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Section 5–4: Power
Coach Kelsoe
Pages 179–181
• Relate the concepts of energy,
time, and power.
• Calculate power in two different
• Explain the effect of machines on
work and power.
Rate of Energy Transfer
• The rate at which work is done is
called power.
• Formally, power is defined as is a
quantity that measures the rate at
which work is done or energy is
• The formula for power can be
written in two ways:
– P = W/Δt
• P = power, W = work, t = time
– P = Fv
• P = power, F = force, v = velocity
Rate of Energy Transfer
• Keep in mind that W = Fd, so
power then can be solved from the
equation P = Fd/Δt.
• The SI unit of power is the watt,
abbreviated with the letter W. It is
defined as 1 joule per second.
• The horsepower, hp, is another
unit of power. One horsepower is
equal to 746 W.
Sample Problem
• Power
A 193 kg curtain needs to be raised 7.5
m, at constant speed, in as close to
5.0 s as possible. The power ratings
for three motors are listed as 1.0 kW,
3.5 kW, and 5.5 kW. Which motor is
best for the job?
Sample Problem Solution
• 1. Identify givens and unknowns
– m = 193 kg
– Δt = 5.0 s
– d = 7.5 m
Sample Problem Solution
• 2. Choose the correct equation
– We know that P = W/Δt, but since
W = Fd and F = mgd, we can say
that P = mgd/Δt
Sample Problem Solution
• 3. Calculate
– P = mgd/Δt
– P = (193 kg)(9.81 m/s2)(7.5 m)/5.0 s
– P = 2800 W or 2.8 kW
– So the BEST motor to use would be
the 3.5 kW motor because the 5.5
kW motor would lift the curtain too
fast and the 1.0 kW motor wouldn’t
be fast enough.
• Power