Chapter 5: Work and Machines
Compare the effort exerted
by a backpacker moving over
level ground to that exerted
by a backpacker moving
uphill.
 How do you think the weight
of the backpack affects the
amount of force needed to
move it?

 Explain the meaning of work.
 Describe how work and energy are
related.
 Calculate work.
 Calculate power.
 Work: transfer of
energy that occurs
when a force makes an
object move
 For work to occur, two conditions must
be met:
1. an object must move
2. the motion of the object must be in the
same direction as the applied force on
the object.
 Work and energy are related.
 Energy is always transferred
from the object doing the
work to the object on which
the work is being done.
 Work is done on an object only when a
force is being applied to the object
and the object moves.
 If the object does not move, then no work
is being done!
 Work equals force times distance
 W = Fd
 W  work, measured in joules (J)
 F  force, measured in newtons (N)
 d  distance, measured in meters (m)
 W = Fd
 F = W/d
 d = W/F

You push a refrigerator with a force of 100 N. If
you move the refrigerator a distance of 5 m while
you are pushing, how much work are you doing?

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?

The brakes on a car do 240,000 J of work in
stopping the car. If the car travels a distance of 50
m while the brakes are being applied, what is the
force the brakes exert on the car?

Suppose you give a book a push and it slides along
a table for a distance of 1 m before coming to a
stop.
 Work is only being done to the book when your hand is
in contact with it!
 You would not use 1 m in the equation, you would use
the distance the book moved while your hand was
touching it.
 Power: amount of work done in a
certain amount of time
 rate at which work is done
 Power equals work divided by time.
 P = W/t
 P  power, measured in watts (W)
 W  work, measured in joules (J)
 t  time, measured in seconds (s)
 P = W/t
 W = Pt
 t = W/P

You do 900 J of work in pushing a sofa. If it took 5
s to move the sofa, how much power did you
use?

If a runner’s power is 130 W as she runs, how
much work is done by the runner in 10 minutes?

The power produced by an electric motor is 500
W. How long will it take the motor to do 10,000 J
of work?
 Doing work is a way of transferring
energy from one object to another.
 Power is the rate at which work is done
 Power is also the rate at which energy is
transferred
 Power equals energy transferred divided
by time.
 P = E/t
 P  power, measured in watts (W)
 E  energy transferred, measured in joules (J)
 t  time, measured in seconds (s)
 P = E/t
 E = Pt
 t = E/P
 A color TV uses 120 W of power. How
much energy does the TV use in 1 hr?
 Describe a situation in which a force is
applied, but no work is done.