Chapter 5
Work
and
Machines
Work
What is work?
• Work is the transfer of energy that occurs when a force
makes an object move.
Work = Force × Distance
W = Fd
• Units for work: Newton•meter (N•m) or Joule (J)
In order to be working, two
conditions must be met:
1. A force must be applied.
2. The force must make an object move in the same
direction as the force.
• If the object does not move, then no work is
being done.
Work
Example:
• You push a refrigerator with a force of 100-N. If
you move the refrigerator a distance of 5-m, how
much work do you do?
Work = Force × Distance
Work = (100-N) × (5-m)
Work = 500-J
Work
Example:
• A force of 75-N is exerted on a 45-kg couch and
the couch is moved 5-m. How much work is done
in moving the couch?
Work = Force × Distance
Work = (75-N) × (5-m)
Work = 375-J
Work
Example:
• A lawn mower is pushed with a force of 80-N. If
12 000-J of work are done in mowing a lawn, what
is the total distance the lawn mower was pushed?
Work = Force × Distance
12000-J = 80-N × Distance
Distance = (12000-J) ÷ (80-N)
Distance = 150-m
Work
Example:
• The brakes on a car do 240 000-J of work in
stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the
total force the brakes exert on the car?
Work = Force × Distance
240000-J = Force × 50-m
Force = (240000-J) ÷ (50-m)
Force = 4800-N
Work
• Power is the rate at which work is done, or the amount of
work done per unit time.
Power = Work ÷ Time
P = W/T
• Units for power: N•m/sec or J/sec or Watt (W) or
kilowatts (kW) for large quantities.
How we measure work in the U.S.
• In the U.S., we measure power in another unit,
horsepower.
– Horsepower gets its name from “horses” since they were the
most common source of power in the 18th century.
• 1 horsepower (hp) = 745.56 watts
Work
Example:
• You do 900-J of work pushing a sofa. If it took 5seconds to move the sofa, what was your power?
Power = Work ÷ Time
Power = (900-J) ÷ (5-sec)
Power = 180-W
Work
Example:
• To lift a baby from a crib 50-J of work are done.
How much power is needed if the baby is lifted in
0.5-seconds?
Power = Work ÷ Time
Power = (50-J) ÷ (0.5-sec)
Power = 100-W
Work
Example:
• If a runner’s power is 130-W, how much work is
done by the runner in 10-minutes?
Power = Work ÷ Time
130-W = Work ÷ 600-sec
Work = (130-W) × (600-sec)
Work = 78000-J
Work
Example:
• The power produced by an electric motor is 500W. How long will it take the motor to do 10 000-J
of work?
Power = Work ÷ Time
500-W = 10000-J ÷ Time
500-W × Time = 10000-J
Time = (10000-J) ÷ (500-W)
Time = 20-sec
Work
• Recall that energy is the ability to cause a change, you
can also think that energy is the ability to do work.
• When you do work on an object, you increase its energy.
Work Continued…
• Energy is always transferred from the object that is doing
the work to the object on which the work is done.
• Power is also the rate at which energy is transferred.
Power = Energy Transferred ÷ Time
P = E/t
• Example:
– A light bulb: Energy is transferred from the circuit to the
lightbulb filament. The filament converts the electrical energy
into heat and light. The power used by the lightbulb is the
amount of electrical energy transferred to the lightbulb each
second.