the lower book?
The force due to gravity is a non-contact force due to the attraction of the masses of the objects. This force is incredibly
small unless a very massive object, like Earth, is one of the masses. In this scenario, the force by the upper book on the
lower book is only acting when the books are in contact. So this is a contact force of the upper book pushing down on the
lower book, NOT a force due to gravity.
8. Describe the size of the forces on the upper book.
The two forces on the upper book are the same size so that the forces on the upper book balance.
9. Are there any forces acting on the lower book that have the same size as a force acting on the upper book?
Please explain and provide examples.
The force on the lower book by the upper book is the same size as the force on the upper book by the lower book. These
are the only two forces that are the same since the mass of the books is different; the gravitational force by Earth on the
books will be different.
10. Compare the force diagram for the lower book to the force diagram for the same book in part A. (before the
upper book was added)
Which of the forces changed when the upper book was added and which remained the same?
The gravitational force on the lower by Earth is the same size as before. The upper book is pushing downward (normal)
force on the lower book. The normal force by the table has increased in size so that it continues to balance the other two
downward forces on the lower book.
Teacher Check In _________________
Newton’s Third Law of motion states: For every force applied to an object, that object applies an equal but
opposite force back. For example, if I push on you, you push back with an equal but opposite force.
11. Based on the questions you answered above and Newton’s Third Law of motion, which two forces in your force
diagrams of the upper book and lower book are third law pair forces?
The normal force on the lower book by the upper book and the normal force on the upper book by the lower book are
Newton third law pair forces.
12. Are the two forces you chose in #11 equal in size and opposite in direction?
Yes.
13. Can the top or bottom books push on themselves? Explain why or why not.
No, forces are interactions between two objects and in this case we are considering each book a single object.
14. Why do the two forces in a Third Law force pair need to be on different force diagrams?
The third law force pairs are the forces the two objects exert on each other, so they will be in separate diagrams for each
of the objects involved in the interaction.
15. Are the two forces you chose in #11 on different force diagrams? If not correct your answer to #11.
Yes.
16. What do you notice about the force type and the “on by” objects of your Third Law force pair?
The force type is the same, Normal in this case. The on__ by___ objects are switched.
Name__________________________________
Per _________
Requirements for Newton’s third law force pairs:
 The two forces must be equal in size and opposite in direction
 Each of the forces must be on a different force diagram
 The two forces in the force pair must be the same type of force
 The “on by” object will be reversed.
17. Does your third law force pair meet the requirements listed above? If not correct #11.
Yes.
Teacher Check In _________________
II. Introduction of “System” in force diagram
1. On drawing C, circle the upper and lower book to make a
system of books. Name the system “System U-L.” Objects in
the system are treated as one single object.
2. Draw a force diagram for the System U-L of books on
drawing C.
3. Compare the force diagrams in drawing C and B, answer the
following questions
a. Are there any forces that remain the same in size and/or
direction?
Yes, the Normal force by the table that was acting on the lower book,
but is now acting on the system of books, is the same size and
direction as before.
Drawing C
Upper book-U
Lower book-L
Force Diagram for the system of books
FN
On system U-L
By Table
FG
On system U-L
By Earth
b. Are there any forces that change in size and/or direction?
Yes, the gravitational force on the system by Earth is now larger. It is
in the same direction but it’s size is a combination of the gravity
forces on the books individually.
c. Are there any forces from your drawing in B that are not included in your drawing for the System U-L?
Yes, the normal force on the lower book by the upper book and the normal force on the upper book by the lower book are
no longer in the diagram because these forces are internal to the system and so cannot act on the system.
4. Justify why the Newton’s third law force pairs between objects in a system do NOT appear on the force diagram
for the system of objects.
The third law force pairs in this scenario are now contained within the system of two books so do not appear on the
system diagram. Since these forces were equal and opposite forces to begin with, they internally cancel out and so
leaving them out of the diagram has no impact on the sum of forces.
Teacher Check In _________________