physics PAPER WORK

Lab 10: Newton’s Third Law

Purpose

To study action and reaction by observing collisions between moving balls of different masses

Background

In the past, cannons were an important weapon for reaching the enemy at a distance. However, soldiers had to be careful not to stand behind a cannon when it was fired. The heavy cannon would “kick” back or roll backward with each shot. This “kick” back is an example of Newton’s third law of motion: For every action (the cannon ball firing forward at high speed) there is an equal and opposite reaction (the cannon moving backwards at low speed).

Skills Focus

Predicting, interpreting data, applying concepts, drawing conclusions

Procedure

1. Start Virtual Physics and select Newton’s Third Law from the list of assignments. The lab will open in the Mechanics laboratory.

2. The laboratory will be set up with two balls of the same mass on a table. The balls will move towards each other. As they collide, they will exert forces on each other which are equal and opposite.

Predicting When a heavier ball hits a lighter ball, how do the forces they exert on each other compare? What is the resulting motion of the balls?

3. The initial velocity of all of the balls will be the same for the first three trials, but the masses of the balls will change. You will observe what happens in the collisions and record the final velocities of each ball.

4. Click the Start button to start the balls in motion. After the balls bounce off each other and move a short distance away from each other, click the Pause button to stop the experiment. Note the velocity of each ball in the display panel below the table. You can display the velocity of the second ball by clicking on the ball, or by clicking on the Tracking arrows located in the lower right corner of the display to change the display. Record the velocities in the table on the next page. Describe what happened to each ball in the reaction box.

5. Click the Reset button to reset the experiment before trying new masses. Use the table to keep track of what mass to use for each trial. Use the Parameters Palette to change the masses of the balls. Uncheck the Balls Same Mass and Diameter box to be able to change each mass separately.

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6. Now change the masses and initial velocities of the balls so you can get one ball to totally stop after a collision when they both start out moving (Trial 4). Also conduct a trial (Trial 5) where one of the balls starts at rest and is still at rest after the other ball collides into it.

Table

Trial 1	Mass (kg)	Velocity Before	Velocity After	Reaction
Ball 1	10	10
Ball 2	10	10
Trial 2
Ball 1	20	10
Ball 2	10	10
Trial 3
Ball 1	50	10
Ball 2	1	10
Trial 4
Ball 1
Ball 2
Trial 5
Ball 1
Ball 2

Analyze and Conclude

1. Interpreting Data Describe what happened when the masses of the balls were the same, and when they were very different.

2. Applying Concepts Explain why a lighter ball has more velocity after a collision with a heavy ball than it had before. Where did that velocity come from? Hint: Think about Newton’s second law of motion.

34 Newton’s Third Law

Virtual Physics Lab Workbook, by Brian F. Woodfield, Steven Haderlie, Heather J. McKnight, and Bradley D. Moser. Published by Pearson Learning Solutions.

Copyright © 2008 by Pearson Education, Inc.

3. Drawing Conclusions Does the data prove your earlier prediction?

Explain.

4. If forces are acting on each object in every collision, how can a ball stop or not even move, as you observed in Step 6?

5. Drawing Conclusions Why don’t the forces of action and reaction cancel each other? Why isn’t the net force zero if the forces are the same in each direction?

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ISBN 1-269-73240-4

Newton’s Third Law

Newton’s Third Law

ISBN 1-269-73240-4

ISBN 1-269-73240-4

Newton’s Third Law