Teaching Physics with the Physics Suite

Edward F. Redish

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Buying wire

Wire is often delivered wrapped on a large cylindrical spool. Suppose such a spool is supported by resting on a horizontal metal rod that is pushed through a hole that runs through the center of the spool. A worker is pulling some wire off the spool by exerting a force on it something like is shown in the diagram at the right. (If you have ever bought wire in a hardware store, this is the way they usually store and dispense it.)

Suppose the spool rotates on the rod essentially without friction. The spool is approximately a uniform cylinder with a mass of 50 kg and a radius of 30 cm. The worker pulls on the wire for 2 seconds with a force of 30 Newtons. At the end of the 2 seconds he immediately clamps on a brake which very quickly stops the spool's rotation. Just before he put the brake on, how fast was the spool rotating? How much wire did he pull off the spool?

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Page last modified October 15, 2002: R14