F&M Activity IV-D2

How do Objects in Motion Interact?	Name:

Group:

Class Period:

In a children's game called king of the hill, children try to push each other off of a small hill. Often two kids will be locked together, one forcing the other down the hill. What forces are the two children exerting? How do they compare?
Materials: 2 bathroom scales (shared with other groups), chair with wheels, 2 carts, long track, blocks to put in carts, spring scale, 2 MBL force sensors (to share with another group), small piece of foam rubber, tape, block of known weight (to be used for calibration).
Imagine two people holding bathroom scales. One sits in a chair with wheels, and the other stands. For the two situations described below, predict whether you think the scales will show the same or different maximum readings. If you think the maximum readings will be different, state which one will be larger. On each picture, we have drawn a force arrow to represent the strength of the force that the person on the left pushes on the person to the right. You draw the other arrow, representing the force that the person on the right exerts on the person to the left. Explain your reasoning in each case.
Person standing against wall pushes with arms extended against the sitting person's scale. Standing person stays put while the sitting person moves away.

Prediction:
Standing person runs while continuously pushing against the sitting person's scale. They both move.

Prediction:
In either of the two situations above, do you think it would matter if the people are the same or different sizes? Explain your reasoning.
Test your predictions, using people of approximately the same size. Record the maximum reading on each scale.
Person standing against wall pushes with arms extended against the sitting person's scale. Standing person stays put while the sitting person moves away.
Observation:
Standing person runs while continuously pushing against the sitting person's scale. They both move.
Observation:
Repeat with two people of significantly different size. How do your results change, if at all?
Summarize your findings. In each case, how does the strength of the force exerted by the standing or running person compare to the strength of the force exerted by the seated person?
Imagine two carts colliding on a track. What happens when they each exert a force on the other? Assume that both carts are moving with the same speed before the collision. For each of the five situations pictured below, predict whether the force that cart A exerts on cart B during the collision is equal in strength, larger, or smaller, than the force that cart B exerts on cart A. Briefly explain your reasoning. In the "crash" picture we have drawn a force arrow representing the force cart A exerts on cart B. You draw the force arrow that represents the force cart B exerts on cart A.

Prediction:

Prediction:

Prediction:

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To test your predictions you will need a way of measuring the forces during the collision. You can do that with the MBL equipment. You will need two force probes, and may have to share with another group. Open Act IV-D2 MBL, and follow these instructions.

The MBL graphs are already set-up so that one force-time graph will be positive and the other will be negative, so you can tell the two apart. Make sure both force probes are set to 50 N.
This experiment works much better when you tape a foam rubber bumper on one hook or use a special bumper to substitute for the hook. The foam rubber or the tape must not touch the black plastic shell of the force sensor.

Start collecting data, and push one cart (call this cart A) so it collides with the other (cart B).

Click the Zero button above the graph window before each experiment. Do not press the "Tare" button on the side of the force sensor or you will get inaccurate measurements.

If necessary, change the scale on your graph.

If you are having difficulty collecting good data for this experiment, call over a staff member who may be able to offer some suggestions.
When you have a good graph, take a snapshot and paste it below.

Observation:

From your MBL data, what conclusions can you draw regarding how the strength of the force that cart A (the moving cart) exerts on cart B (the stationary one) during the collision compares to the force that cart B exerts on cart A?
Collect MBL data for each of the other four situations for which you had made predictions, and which are summarized below in pictures. For each situation, summarize your findings from the MBL data. How did the force that cart A exert on cart B during the collision compare with the force that cart B exerted on cart A? You may wish to take snapshots of the MBL graphs. Remember to “zero” all sensors before each trial.

Observation:

Observation:

Observation:

Observation:

Summarize your overall findings. For each of the situations, how does the strength of the force that one cart exerts on the other during the collision compare with the strength of the force that the second car exerts on the first?
Do your conclusions depend on whether the masses of the carts are equal or unequal, or whether one or both carts are moving?
Open your Cycle IV Idea Journal. Based on evidence gathered during this activity, add to the idea entitled Comparative Strengths of Forces of Interacting Objects.

F&M Activity IV – D2

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College of Sciences

1998-11-02