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24

meaning of the word “work” is quite different from the common meaning. A

student standing at rest and holding several books is doing no work although he

or she will feel tired after a time. Technically speaking, work is accomplished

only when a force acts though a distance.

1

As a physical concept work is the result of the action of a force. We

define the work done by the force F as the product of the magnitude of the

force and the parallel distance through which the force acts. This concept

may be written as

work = force ⋅ parallel distance

or

W = Fd

Hence work requires not only a force but also motion or the movement of an

object through a distance. For example, when a person is mowing the lawn, only

the force component parallel to the lawn (the horizontal force) is used to

accomplish work. The vertical component of the force is doing no useful work,

since the force is tending to push the lawn mower into the ground. Since work is

the product of a force and a distance, the units of work are the units of force

times length. In the SI system the units of work are the newton – meter (N ⋅ m,

force ⋅ length). This unit is given the special name of joule.

2

When a force is applied to change the velocity or direction of an object,

work is done against inertia. If we lift an object at a constant velocity there is no

net force on it because it is not accelerated. The weight of the object mg presses

down, and we push up with an equal and opposite force. The distance parallel

to the applied upward force is the height h that we lift the object. Thus the work

done against gravity is

W = Fd, W = mgh

if we lift a 10 – kg mass a distance of 2 m, the work done is

W = mgh = 10kg ⋅ 9.8 m/s

2

⋅ 2m = 196 J

Friction always opposes motion. Hence to move something, we have to apply a

force. The work done by the applied force against friction is W = Fd.

3

The rate of doing work is called power. Power is calculated by

dividing the work done by the time required to do it. Power can be written as

work W

power = , P =

time t

Because work is the product of force and distance (W = Fd), power may be

written in terms of these quantities

24 meaning of the word “work” is quite different from the common meaning. A student standing at rest and holding several books is doing no work although he or she will feel tired after a time. Technically speaking, work is accomplished only when a force acts though a distance. 1 As a physical concept work is the result of the action of a force. We define the work done by the force F as the product of the magnitude of the force and the parallel distance through which the force acts. This concept may be written as work = force ⋅ parallel distance or W = Fd Hence work requires not only a force but also motion or the movement of an object through a distance. For example, when a person is mowing the lawn, only the force component parallel to the lawn (the horizontal force) is used to accomplish work. The vertical component of the force is doing no useful work, since the force is tending to push the lawn mower into the ground. Since work is the product of a force and a distance, the units of work are the units of force times length. In the SI system the units of work are the newton – meter (N ⋅ m, force ⋅ length). This unit is given the special name of joule. 2 When a force is applied to change the velocity or direction of an object, work is done against inertia. If we lift an object at a constant velocity there is no net force on it because it is not accelerated. The weight of the object mg presses down, and we push up with an equal and opposite force. The distance parallel to the applied upward force is the height h that we lift the object. Thus the work done against gravity is W = Fd, W = mgh if we lift a 10 – kg mass a distance of 2 m, the work done is W = mgh = 10kg ⋅ 9.8 m/s2 ⋅ 2m = 196 J Friction always opposes motion. Hence to move something, we have to apply a force. The work done by the applied force against friction is W = Fd. 3 The rate of doing work is called power. Power is calculated by dividing the work done by the time required to do it. Power can be written as work W power = , P= time t Because work is the product of force and distance (W = Fd), power may be written in terms of these quantities

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