[Illustration: FIG. 93.--Rolling barrels up a plank.]
150. When do we Work? Whenever, as a result of effort or force, an
object is moved, work is done. If you lift a knapsack from the floor
to the table, you do work because you use force and move the knapsack
through a distance equal to the height of the table. If the knapsack
were twice as heavy, you would exert twice as much force to raise it
to the same height, and hence you would do double the work. If you
raised the knapsack twice the distance,--say to your shoulders
instead of to the level of the table,--you would do twice the work,
because while you would exert the same force you would continue it
through double the distance.
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