1. Simple Machines
Making Work Easier

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Simple 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
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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
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4. 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
So for our plane
MA = 15 m/3 m = 5
Let’s say S = 15 meters, H = 3 meters
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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
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1st Class Lever
The fulcrum is located between the effort and the load
Direction of force always changes
Examples are scissors, pliers, and crowbars
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2nd Class Lever
The resistance is located between the fulcrum and the effort
Direction of force does not change
Examples include bottle openers and wheelbarrows
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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
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9. 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
For our example,
MA = 10/5 = 2
Distance from effort to fulcrum: meters
Distance from load to fulcrum: 5 meters
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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
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11. 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
So for our wheel and axle MA = 10”/2” = 5
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Screw
An inclined plane wrapped around a rod or cylinder
Used to lift materials or bind things together
Trade off: the distance (number of times you spin it) increases as effort decreases
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13. 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
So for our screwdriver
MA = 3.14”/0.1” = 31.4
Circumference = ∏ x 1” = 3.14”
Pitch = 1/10” = 0.1”
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Wedge
An inclined plane placed back to back
Used to cut or force material apart
Often used to split lumber, hold cars in place, or hold materials together (nails)
They are one of the least efficient simple machines
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15. 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
So for our wedge,
MA = 6”/2” = 3
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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
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17. Mechanical Advantage: Pulley
The Mechanical Advantage of a pulley is equal to the number of ropes supporting the resistance force(load).
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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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 = F x d
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 can lift heavy objects.
Inclined Plane:
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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Efficiency
The efficiency is a ratio that measures how much work the machine produces versus
how much work goes in
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%
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