Pulleys

When I was in college, many years ago, I studied physics. Somewhere along the way we learned about pulleys.

The purpose of a pulley is to change the direction of a force. Ideally it will do so with very little friction, but the purpose is still nothing more than changing the direction of a force. When you combine multiple pulleys in the right configuration, you effectively trade force for distance in the equation for Work: W = FD (Work equals the Force applied multiplied by the Distance over which the force is applied). The basic principle is that the necessary force goes down as the distance traveled goes up. A collection of pulleys increases the distance (you have to pull more rope) which reduces the required force by the same factor (i.e., four times more rope means 1/4 as much force).

In one pulley-related assignment, there was a problem where one pulley was replaced by a simple iron bar which was assumed to be frictionless. Instead of going around a pulley, the rope went around this frictionless iron bar. Since a pulley changes the direction of a force, and the bar changed the direction of the force, I treated the iron bar as if it where a pulley. The professor marked my answer wrong. When I discussed it with him, he said that since the bar was not a pulley, it did not add to the mechanical advantage of the system. He rejected my reasoning that since the bar changed the direction of the force just like a pulley, and since the bar increased the distance traveled by the rope just like a pulley, then the bar served exactly the same function as a pulley.

Although I got nowhere with that professor, I have since found that my analysis was in fact correct. You can replace a pulley with a metal bar, a metal ring, or even a loop of rope (you can actually make a “block and tackle” arrangement with nothing but rope). As long as the direction of the force is changed with relatively little friction added to the system, the physical principle is the same.

A doctorate does not prove that someone knows what he’s talking about.

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