EXPERIMENT 7.3

PURPOSE:

To investigate action and reaction forces and observe Newton's third law of motion.

MATERIALS:

• A plastic, 2 liter bottle
• A stopper that fits the bottle (It could be rubber or cork, but you cannot use the screw-on cap. It has to be something that plugs up the opening of the bottle but can be pushed out by a pressure buildup inside the bottle. Modeling clay can work as well. You could also try a large wad of gum, as long as the gum has dried out and has the texture of firm rubber.)
• A cup of vinegar
• Two teaspoons of baking soda
• Aluminum foil
• Four pencils
• Eye protection such as safety goggles or safety glasses

BACKGROUND:

You will want to do this experiment outside, because it is loud and messy!

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.

In this experiment, we will observe Newton's Third Law in action through a chemical reaction that produces gas, creating pressure inside a closed container. When the pressure builds up enough to push out the stopper, the escaping gas pushes in one direction (action), and the bottle moves in the opposite direction (reaction).

The chemical reaction between vinegar (acetic acid) and baking soda (sodium bicarbonate) produces carbon dioxide gas. As this gas builds up in the sealed bottle, pressure increases until it forces the stopper out, demonstrating Newton's Third Law.

QUESTION:

What happens when a chemical reaction produces a lot of pressure inside a closed container? How can Newton's third law explain this?

HYPOTHESIS:

Write your prediction for what will happen when a chemical reaction causes a pressure buildup in a closed container:

PROCEDURE:

1. Pour about a cup of vinegar into the 2 L bottle.

2. Put the four pencils on the ground parallel to each other and about 2 inches apart. Lay the bottle on its side on top of the pencils. Do this carefully so that none of the vinegar spills out.

3. Take the aluminum foil and make a long, thin trough. The trough should be thin enough to fit inside the mouth of the bottle.

4. Once you have made the trough, fill it with 2 teaspoons of baking soda.

5. Gently push the trough into the bottle, so that it floats on top of the vinegar. Try to spill as little baking soda as possible.

6. Be careful to stay on the side of the bottle. You should not be in front of or behind it! Carefully use the stopper to plug up the mouth of the bottle.

7. Staying to one side of the bottle, roll the bottle to the side, allowing the baking soda to mix with the vinegar. For best results, make sure the bottle stays on the pencils.

8. Stand away to one side of the bottle. BE SURE TO STAY TO THE SIDE OF THE BOTTLE. DO NOT GET IN FRONT OF OR BEHIND IT!

   Record what happens:

9. Clean up and put everything away.

VIDEO DEMONSTRATION:

Watch this video demonstration of Newton's Third Law experiment:

DATA COLLECTION:

CONCLUSION:

What happened in the experiment? Write a paragraph explaining what happened in terms of Newton's third law of motion. Make connections to the text.

ANALYSIS QUESTIONS:

EXTENSION ACTIVITIES:

Try these additional activities to further explore Newton's Third Law:

1. Try the experiment with different amounts of vinegar and baking soda. How does this affect the reaction?
2. Design a water rocket using a plastic bottle and compressed air. How does this demonstrate Newton's Third Law?
3. Research how Newton's Third Law applies to rocket propulsion in space.