Question

Anyone answer the 3 questions?

Experiment:

Tie one end of the fishing line to a chair.

String the other end of the fishing line through the straw and tie the line to a 2nd chair spaced ft. away.

Inflate the balloon. Hold it closed with your fingers, and tape it to the straw.

Slide the straw/balloon back so that the mouth of the balloon is facing the nearest chair.

Let go of the balloon and observe what happens.

Questions:

1. Explain what caused the balloon to move in terms of Newton's 3rd Law.

2. What is the force pair in this experiment? Draw a free body diagram (FBD) to represent the unbalanced) forces on the balloon/straw combination.

3. Add some mass to the straw by taping some metal washers to the bottom and repeat the experiment. How does this change the motion of the assembly? How does this change the FBD?

Answer 1

It might help if you drew a bunch of diagrams. The problem is that you have to see this happening as a pair of forces.

Diagram One
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Draw a balloon with the fat part up and the neck down. Show the neck squeezed together where your fingers hold the neck shut.

Diagram two
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The neck straightens out when you let go. Draw an arrow pointing down. The start of the arrow (the feather end if you are a good artist) is closest to the neck. The pointy end is going downward. Draw a little cloud at the bottom of the neck. The cloud is the air escaping.

Diagram Three
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Here is the tough part. As the air escapes, the pressure inside the balloon becomes less. It is still more than the air pressure. The second arrow comes from the pressure inside the balloon pushing against the point opposite where the air inside the balloon is escaping. So diagram 3 looks just like diagram two except that there is an upward arrow pushing against the inside top of the balloon.

Draw that. Your diagram should have two arrows.

The balloon moves upwards because the force against the top of the balloon is greater than the atmospheric force going down and the force of gravity.

Comment
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Newton's third law says that for every action (escaping air) there is an equal and opposite reaction (force inside the balloon). The force inside the balloon must be greater because the balloon moves upward.

There are always 2 arrows when using Newton's third law. Handy thing to know.