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Notes 14-1 “Work and
Power”
I. Work
A. Work is done only when a force moves an
object.
B. If you push, pull, lift, or throw an object
you are doing work.
C. A force acting through a distance is work.
D. Work is not done every time a force is
applied.
a. a force can be applied without work
being done.
Ex) If you push against a wall as hard as you can
for 1 minute straight are you tired? Have you
done work?
E. If the force being applied is in a direction
different from the direction of motion no
work is done.
F. the direction of movement must be the
same as the direction of force applied for
work to be done.
Work = Force x Distance
G. The force needed to lift an object up must
be equal to the force pulling down on the
object (its weight)
Ex) How much work must be done to lift a 20 kg
object 10 m if it takes a force of 220 N? What is
the weight of this object?
Work = F x D
Work = 220 N x 10 m = 2200 J
Weight = m x g
Weight = 196 N
Weight = 20 kg x 9.8 m/s/s
Ex) How much force is needed to move an object
225 cm if the amount of work done is 880 J?
225 cm x 1 m
= 2.25 m
100 cm
Work = F x D
F = 391.1 N
880 J = F x 2.25 m
2.25 m
2.25 m
II. Power
A. Like speed, acceleration, and velocity
power tells you how fast something is
happening.
B. Power is the rate at which work is done,
or the amount of work per unit of time.
P= W
T
C. Units are equal to Joules/sec or Watts.
a. 1 watt = 1 J/s
Ex) A 50 watt light bulb does 50 J of work every
second.
D. Large quantities of power are measured in
kilowatts (kw)
a. 1 kw = 1000 watts
Ex) How much power is used when a bulldozer
moves dirt with a force of 820N, over a distance
of 5 m, and it takes the bulldozer 70 s to do the
job?
III. Horsepower
A. 1 hp = 746 watts.
B. Horses were used as the comparison for
power output since they were the most
commonly used source of power in the
1700’s.
C. A 10 hp motor produces as much power
as 10 strong horses.