University of Virginia
Physics Department

## Newton's Third Law: Action and Reaction

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.10

Objectives

Students will

• identify action-reaction force pairs;
• understand applications of Newton's Third Law of Motion.

Motivation for Learning

Discrepant Event - Soda Bottle Rocket

Materials
• 2-liter plastic soda bottle, empty
• Cork to fit bottle
• 200 ml vinegar
• 3 tablespoons baking soda
• 3 sheets aluminum foil, each 3 cm x 10 cm
• Safety goggles

Procedure

 *Take care that the bottle opening does not face in the direction of any people or breakable objects during this demonstration. Wear goggles throughout*

1. Pour 200 ml vinegar into the plastic soda bottle. Place a cork in the mouth of the container.
2. Make a v-shaped envelope out of the sheets of foil, folding one sheet over the top of the other to make the envelope thicker. This envelope should be small enough to fit through the mouth of the bottle.
3. Fill the envelope with 3 tablespoons of baking soda.
4. Remove the cork from the soda bottle, taking care not to spill the vinegar. Insert the aluminum envelope through the mouth of the bottle, being careful not to mix the vinegar and baking soda.

5. Quickly reseal the bottle.
6. Standing to the side, rapidly rotate the bottle so that the baking soda mixes with the vinegar. Stand back and observe.
7. The cork should shoot from the mouth of the bottle forward, in turn propelling the bottle backwards. The action is the cork, the reaction is the bottle movement.

Optional Bottle Rocket Launcher

For full and detailed instructions on the construction of this device, click here.

This device also uses a 2-Liter bottle as a rocket, but uses a more complicated mechanism to achieve much higher air pressure within the bottle for greater propulsion.

Materials

• launch apparatus (see above)
• 2 liter bottle
• water
• bicycle pump (foot pump with pressure gauge works well)
• pressure gauge (either built into apparatus or attached to bicycle pump)
• 4 anchor spikes and hammer

Launch Procedure

1. Put a spike in each of the anchor holes and use the hammer to secure them into the ground. Attach the bicycle pump to the tire valve stem.
2. Check to make sure that the opening of your empty 2-liter soda bottle will fit over the PVC pipe. It should be a close fit, but some bottles are irregular and may not work.
3. Add varying amounts of water to the bottle (0% to 100% full). Quickly turn the bottle upside down so that the neck is over the air inlet hole.
4. Pivot the 1' steel bars inward so that they go over the bottle lip and under the 3" steel bar.
5. Tighten the wing nut above the compression spring to secure the bottle and make a tight seal.
6. Using the bicycle pump, increase the pressure in the bottle to about 40 psi. Be sure that nobody is above the bottle, then pull the string to remove the steel bars and thus launch the rocket.
7. Observe the motion and record your results.

Note: The industrial specifications for the pop bottles maintain that the bottles can withstand 180 psi. For maximum safety, do not exceed 90 psi.

Background Information

Newton's Third Law of Motion states that for every action done by a force, there is an equal and opposite reaction by another force. Forces always occur in pairs; one force is called an action force and the other is called the reaction force. In the demonstration detailed above, the action force is caused by carbon dioxide from the reaction between vinegar and baking soda. The reaction gas causes pressure to build inside the soda bottle, eventually pushing the cork from its mouth. A result of this action is the movement of the soda bottle in the opposite direction. This movement is caused by the reaction force that stems from the original action. There are many other examples of force pairs in everyday life. When you walk, your feet push on the ground with a force. In return, the ground pushes on you with an equal force, propelling you forward. A plate resting on a table exerts a downward force on that table due to gravity. In return, the table exerts an upward normal force on the plate.

In this experiment, the students are asked to explore two different action-reaction pairs. Part one involves the motion of a balloon along a piece of fishing line. If an inflated balloon is caused to release its air slowly, the force of the air leaving the balloon will result in a reaction force, which launches the balloon forward. In part two, the students will combine Alka Seltzer tablets and water in a sealed Kodak film canister. This combination will produce carbon dioxide, which will force the lid from the canister. Similar to the soda bottle demonstration, the forward motion of the lid will induce a backward, reaction motion of the canister. However, part two involves a second canister, which will move as a result of the initial action, allowing it to be more visible. The students should try each of these experiments a few times to completely conceptualize action-reaction force pairs. Before they begin the experiment, discuss and brainstorm with them other examples of these force pairs that are found in the home or classroom.

### 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.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.