• California State University, Northridge

• PHYS 170

UNIT CF

Developing Ideas

ACTIVITY 3: Comparing Forces During

Interactions

Purpose

In all previous activities in this module, you have used ideas about

forces to examine what happens to only one of the objects involved

in a contact push/pull interaction. For

example, when a stationary lowfriction cart is given a push, we say a

force is applied to the cart and the

effect of this force is to start the cart

moving. However, there is another

object involved—the hand! Does the cart also exert a force on the

hand while they interact? If so, how do these forces compare?

In this activity you will examine the forces that two objects exert on

each other during a contact push/pull interaction between them, to

see if there is a general rule that can be applied to any such

situation.

When two objects interact in a

contact push/pull interaction, how do

the forces they exert on each other

compare?

Initial Ideas

A small, empty, pick-up truck

and a large, fully loaded, box

van are about to collide. The

box van has a greater mass,

but the pick-up has a higher

speed. What forces do you think they will exert on each other while

they are colliding, and how do you think those forces will compare?

© 2017 Next Gen PET CF-25

Unit CF

During the short period of the collision, do you think both

vehicles will apply a force to the other? How would you know if

they did?

How do you think these forces would compare? To show your

thinking, use the pictures below to draw two separate force

diagrams, one for each of the two vehicles at a moment in time

during the collision. Briefly explain why you drew any force

arrows as you did.

_ Draw your two force diagrams on a presentation board and

participate in a class discussion about your ideas. Make a note

of any ideas or reasoning that differs from those of your group.

Collecting and Interpreting Evidence

Exploration #1: Do both objects in an interaction apply forces?

You will need:

2 low friction carts (share with another group if necessary)