Page 206 - Physics

Physics for Secondary Schools

These conditions are illustrated in Figure 9.3.

No motion Walking

Direction
of motion

Force

Work is being done when lifting the No work is done when you are just No work is done while moving an object
object upwards holding an object horizontally
(a) Lifting or lowering (b) Force holding an object (c) Distance moved is
an object. Work is done but an object does not perpendicular to the
move. No work is done force. No work is done

Figure 9.3: Illustration of the conditions for the work to be done

(a) If an object is lifted or lowered shown component of the force that is parallel
in Figure 9.3 (a), a vertical force is to the distance moved.
exerted on the object and it moves (f) If the force and distance moved are
parallel to the force so work is done. perpendicular (at a right angle) to each

(b) If an object is simply held in the air other, no work is done.
shown in Figure 9.3 (b), an upward
force is exerted on the object but the Example 9.1
object is not moving. Thus, no work A force of 80 N pulls a box along a
is done. smooth and level ground through a

(c) If an object is held while walking distance of 5 m. Calculate the work
across the room as displayed in done by the force.
Figure 9.3 (c), an upward force is
exerted on it and the object is moving. Solution
However, the movement is not parallel Force (F) = 80 N, distance (d) = 5 m,
to the force, no work is done. work done (W) is given as:

(d) If the force and distance moved by W = F × d
the object are in opposite directions, = 80 N ×5 m
work is done by the force.
(e) If the force and distance moved are at = 400 J
some angle, work is done only by the Therefore, the work done is 400 J.

200
Student’s Book Form One

Physics Form 1 Final.indd 200 16/10/2024 20:58