Mechanical Advantage & Efficiency

• A box of San Francisco 49ers jerseys is pushed
with 80 Newtons of force for 5 meters up a
ramp into a truck with a height of 1 meter to
be taken to the thrift store. How much work
did the person do on the box?
Mechanical Advantage &
•How do machines do work for us?
•We can compare the amount of force put
into a machine with the amount of force it
•This ratio is a machine’s mechanical
Mechanical advantage =
(output force)
(input force)
•Can openers are simple
machines that multiply the
amount of force we input into
opening a can
•If you use 5 N of force to open a
can, and the opener exerts 15 N
of force, what is the opener’s
mechanical advantage?
Output Force
Input Force
15N/3N = 3
•Your force is tripled!
• A lever is used to lift a heavy rock. The
mechanical advantage of the lever is 4 and the
lever applies a force of 800 N to the rock.
What is the force applied to the lever?
Consider this pulley:
•If only the direction of
force is changed, what is
the mechanical
• 1/1 = 1
•No force is multiplied,
but the work is easier.
In elevators…
Here is where
the people go
• When using a pulley with a mechanical
advantage of 1.5, a worker applies 250 N to
the pulley. The pulley is used to lift a heavy
crate. How much force does the pulley apply
to the crate?
• A 100 N force is applied to a machine to lift an
object with a mass of 45 kg. What is the
machine’s mechanical advantage?
• What comes to mind?
• Efficiency: the percentage of work input used
in work output
• In other words…
Efficiency = (work output)
(work input)
x 100%
The Internal Combustion Engine
15%-25% Efficient
Where does the rest of the energy go?
Resisting drag
Overcoming friction
Powering other parts of the car
• A tank of gas contains 970 kJ of energy. If the
engine’s work output is 213 kJ, what is the
efficiency? If it costs $44.00 to fill up the tank,
how much of the cost is lost to inefficiency?
• If a compact fluorescent light bulb uses 75
Joules of energy to produce 12 Joules of work,
how much more efficient is it than an
incandescent light bulb that uses 130 Joules to
do the same?
• A 100 Newton force is applied over 10 meters,
and raises a 500 Newton object 1.5 meters.
What is the machine’s efficiency?

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