EMT has it backwards: pulleys, and all other simple machines, leave

*work *unchanged while

*altering force and distance*. In all cases, the force is multiplied by a particular factor (which can be greater or less than one) and the distance is divided by the

*same *factor.

For MCAT-style pulley systems, output force is input force multiplied by the number of ropes that are effectively pulling on the object to be moved; equivalently, by the number of ropes that must (not just might) shorten in order for the resistance to move. Another hint: if none of the pulleys is free to move, then the factor is 1: force and distance remain the same. There is a formulaic way to know what the multiplier is by inspecting the pulley diagram, but I'm not going to write it out because it is more trouble than it's worth -- just look at the picture given with the problem.

The other simple machines:

Inclined plane: the force multipler (relative to straight lifting) is the sine of the angle of the plane with the horizontal. Distance is divided by the same factor.

Wedge: similar to an inclined plane; the force multiplier is length divided by width; distance is divided by the same factor. *Unlikely on the MCAT.*

Lever: the force multiplier is ratio of distance from fulcrum to object, to distance from fulcrum to applied force. Look at the picture to make sure you know whether distance goes up or down (hence force down or up). This treatment works even when the force and the object are on the same side of the fulcrum. Note that "fulcrum" just means pivot point.

Wheel and axle: force multiplier is ratio of radii. To figure whether to divide or multiply, look at the picture to see whether distance the object travels is greater or less than the distance over which the force is applied. *Unlikely on the MCAT.*

Gears: the force multiplier is the ratio of the numbers of teeth on the two wheels, or (equivalently) the ratio of the radii. Again, look at the picture to see which increases, force or distance. *Unlikely on the MCAT.*

Screw: an inclined plane wrapped around a rod. *Unlikely on the MCAT*. For completeness: the force multiplier is 2 x pi x radius x (turns/unit length). *Don't worry about this one at all.*

It is possible to combine two or more of these: for example, a cam is an inclined plane combined with a wheel and axle. But don't worry: such a problem is very unlikely to appear on the MCAT, and if it did it would necessarily be a very simple setup.

Hydraulic jack: not usually considered to be a simple machine like the aforementioned, but effectively the same. Force multiplier is the ratio of the areas of the two pistons. Recall that force changes, not pressure. Assuming that the change in height is negligible, it doesn't matter what liquid is used.

Any time one of these devices appears, you should look to see whether the object is moving more or less than the applied force. If the object moves more than the input, then the force on it is less than the input force, and vice versa. This is important: often the easiest way to solve the problem given is to look at distance traveled.

This recap is intended for people who started out with at least some idea what these machines are. It is not intended as de novo instruction; for that, you'll need a textbook or your teacher.

If there is another question pending, I will answer it, but I've lost the point in all the clutter. If anyone is still confused, go ahead and post another question.

Shrike

TPR physics, verbal, bio