Simple Machines
Machines
Machines
•
• Have few or no moving
parts
• Make work easier
• Can be combined to
create complex
machines
• Six simple machines:
Lever, Inclined Plane,
Wheel and Axle, Screw,
Wedge, Pulley
Machines Make Work Easier by:
Make work easier by:
1) decreasing force by increasing
distance
2) increasing force by decreasing
distance
3) force and distance stay the same but
the direction is different.
Mechanical Advantage
• We know that a machine multiplies whatever
force you put into it:
- Using a screwdriver to turn a screw
- Twisting a nail with pliers
- Carrying a box up a ramp instead
of stairs
• The amount that the machine multiplies that
force is the mechanical advantage of the
machine
• Abbreviated MA
Mechanical Advantage
• (IMA) Ideal MA: This is the MA of a
machine in a world with no friction, and
no force is lost anywhere
• (AMA) Actual MA: This is simply the MA
of a machine in the world as we know it
- Force is lost due to friction
- Force is lost due to wind, etc.
• Can we have an ideal machine?
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Mechanical Advantage
MA = output force
input force
IMA = input distance
output distance
Mechanical advantage is a ratio so
there is no unit.
Inclined Planes
• A slope or ramp that
goes from a lower to
higher level
• Makes work easier by
taking less force to
lift something a
certain distance
• Trade off: the
distance the load
must be moved would
be greater than
simply lifting it
straight up
Wedge
• An inclined plane
on its side
• Used to cut or force
material apart
• Often used to split
lumber, hold cars
in place, or hold
materials together
(nails)
Efficiency
• The efficiency is a ratio that measures how much
work the machine produces versus
how much work goes in
Efficiency =
Work Output
X 100%
Work Input
Efficiency =
Actual MA
X 100%
Ideal MA
• Example: We have an inclined plane
with an ideal MA of 3. We measure
our real-life inclined plane and find
an MA of 2.
Efficiency = Actual MA/Ideal MA x 100%
= (2/3) X 100%
= 66.66%
Mechanical Advantage: Inclined Plane
• The mechanical
advantage of an
inclined plane is the
length of the slope
divided by the height
of the plane, if effort is
applied parallel to the
slope
MA =
Length of Slope
Height of Plane
• So for our plane
MA = 15 feet/3 feet
=5
• Let’s say S = 15 feet, H =
3 feet
Mechanical Advantage: Wedge
• Much like the
inclined plane, the
mechanical
advantage of a wedge
is the length of the
slope divided by the
width of the widest
end
MA =
2"
6"
Length of Slope
Thickness of Widest End
• So for our wedge,
MA = 6”/2” = 3
• They are one of the
least efficient simple
machines
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Screw
• An inclined
plane wrapped
around a rod or
cylinder
• Used to lift
materials or
bind things
together
Wheel and Axle
• A larger circular wheel
affixed to a smaller
rigid rod at its center
• Used to translate force
across horizontal
distances (wheels on a
wagon) or to make
rotations easier (a
doorknob)
• Trade off: the wheel
must be rotated
through a greater
distance than the axle
Pulley
• A rope or chain free to
turn around a
suspended wheel
• By pulling down on the
rope, a load can be
lifted with less force
• Trade off: no real trade
off here; the secret is
that the pulley lets you
work with gravity so
you add the force of
your own weight to the
rope
Mechanical Advantage: Screw
• The Mechanical
advantage of a screw
is the circumference
of the screwdriver
divided by the pitch
of the screw
• The pitch of the
screw is the number
of threads per inch
MA =
•
Diam.=1"
Circumference of Screwdriver
Pitch of Screw
So for our
screwdriver
MA = 3.14”/0.1”
= 31.4
10 threads
per inch
Circumference = ∏ x 1” =
3.14”
Pitch = 1/10” = 0.1”
Mechanical Advantage: Wheel and Axle
• The mechanical
advantage of a wheel
and axle system is
the radius of the
wheel divided by the
radius of the axle
Radius of Wheel
MA = Radius of Axle
2"
10"
• So for our wheel and
axle MA = 10”/2” = 5
Mechanical Advantage: Pulley
• The Mechanical
Advantage of a pulley
is equal to the
number of ropes
supporting the pulley
• So for the pulley
system shown there
are 3 ropes
supporting the
bottom pulley
MA = 3
• This means that if
you pull with a force
of 20 pounds you will
lift an object weighing
60 pounds
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Lever
• A rigid board or rod
combined with a
fulcrum and effort
• By varying position
of load and fulcrum,
load can be lifted or
moved with less
force
• Trade off: must
move lever large
distance to move
load small distance
• There are 3 types of
levers
2nd Class Lever
• The fulcrum is
located between
the effort and the
load
• Direction of force
always changes
• Examples are
scissors, pliers,
and crowbars
3rd Class Lever
• The effort is
located between
the fulcrum and
the resistance
• Direction of force
does not change,
but a gain in
speed always
happens
• Examples include
ice tongs, tweezers
and shovels
• The resistance is
located between
the fulcrum and
the effort
• Direction of force
does not change
• Examples include
bottle openers and
wheelbarrows
Mechanical Advantage: Lever
• The mechanical
advantage of a lever
is the distance from
the effort to the
fulcrum divided by
the distance from the
fulcrum to the load
Distance, effort - fulcrum
MA = Distance, load - fulcrum
• For our example,
MA = 10/5 = 2
1st Class Lever
• Distance from effort to fulcrum:
10 feet
• Distance from load to fulcrum:
5 feet
The trick is WORK
• Simple machines change the amount of
force needed, but they do not change the
amount of work done
• What is work?
• Work equals force times distance
• W=Fxd
• By increasing the distance, you can
decrease the force and still do the same
amount of work
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Examples:
• Lever:
• Work is equal on both
sides of a lever. You
move the long end a
LARGE distance with
SMALL force. The other
end moves a SMALL
distance with a LARGE
force, which is why it
•Inclined Plane:
can lift heavy objects.
•It takes a certain amount of
work to get the cabinet into the
truck. You can either exert a
LARGE force to lift it the
SMALL distance into the truck,
or you can exert a SMALL force
to move it a LARGE distance
along the ramp.
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