Page 6 - Fisika Terapan for Engineers and Scientists

206 CHAPTER 7 Work and Energy

Force parallel to
This force F has only (a) the motion does
an x component, F . positive work.
x
positive work
motion
F F F F

x x
(b)
For work to be done
by a force, there must
be a displacement. motion negative work
F F
FIGURE 7.1 You do work while pushing an automobile along a
road with a horizontal force F.
x
Force antiparallel
to the motion does
negative work.

FIGURE 7.2 (a) The work you do on the automobile is positive if
you push in the direction of motion. (b) The work you do on the auto-
mobile is negative if you push in the direction opposite to the motion.

Note that in Eq. (7.1), F is reckoned as positive if the force is in the positive x
x
direction and negative if in the negative x direction.The subscript x on the force helps
us to remember that F has a magnitude and a sign; in fact, F is the x component of
x
x
the force, and this x component can be positive or negative. According to Eq. (7.1), the
work is positive if the force and the displacement are in the same direction (both positive, or
both negative), and the work is negative if they are in opposite directions (one positive,
the other negative). When pushing the automobile, you do positive work on the auto-
mobile if you push in the direction of the motion, so your push tends to accelerate the
automobile (Fig. 7.2a); but you do negative work on the automobile (it does work on
you) if you push in the direction opposite to the motion, so your push tends to decel-
erate the automobile (Fig. 7.2b).
Equation (7.1) gives the work done by one of the forces acting on the particle. If
several forces act, then Eq. (7.1) can be used to calculate the work done by each force.
If we add the amounts of work done by all the forces acting on the particle, we obtain
the net amount of work done by all these forces together. This net amount of work
can be directly calculated from the net force:
W F x
net,x
In the SI system, the unit of work is the joule (J), which is the work done by a force
of 1 N during a displacement of 1 m. Thus,
1 joule 1 J 1 N m

Suppose you push your stalled automobile along a straight road
EXAMPLE 1
(see Fig. 7.1). If the force required to overcome friction and to
keep the automobile moving at constant speed is 500 N, how much work must
you do to push the automobile 30 m?