Chapter 8
Work and Machines
Work is done when an object is
moved through a distance in the
direction of the applied force.
examples: -lifting book bag
-pushing chair back
NO work: -holding up a brick
-“piggy-back” ride
- pushing a wall
The amount of work done can
be calculated:
Work = force distance
W=Fd
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Gilligan bench pressed a
weight set. He applied a 70 N
force on the bar and lifted it
0.25 meters. How much work
did he do?
W=Fd
W = 70 N x 0.25 m
W = 17.5 Nm
Or
17.5 J
The unit for work is the joule (J)
1 joule (J) = 1 N m
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Practice:
How much work does it take
for you to go up steps that are
3 meters high?
W=Fd
W = _______ N 3 m
W = _____ Nm
or
W = _____ J
(practice problems)
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Power is the rate at which
energy is transferred.
To calculate power, divide the
amount of work done by the time
it takes to do that work:
P=
The watt (W) is a unit used to
express power.
1 watt (W) = 1 J/s
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Power is the amount of work done
in a unit of time, or
the rate of work.
Or, since work is force times
distance;
Power = Force
distance
time
Examples:
1. If it takes you 10 s to do 150
J of work on a box to move it
up a ramp, what is your
power output?
P=w/t
P=150 J / 10s
P=15 J/s OR 15 W
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2. What is the power of a
machine that applies a 50 N force
for 3 m in 5 seconds?
P=F d / t
P=50 N 3m / 5s
P=30 Nm/s OR J/s OR W
Power is also the rate energy is
converted from one form to
another.
 A 100 watt light bulb
converts 100 joules of
electrical energy in 1
second to thermal and
electromagnetic (light)
energy.
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Horsepower is an older unit for
describing power (car engines).
 It is NOT an SI unit!
A machine is a device that
makes work easier by changing
the size or direction of a force.
A machine makes work easier by:
 change amount of force you
exert;
 change the distance you exert
a force;
 change direction of your force.
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The force you exert on a machine
is called input force (or effort).
The force exerted by a machine is
called output force (or resistance).
The mechanical advantage is
the number of times the
machine multiplies force.
Mechanical Advantage = output force
input force
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A machine’s efficiency is a
comparison of the work output
with the work input.
Efficiency =
x 100%
What is the efficiency of a
machine whose work input is
100 J and work output is 30 J?
work output
Efficiency = work input
100%
Efficiency = 30 J / 100 J
100%
Efficiency = 30 %
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Types of simple machines:
1. inclined plane
2. wedge
3. screw
4. lever
5. wheel and axle
6. pulley
1. An inclined plane is a flat
slanted surface.
a. The input force is equal to
the pushing or pulling force
on an object.
b. The output force is equal
to the weight of the object.
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c. Inclined plane allows you
to exert the input force
over a greater distance.
mechanical advantage = length of incline
height of incline
If the length of an inclined plane
doubles, but the height is
unchanged, the mechanical
advantage doubles.
Examples: ramp, handicap ramp,
stairs, hill, ladder,
sink basin
2. A wedge is a pair of
inclined planes that move.
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a. A wedge applies an
output force that is
greater than the input
force, but over a greater
distance.
Mechanical advantage = length of wedge
greatest thickness
b. Examples: knife, axe,
zipper, plow, nail
3. A screw is an inclined plane
wrapped around a cylinder.
As the screw turns, a small
force is applied over a long
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distance (over the inclined
plane of the screw).
a. The screw applies a large
force through the short
distance it is pushed.
b. Fine threads have a long
inclined plane and can apply
a great force.
Examples: nut, bolt, screw,
jar lids, faucet valve
4. A lever is a bar that pivots at
a fixed point called a
fulcrum.
a. First class lever- the
fulcrum is between the input
force and the load.
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Changes direction of
input force.
examples: see-saw
pry bar, oar, balance
b. Second class lever- the
load is between the input
force and the fulcrum.
examples: wheel barrow,
bottle opener
c. Third class lever- input
force is between the load
and the fulcrum.
Output force always less
than input force.
examples: elbow, broom
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mechanical advantage of
a lever:
d.
distance to input force
MA = distance to output force
5. A wheel and axle consists
of two circular or
cylindrical objects fastened
together and rotate on a
common axis.
a. The wheel has a large
diameter, and the axle
has smaller diameter.
b. Mechanical advantage:
radius of wheel
MA = radius of axle
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examples:
wheels,
screw driver, door knob
6. A pulley is a grooved wheel
with a rope wrapped around
it.
a. A Fixed pulley is attached
to a structure. It changes
direction of force, NOT
amount of force
b. A movable pulley is
attached to the object you
wish to move. It changes
amount of force.
c. Count the number of ropes
supporting an object for the
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mechanical advantage (see
pg. 224).
A compound machine is made
of two or more simple
machines.
block and tackle (2 or
more pulleys),
gears (a modified wheel and axle –
Examples:
work together),
can opener, scissors
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