Work, Power, and
Machines
Chapter 14
Work & Power
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Section 14-1
Work
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Work – the product of force and distance
Work = Force x Distance
For a force to do work on an object, some of
the force must act in the same direction as the
object moves. If there is no movement, no
work is done.
Any part of a force that does not act in the
direction of motion does no work on an
object.
Joules
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A joule is the SI unit for work
When a force of 1 newton moves an object 1
meter in the direction of the force, 1 joule of
work is done.
Calculating Work
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Your family is moving to a new house. While
you lift a box straight up 1.5 meters to put it
on the moving truck, you exert an upward
force of 200 N. How much work was done?
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Work = Force x Distance
Work = 200 N x 1.5 meters = 300 N·m
Work = 300 Joules
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Power
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Doing work at a faster rate requires more power.
To increase power, you can increase the amount of
work done in a given time, or you can do a given
amount of work in less time.
The SI unit for power is the watt. (Joules/sec)
Work
Power = Time

Horsepower – one horsepower is equal to about 746
watts
Calculating Power
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If it took you exactly 1 second to lift the box into the
moving truck from the last problem, how much
power was required?

Power = work/time
Power = 200 N x 1.5 m /1 sec
Power = 300 Joules/1 sec
Power = 300 Joules/sec
Power = 300 Watts
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Try one…
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You lift a book from the floor to a bookshelf 1.0 m
above the ground. How much power is used if the
upward force is 15.0 N and you do the work in 2.0
seconds?

Power = Work/Time
Power = (15.0 N x 1.0 m)/ 2.0 sec
Power = 15.0 Joules/2.0 sec
Power = 7.5 Watts
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Work & Machines
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Section 14-2
Machines Do Work
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Machines make work easier to do. They
change the size of a force needed, the
direction of a force, or the distance over
which a force acts.
Because of friction, the work done by a
machine is always less than the work done on
the machine.
Work Input to a Machine
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Input force – the force you exert on a
machine.
Input distance –the distance the input force
acts through
Work input – the work done by the input force
acting through the input distance
Work Output of a Machine
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Output force – the force that is exerted by a
machine
Output distance the distance the output force
is exerted through
Work output – Output force x Output distance
Mechanical Advantage and Efficiency
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Section 14-3
Mechanical Advantage
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Mechanical advantage is the number of times
that the machine increases an input force.
Actual mechanical advantage – the ratio of
the output force to the input force
Actual mechanical advantage =
Output Force
Input Force
Try one…
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You test a machine and find that it exerts a
force of 5N for each 1 N of force you exert
operating the machine. What is the actual
mechanical advantage?
Actual mechanical advantage =
 AMA = 5 N ÷ 1 N
 AMA = 5
Output Force
Input Force
Ideal Mechanical Advantage
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IMA – the mechanical advantage in the
absence of friction
Because friction is always present, the actual
mechanical advantage of a machine is always
less than the ideal mechanical advantage.
Input distance
Ideal mechanical advantage = Output distance
Try one…
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A woman drives her car up onto wheel ramps
to perform some repairs. I she drives a
distance of 1.8 meters along the ramp to raise
the care 0.3 meter, what is the ideal
mechanical advantage of the wheel ramp?
Input distance
IMA = Output
distance
IMA = 1.8 meters ÷ 0.3 meters
IMA = 6
Efficiency
The percentage of the work input that
becomes work output
 Because there is always some friction, the
efficiency of any machine is always less than
100 percent.
Work output
Efficiency = Work input x 100%
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Try one…
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You have just designed a machine that uses
1000 J of work from a motor for every 800 J
of useful work the machine supplies. What is
the efficiency of the machine.
Work output
Efficiency = Work input x 100%
Efficiency = (800 J ÷ 1000 J) x 100%
Efficiency = 0.80 x 100%
Efficiency = 80%
Simple Machines
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Section 14-4
Simple Machines
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The six types of simple machines are the
lever, the wheel and axle the inclined plane,
the wedge, the screw, and the pulley.
Levers
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A rigid bar that is free to move around a fixed
point.
Fulcrum - the fixed point the bar rotates
around
Input arm – the distance between the input
force and the fulcrum
Output arm – the distance between the output
force and the fulcrum
Levers
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To calculate the ideal mechanical advantage
of any lever, divide the input arm by the
output arm.
First class- screwdriver used to open a paint
can
Second class – wheelbarrow
Third class - broom
Wheel and Axle
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Consists of two disks or cylinders, each one
with a different radius
To calculate the ideal mechanical advantage
of the wheel and axle, divide the radius (or
diameter) where the input force is exerted by
the radius (or diameter) where the output
force is exerted.
Inclined Planes
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A slanted surface along which a force moves
an object to a different elevation.
The ideal mechanical advantage of an inclined
plane is the distance along the inclined plane
divided by its change in height.
Wedges
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A V-shaped object whose sides are two
inclined planes sloped toward each other
A thin wedge of a given length has a greater
ideal mechanical advantage than a thick
wedge of the same length
Screws
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An inclined plane wrapped around a cylinder
Screws with threads that are closer together
have a greater ideal mechanical advantage.
Pulleys
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A simple machine that consists of a rope that
fits into a groove in a wheel.
The ideal mechanical advantage of a pulley or
pulley system is equal to the number of rope
sections supporting the load being lifted.
Three types – fixed pulley, movable pulley
and pulley system
Compound Machines
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Compound machine – a combination of two
or more simple machines that operate
together.
Examples – car, washing machine, clock