Activity 5. Pull each other

Question

Materials: 2 rubber bonds and ruler/ tape measure

Procedure:

1. Tie the two rubber bond together.

2. Each person should pull back the end of the rubber bond. Record the two readings using a ruler or tape measure.

3. Pull harder and record the two readings.

4. Continue to pull on both rubber bond but let the rubber bond move toward one person.

5. Try pull in each such a way that the two rubber bonds have different readings.

Think about it:

1. What did you notice about the reading in step 2 to 4?

2.What can you conclude about the pair of forces in each situation?

Answer 1

This is a classical "tug of war" problem. I would rather use not the rubber bond but wide rubber bands.

If you do all measurements in a right way, you would notice that measured elongations of the bands are the same. Hence, using Hooke's law we conclude that magnitudes of forces are the same.

This is exactly what Newton's third law states: When two bodies interact, they exert on each other forces of equal magnitudes and opposite directions.

Then the question arises: Wait a second, how come? We always have the winner in a "tug of war" competition. If magnitudes of forces the same, how is it possible to win?

The answer is simple. The Newton's third law we can apply only to TWO interacting bodies! In case of "tug of war" there are 3 bodies, i.e. we need also to consider the surface of the earth. In the game the individual, who has better connection with the surface of the earth (hence, greater static friction), will win!!!