F&M Activity IV-D1

How do Objects at Rest Interact?	Name:

Group:

Class Period:

To start your exploration of how objects interact, you will look at a fairly simple case - two objects at rest. When two people are arm-wrestling, and neither arm is moving, what kind of forces are they exerting? How do their forces compare?
Materials: 2 bathroom scales (shared with other groups), 2 spring scales (shared with other groups), long rubber band, stiff spring, string
Hold a bathroom scale vertically against the wall and push. Then push harder and notice what happens to the scale reading. Let us assume throughout this activity that the reading on the scale indicates the strength of your force (or push) against it.
Imagine four situations in which two people are holding bathroom scales. Assume the people use their scales to push hard against each other, but not hard enough to cause either of them to move. As each person pushes, she notes the reading on her scale, which we assume measures the strength of the force with which she is pushing against the other person. In the diagrams below, we have drawn a force arrow whose length represents the strength of the force that the left person exerts on the right person. (A longer arrow means a stronger push.)

For each of the situations, predict how the readings on the scales will compare--are they equal, or is one larger than the other, and if so, which is larger? Draw an arrow representing the force that the person on the right exerts on the person to the left. (It should be longer, shorter, or equal in length to the force arrow already drawn, and in the appropriate direction.) Briefly explain how you decided each case.
Same size people push hard on each other.

Force that left person pushes

on right person	Prediction:

One person is bigger; people push hard on each other.

Force that left person pushes

on right person	Prediction:

Smaller person pushes, the bigger person resists.

Force that left person pushes

on right person	Prediction:

Bigger person pushes, the smaller person resists while braced against a wall.

Force that left person pushes

on right person	Prediction:

Now try it! Use scales and people to test your predictions. Try not to squeeze the scales when you hold them, just push with them. (If one person in your group is not much larger in size than the others, perhaps you can “borrow” one person from another group.) Summarize your observations below.
Same size people push hard on each other.
Observation:

One person is bigger; people push hard on each other.
Observation:

Smaller person pushes, the bigger person resists.

Observation:

Bigger person pushes, the smaller person resists while braced against a wall.
Observation:

Perhaps you noticed some differences in the various situations. In cases (a) and (b), both people were actively pushing. In cases (c) and (d), one person seemed to actively push, while the other person was trying to resist the push. However, in all cases, both people were exerting a force against their scales. You certainly should have felt that in your muscles!

Summarize your findings. Do you see a pattern in your data? For each situation, how does the strength of the force exerted by one person compare with the strength of the force exerted by the other person? Does it make any difference whether one person seems to be actively pushing, while the other seems to just be resisting? Does the size of the person seem to matter? Check your findings with at least one other group.

Imagine four new situations illustrated below in which each person is pulling on a spring scale instead of pushing on a bathroom scale. Also, each person's pull is still less than the maximum reading on the scale (otherwise the scales will break). In each case we have drawn an arrow that represents the strength of the force that the person on the left exerts on the person to the right.

For each of the situations, predict how the readings on the scales will compare—will they be equal, or will one be larger than the other? If so, which reading is larger? Draw an arrow representing the force that the person on the right exerts on the person to the left. (It should be longer, shorter, or equal in length to the force arrow already drawn, and in the appropriate direction.) Briefly explain how you decided each case.
Same size people are pulling on spring scales.

Scales hooked together	Prediction:

One person is bigger; both are pulling

Scales hooked together	Prediction:

Rubber band is in the middle between the two spring scales.
Prediction:

Rubber band and stiff spring are in the middle.
Prediction:

Now try it! Use scales, the rubber band and stiff spring to test your predictions. Be careful not to pull hard enough to break the scales. Fill in your observations below.
Same size people are pulling on spring scales.

Scales hooked together
One person is bigger; both are pulling

Scales hooked together
Rubber band is in the middle between the two spring scales.
Rubber band and stiff spring are in the middle.
Summarize your findings from the previous set of experiments. For each situation, how does the strength of the force exerted by one person compare with the strength of the force exerted by the other person? What effect(s) did the rubber band and/or stiff spring have on these conclusions?
In this activity you examined several situations in which a person was exerting a force by either pushing or pulling on something (for example, a scale). Do you think it is possible for a person to exert a force without pushing or pulling on something? If so, give some examples. If not, why do you not think so?
Let us introduce a definition. Two objects that exert pushes or pulls on each other are said to interact with each other. An interaction always involves two objects that are doing the interacting. The objects can be people, springs, scales, or any of a number of different things. Consider the following statement:

"When two objects interact, the strength of the force that one object exerts on the other is equal to the strength of the force that the second object exerts on the first, and in the opposite direction."

Based on evidence gathered in this activity, how would you respond to that statement?
Open up your Cycle IV Idea Journal. Add to your idea under the pocket headed Comparative Strengths of Forces of Interacting Objects.

F&M Activity IV – D1

4	©2000 CPU Project

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©2000 CPU Project	1

College of Sciences

1998-11-02