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Simple Machines
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Types of Simple Machines:
levergear
rampwheel
pulley
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What Do Simple Machines Do?
Simple machines make it easier to do work.
They allow you to use less force – TO GET THE WORK DONE!
Example: Your body alone could never produce enough force to lift a car. A mechanical jack is a combination of simple machines that allows you to exert more force to
lift the car...
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Less Work?
Although you use less force, you usually need to move something more
distance.
Actually, you may end up doing more total work, but with less force it will be
easier or more manageable.
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It’s a trade off, you gain one thing, but lose another...
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Machine Input & Output
When describing many types of systems, the terms input and output are often used.
Input: refers to what you are doing.
Output: refers to what the machine is doing.
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Input/output could refer to many types of quantities.
Ex: You push on a lever with 100N of force
Finput = 100N
Ex: A pulley system lifts a boulder 1m up.
doutput = 1m
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Efficiency
One way to describe, rate, or quantify a machine is by finding its efficiency.
Efficiency is the ratio of output work/input work.
Output workEfficiency = ----------------- x 100
Input work
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Efficiency
WorkOutput
WorkInput
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Efficiency Example:
A man uses a combination of pulleys to
lift a 500N object. He pulls with a force of 70N, and reels in 10m of rope.
However, the weight itself only moves 1m
upward. What is the efficiency of the pulley
system?
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Solution:
W = F x d
(what you do!)Winput = (70N) ( 10m) = 700J
(what the machine does) Woutput = (500N) (1m) = 500J
efficiency = Woutput / Winput
eff = 500J / 700J = .71
71%
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Question:Q: The efficiency wasn’t 100%. Why not?
Where did the “missing” work go?
A: Some energy was “lost” due to friction. The energy is
“unrecoverable” - we can’t use it… It takes work to spin or rotate the pulleys. Air resistance is minimal.
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Ideal Mechanical Advantage:
dinput
IMA = ----------------
doutput
IMA is a ratio of input and output distances. It tells how much your effort is “multiplied” by
using the machine! This is without friction – Mr. C’s perfect Physics world!
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Actual Mechanical Advantage:
Foutput
AMA = ------------------
Finput
AMA is a ratio of output and input forces.
The AMA will never be greater than the IMA because of friction.
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Example:
Using the information from the previous pulley example, find the AMA and IMA.
AMA = Foutput/Finput
= 500N / 70N = 7.14
IMA = dinput/doutput
=10m / 1m =10
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The Ideal LeverA car jack is used to lift a car. The force on the car is 4500 N. The force applied by the lifter is
just 300 N. Each time the lifter presses the lever down, they move their end of the lever a distance
of 0.8 m. If this lever is ideal, how far does the car rise for each press of the lever?
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The Ideal LeverSince the lever is ideal, we won’t lose any of our work to friction, it will all go into lifting
the car.
Winput=Woutput
Finput dinput = Foutput doutput
(300 N) (0.8 m) = (4500 N) doutput
solve for doutput
doutput = 0.053 m = 5.3 cm
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Compound Machines:A compound machine combines one or more
simple machines.
Ex: A mechanical watch where a lever may be
attached to a gear, pulley, or spring.
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