Page 205 - Physics

Work, energy and power

The SI unit of work is joule (J) which is equivalent to newton metre (Nm).

One joule is defi ned as the work done when a force of one Newton (N) moves an
object through a distance of one metre in the direction of the force.

Other units used for measuring work are:
1 kilojoule (kJ) = 1 000 joules (J) and 1 Megajoule (MJ) = 1 000 000 J.

From the Physics point of view, work is specifi cally meant to describe what is
accomplished by the action of a force that makes an object move through a certain
distance in the direction of the force. For example, a girl does work by lifting up books
from the fl oor, as shown in Figure 9.2 (a). But in Figure 9.2 (b), the girl is pushing a
wall which is not moving and hence, no work is done by pushing the wall because there
is no distance moved in the direction of the push.

(a) Lifting a pile of books (b) Pushing against a wall
from the fl oor
Figure 9.2: Illustration of work

For example, if a boy applies a force of 3 N on a box and the box is pushed through a
distance of 1.5 m, work is done on the box. The magnitude of this work is 4.5 J. Lifting
a bag onto your head and lifting the books from the fl oor are examples of work, since,
there is a distance moved in the direction of force. Thus, there are two conditions for
work to be done.
1. There must be a force acting on an object.
2. The object must move along the direction of the applied force.

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