University of Virginia
Physics Department

Newton's Third Law: Action and Reaction

A Physical Science Activity

Student Activity

Materials

 Safety goggles Towel/sponge Meter stick Long, cylindrical balloon (average size) 3 empty Kodak 35 mm film canisters Scotch tape 2 Alka Seltzer tablets 1 plastic straw One 1-2 oz weight/washer Fishing line (5 m) 2 chairs or lab stools Hot Wheels car track or model railroad track, 70 cm long (N-gauge) Water, 100 ml

Procedure

Part 1

1. Cut a long piece of fishing line (5 m). Tie one end to the leg of a chair.
2. Inflate the balloon. While one person holds the opening closed, tape a straw to the balloon parallel to its length.
3. String the other end of the fishing line through the straw, and tie that end to the leg of another chair approximately 5 m from the first.
4. Hold the inflated balloon near one chair, with the opening facing the chair, and release it.
5. Repeat this three times, each time trying to make the balloon travel farther along the fishing line.

Part 2

1. Place the railroad or Hot Wheels track on the table. Make sure that both ends of the track are pointing away from people.
2. Place an empty film canister, with its cap, on the track. It should fit cleanly between the two rails of the track.
3. Pour water in another canister to a depth of about 0.5 cm.
4. Place 1/3 tablet of Alka Seltzer into the canister and shake it for a second or two.
5. Quickly place the canister on the track so that the caps of the two canisters are touching. Step back. The "loaded" canister should explode, pushing on the empty canister.

6. If the canister does not fire within two minutes, carefully lift it from the track and slowly release any internal pressure. Load another film canister and repeat.
7. Measure the distance traveled by the bullet (empty canister) and the cannon (full canister). Put the data in the data table (see below).
8. Fill the empty canister with water and place it on the track.
9. Reload the canister acting as a cannon as described in steps 3 and 4. Place it quickly on the track and step away.
10. Measure the distance traveled by the bullet (canister + water) and the cannon.
11. Pour the water out of the bullet canister and place the weight or washer inside. Place the canister on the track.
12. Repeat step 9. Measure the distance traveled by the bullet (canister + weight) and the cannon.

Data Sheet

 Distance by bullet (m) Distance by cannon (m) Trial I (empty bullet) Trial II (canister + water) Trial III (canister + weight)

Questions

1. Explain what caused the balloon to move across the room in Part I in terms of Newton's 3rd Law. Identify the force pair in this situation.

2. What caused the "bullet" canister to move in Part II? Identify the force pair.

3. Explain the difference in the distance traveled by the "bullet" in each of the three trials. Compare this with the distance traveled by the "cannon."

4. Cannons that fire bullets are always very massive in comparison with the bullet. Explain why this is necessary.

5. Identify the action-reaction pairs in the following situations:

a. A man steps from his boat to the dock.

b. A textbook rests on a school desk.

c. A tennis racket contacts a tennis ball.