Force Multiplier

Who's Stronger?

The person pulling on the rope will make the other two people come together, no matter how hard they resist.


  Two cylindrical bars about equal length. Drill or groove one end of one bar to hold the ends of rope.

  One rope: about 40 feet.


( 5 min.)

Tie ends of rope at the two ends of one bar. While two people standing 1~2 feet apart face each other holding bars, the third person wraps the loop around the both bars three times. Now the third person may pull while the other two try to resist.

Wrap the rope around more times and try again; wrap the rope fewer times and try.

Work equals force times distance.

In the absence of friction, energy is conserved and the work done by the pulling person is equal to the work done on the holding people. The person who is pulling moves a large distance and exerts a small force while the two bar holders each experience a large force over a small diatance.

Increasing the number of times the rope wraps around reduces the force and increases the distance even further.


Julia Fong



©1997 The Exploratorium 3601 Lyon Street San Francisco, CA 94123