Work and Machines
Machines make work easier (lessen
the force)
Machines DO NOT DO MORE WORK
(less work out of a machine than in
due to friction)
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
Machine –
n  a
device that helps do work by
either:
n  changing the size (amount) of force
n  distance of a force
n  changing the direction of the force
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
NOT
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 NOT lessen the amount of work
done,it changes the direction of the force
applied
Work Output
Mechanical advantage
n  The
n 
n  W
work done by a machine
=Fxd
n 
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
MA tells you how much easier the machine
makes the work!
(also can be the force of the load you are lifting)
2
Calculating Mechanical
Advantage Problem
Write the equation
n  A
n  MA
Fill in the values from the
problem
Solve the problem
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?
n 
MA = 350 N
50 N
n 
= Output force
Input force
MA = 350 N
50 N
MA = 7
n 
n 
The output force of the handcart is 7 times the
input force of the man
It is 7 times easier for the man to do the work
with the cart
3
Efficiency
n 
Efficiency compares the output work to
the input work (%)
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
Cannot be greater than 100%(no ideal
machine)
n  Equation = work output/work input X 100
n 
•  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 output/work input X 100
n  Tells you the % of work that the
machine does compared to the
amount of work in
4
Increasing Efficiency
5