6.4. Power http://www.ck12.org
6.4 Power
Objectives
The student will:
- Understand how power is defined in physics
- Be able to solve problems involving power
Vocabulary
- power:The rate at which work is done.
Introduction
In the language of physics, work is defined as force applied over a distance(W=Fâx). No mechanical work is
done when holding a heavy weight for a long period of time. You may grow tired but no mechanical work is done
unless there is a displacement.
Poweris defined as the rate at which work is done. The faster work is performed, the more power is delivered.
The average power is found by dividing the work done (or energy used or energy produced) by the time it takes to
perform the work:Pavg=Wt. If the rate at which the work is done is constant, thenP, of course, is constant.
Power has standard metric units of Joules per second, called Watts(W). These units are named for the Scottish
engineer James Watt (1736-1819), picture in theFigure6.23, who, among many other achievements, perfected the
steam engine.
Check Your Understanding
- In the example above, what power is required for the first lift and for the second lift?
Answer:
First li f t: Pavg=W
t=
F x
t=
( 2500 )( 2. 40 )
3. 00
= 2000 W
Second li f t:Pavg=W
t=
F x
t=
( 2500 )( 2. 40 )
6. 00
= 1000 W
- A weightlifter lifts a barbell a heighthover a period of timet. If he lifts the same weights to the same height over
a period of time 2t, which lift requires the greater power?
Answer:Since the first lift required only half the time of the second lift, the first lift required twice the power of the
second lift. (See 1 above.)
It is also useful to re-express the equation for power in terms of average speed and constant force. Since power is
defined asPavg=Wt =F xt we can also writeF xt asFxtwherextis average speed andFis constant. Thus:Pavg=F v.