11/5/2008
Machines: Machines make work easier but
do not change the amount of work that is
done. Machines either change the amount of
force you exert, the distance over which you
exert your force, or change the direction in
which you exert your force.
Simple Machines
Calculating Efficiency and
Mechanical Advantage
►Multiplying force: Less force is required but it must be
applied over a longer distance.
► Multiplying distance: Machine allows you to work over
a shorter distance but more force is required
► Changing direction: This only changes direction not
the force required or distance applied.
Inclined plane
Does it change the direction of
applied force?
What is the advantage to using
an inclined plane (How does it
affect the work equation)?
Lever
Does it change the direction of
applied force?
What is the advantage to using a
lever (How does it affect the work
equation)?
Wedge
Does it change the direction of
applied force?
What is the advantage to using
a wedge (How does it affect the
work equation)?
Wheel and axle
Does it change the direction of
applied force?
What is the advantage to using a
wheel and axle (How does it
affect the work equation)?
Work Output - Amount of work done by a machine on the
load (weight) is called output of the
machine.
Screw
Does it change the direction of
applied force?
What is the advantage to using
a screw (How does it affect the
work equation)?
Pulley
Does it change the direction of
applied force?
What is the advantage to using a
pulley (How does it affect the
work equation)?
An ideal machine is a hypothetical machine
whose output is equal to its input.
Amount of work done on a machine by a
Work Input - given effort (force) is called input of a
machine.
Keep in Mind:
Work Output is always less than Work Input. (FRICTION!)
Practice
Efficiency - The ratio of useful work output
of a machine to the input of
machine.
efficiency
Wout
Win
100%
MAactual
MAtheoretica l
You do 222 J of work pushing a box up a ramp. If the ramp
does 200 J of work, what is the efficiency of the ramp?
100%
Efficiency of an ideal machine is 100% because there is
no loss of energy in an ideal machine due to friction or
any other means that can waste useful energy. Think
about it! In the real world, there is always friction;
efficiency can never exceed 100%.
You do 1200 J of work with gears. If the gears do 1000 J
of work, what is the efficiency of the gears?
A 500-newton cart is lifted to a height of 1 meter
using a 10-meter long ramp. The worker only has to use 50
newtons of force to pull the cart. What is the efficiency of
the ramp?
The closer a machine’s efficiency to 100% the better the machine.
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11/5/2008
mechanical advantage (MA) – ratio of output force to
input force.
Ideal Mechanical Advantage:
• Ignores effects of Friction.
• Can be calculated.
Actual Mechanical Advantage:
• Includes effects of Friction.
• Cannot be calculated.
• Must be Measured.
MA
Fout
Fin
Since mechanical advantage is the ratio of force
output to force input, the units cancel each other out.
Therefore, mechanical advantage is simply expressed
as a number.
The higher the number, the greater
the mechanical advantage.
Practice
A lever is used to lift a load weighing 200N. The
force needed was 600N. What is the mechanical
advantage of this first class lever?
A 500-newton cart is lifted to a height of 1 meter
using a 10-meter long ramp. Thee worker only has to
use 50 newtons of force to pull the cart. What is the
mechanical advantage of the ramp?
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