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Gravity: A series of pulleys can help workers trade force for distance:

In this diagram, the only thing that changes is the direction of the force the farmer has to apply to lift the weight. He has to apply 400 pounds of force to keep the weight suspended.

The following arrangement changes the amount of force needed. The weight is now suspended by two ropes rather than one. That means the weight is split equally between the two ropes, so each one holds only half the weight, or 200 pounds. So if you want to hold the weight suspended in the air, you only have to apply 200 pounds of force (the ceiling exerts the other 200 pounds of force on the other end of the rope). Yet, though the force has been cut in half, the distance the rope must be pulled has doubled.