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Work is the energy transferred when a force makes an object move
In order for work to take place, a force must be exerted through a
distance.
The amount of work done depends on two things:
• the amount of force exerted and the distance over which the force is
applied.
There are two factors to keep in mind when deciding when work is
being done:
• something has to move and the motion must be in the direction of
the applied force.
Work can be calculated by using the following formula: Work=force x
distance
Work Formula
• Work can be calculated by using the
following formula: Work=force x distance
Work is done on the
books when they are
being lifted, but no work
is done on them when
they are being held or
carried horizontally.
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Power
Power is the amount of work
done in one second
Power Formula
• Power(in watts)=Work(in joules)/time(in
seconds)
– P = W/t
• Power(in watts)=energy transfer(in
joules)/time(in seconds)
– P = E/t
Machine
• A machine is a device that makes doing
work easier
Work and Machines
• Machines make doing work easier
• Machines increase the force that can be
applied to an object.
– Example: A screwdriver increases the force
you apply to a screw.
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• Machines can also increase the
distance over which a force is
applied
– Example: a leaf rake
Mechanical advantage
• The force that is applied to the machine is
the input force (F in)
• The force thatis applied by the machine is
called the output force (F out)
• Machines can make work easier by
changing the direction of an applied
force
– Example: A pulley changes a
downward force to an upward
force
Mechanical advantage
• The ratio of output force to the input
force of a machine is called mechanical
advantage
• ***Machines make the OUTPUT force
greater than the INPUT, making work
easier
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Ideal Machines
• Theoretical machine where there is no
friction
• W in = Wout
Efficiency
• Efficiency is a measure of how
much of the work put into a
machine is changed into useful
output work by the machine
– Machines can be made more efficient
by reducing friction
Mechanical Advantage
Formula
• MA = Output force (in Newtons)
/ Input force (in Newtons)
• MA = Fout/Fin
Efficiency Formula
• efficiency(%) = output work(in
joules/input work(in joules)
x100%
• efficiency = Wout/Win x 100%
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Simple Machines
• A simple machine is a machine that
does work with only one
movement of the machine
First-Class Lever
The fulcum is
between the
input force
and the output
force
-a screwdriver
is an example
Levers
A lever is a bar
that is free to
pivot or turn
around a fixed
point called a
fulcrum
Third-Class Lever
The input force
is between the
fulcrum and the
output force
-a bat is an
example
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Ideal mechanical advantage of a
lever
• IMA = length of input arm (m)/length of the
output arm(m)
• IMA = L in/L out
Fixed pulleys
• The pulley is
attached to
something that
does not move
• Changes only the
direction of force
• IMA is 1
Pulley
• A pulley is a
grooved wheel
with a rope, chain
or cable running
along the groove
Movable Pulleys
• A pulley in which one
end of the rope is
fixed and the wheel is
free to move is called
a movable pulley
• Multiplies the force
• IMA is 2
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Block and Tackle
• a system of pulleys
consisting of fixed and
movable pulleys is
called a block and
tackle
• IMA is equal to the
number of rope
segments that support
the weight
IMA of a wheel and axle
• IMA = radius of wheel (m)/radius of
axle (m)
• IMA = rw/ra
Wheel and Axle
• A wheel and axle is a
simple machine consisting
of a shaft or axle attached to
the center of a larger wheel,
so that the wheel and axle
rotate together
• Doorknobs, screwdrivers,
and faucet handles are
examples
• Usually the input force is
applied to the wheel and the
output force is exerted by
the axle
Gears
• A gear is a wheel and axle
with the wheel having
teeth around its rim
• When the teeth of two
gears interlock, the
turning of one gear causes
the other gear to turn
• When gears of different
sizes are interlocked they
rotate at different rates
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Inclined Planes
• An inclined plane is a
sloping surface, such as
a ramp, that reduces the
amount of force required
to do work.
Screw
• A screw is an inclined
plane wrapped in a
spiral around a
cylindrical post
• IMA of a screw is
related to the spacing
of the threads
– IMA is larger, when
the threads are closer
together
IMA of an inclined plane
• IMA = length of slope (m)/height of slope
(m)
• IMA = I/h
Wedge
• A wedge is an
inclined plane
with one or two
sloping sides
• Changes the
direction of the
input force
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Compound machines
• Two or more
simple
machines that
work together
form a
compound
machine
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