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We say that force is the prime mover in a mechanical system because a net
force acting on an object changes the object’s motion. If the force acts in the
direction of the object’s velocity or in the opposite direction, the force does
work on the object. For example, when a tennis racket strikes a ball, it exerts
a force in the direction opposite the ball’s initial velocity. The racket does
work on the ball and changes its direction and speed. You can calculate the
work done on the tennis ball by using the force applied by the racket and the
distance the ball moves while being pushed by the racket.
A tennis racket exerts a force on a ball
and does work on the ball.
Forces are distributed over areas in fluid systems. The force per unit area is
pressure. Pressure (or pressure difference) is the prime mover in fluid
systems. If the pressure in a fluid is high enough, it can cause a boundary of
the fluid to move, doing work on the boundary. For example, in the power
stroke of an internal-combustion engine, an electric spark ignites the fuel-air
mixture in a cylinder. The mixture burns rapidly, and the gas pressure inside
the cylinder increases by as much as 600 psi. This pressure acting over the
surface of the piston forces the piston to move down the cylinder. During the
power stroke, the expanding gas does work on the piston. You can use the
pressure and the volume to calculate the work done by the gas.
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C HAPTER 2
WORK
In the power stroke of an
internal-combustion engine,
a fuel-air mixture burns
rapidly in a cylinder. The hot,
high-pressure gas expands
and does work on a piston.
In an electrical system, charges (electrons) move through a circuit because
coulomb forces are exerted on the charges. Coulomb forces are caused by
the presence of other charges. These other charges create an electric field
and a potential difference. Potential difference is the prime mover in
electrical systems. In the circuit below, a current (or flow of charge) drives a
DC motor for a model boat. The source of the potential difference that
moves the charge is a 6-V battery. The battery does work in moving the
charge. You can use the potential difference and the amount of charge moved
to calculate the work done by the battery.
In a circuit for a model boat motor, a
battery does work in moving charge
through a potential difference.
C HAPTER 2
WORK
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