Work and Machines
Machines make work easier
Machine –
n  a
device that helps do work by
either changing the size (amount or
distance)of a force or changing the
direction of the force
Machines DO NOT DO MORE WORK
Total work does not change
n 
n 
WORK AND FORCE ARE NOT THE SAME THING!
Work = Force X Distance
Force and distance have an inverse (opposite)
relationship
n  If a machine allows you to apply a smaller
force the distance will increase
n  If a machine reduces the distance the force
will increase
n  If a machine allows you to apply a force in a
different direction it doesn’t change how
much force you have to apply or how far you
move it
Making work easier
n  Machines
n 
Change the amount of force you exert and the
distance the force is exerted
output
force
Input
force
10 N
5N
n  Do
lessen the amount of work
done on the object, it makes it easier
for the person to do the work
1
n 
Making work easier
Work Input
Machines
n  The
n 
n  W
Change the direction in which you exert
force
50N , 3 m
Input
work done on a machine
=Fxd
force
50N , 3 m
n 
Output
force
They do lessen the amount of work done,it
makes it easier for the person to do the work
Work Output
Mechanical advantage
n  The
n 
n  W
work done by a machine
=Fxd
n 
n 
n 
n 
The number of times a machine multiplies the
force
Mechanical ad. = Output force
Input force
If M.A. > 1 the machine output force is greater
than the input force – helpful for lifting heavy
objects
If M.A. < 1 the machine reduces output force but
distance increases (fishing pole)
If MA = 1 the direction of the force changes
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Efficiency
n 
Efficiency compares the output work to
the input work (%)
Calculating Mechanical
Advantage Problem
n  A
mover uses a handcart to lift a
refrigerator. He applies an input
force of 50 N. The handcart applies
an output force of 350N on the
refrigerator. What is the mechanical
advantage of the handcart?
Cannot be greater than 100%(no ideal
machine)
n  Equation = work output/work input X 100
n 
Write the equation
Fill in the values from the
problem
n  MA
n 
= Output force
Input force
MA = 350 N
50 N
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Solve the problem
n 
MA = 350 N
50 N
Machines are not 100%
efficient because of friction
n 
Friction
•  A force that opposes motion – will slow machines
down
•  Occurs when 2 surfaces touch or rub against each
other
MA = 7
n 
The output force of the handcart is 7 times the
input force of the man, but the distance of the
output force is only 1/7 of the distance of the
input force, so the work stays the same
•  May cause heating of machine
The less work that the machine has to do to
overcome friction, the more efficient it is
n  Work output is always less than work input
n 
•  No work is lost
•  Work output + work to overcome friction = work input
There can’t be an ideal
machine
Mechanical Efficiency
n  An
n  Calculated
ideal machine would have 100%
efficiency
n  Since any time 2 surfaces rub
together there is friction, this is
impossible
n  Technologies exist to increase
efficiency by decreasing friction
n  Ideas?
by:
n  Work input/work output X 100
n  Tells you the % of work that the
machine does compared to the
amount of work in
4
Increasing Efficiency
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