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A constant force F acting along a straight line for a distance s requires the amount of work

W = Fs.

For example, the force of gravity on an object of mass m near the surface of the earth is very nearly a constant g times the mass, F = gm. Thus to lift an object of mass m a distance s against gravity requires the work W = gms. The following principle is useful in computing work done against gravity.

The amount of work done against gravity to move an object is the same as it would be if all the mass were concentrated at the center of mass. Moreover, the work against gravity depends only on the vertical change in position of the center of mass, not on the actual path of its motion.

That is,

W = gms

where s is the vertical change in the center of mass.

Example 6
Example 7
Example 8

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Last Update: 2006-11-25