CP PHYSICS - Brookwood High School

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CP PHYSICS
MACHINE NOTES
Machines
 machines are devices that make work easier to do (by transferring
energy)
 two ways:
1) Machine changes the direction of your force.
2) Machine multiplies the force applied to it.
 you do work on the machine and then the machine does work on an
object
 six simple machines:
1)
lever – long rigid bar with a support called a fulcrum
ex. crowbars, car jacks
2)
pulley – special type of lever, has a grooved wheel over
which a rope passes
ex. window shade pull, block and tackle, well, flag poles
3)
wheel and axle – special type of lever, consists of a wheel
attached to an axle
ex. pencil sharpener, egg beater
4)
inclined plane – flat surface with one end higher than the
other
ex. ramp
5)
wedge – inclined plane with either one or two sloping sides
ex. door stops, knives, axes, scissors, chisels
6)
screw – an inclined plane which is spiral, you move the
plane rather than the load
Mechanical Advantage
 indicates how much the machine multiplies your force
 equation:
MA = Fr/Fe
where,
MA = mechanical advantage (no unit)
Fr = resistance force (equals weight of object)
Fe = effort force (force you apply to the machine)
 second form of mechanical advantage known as ideal mechanical
advantage (IMA) – based on an ideal machine which transfers 100% of
your energy
 equation:
IMA = de/dr
where,
IMA = ideal mechanical advantage
de = effort distance (distance you apply a force)
dr = resistance distance (distance machine moves)
CP PHYSICS, MACHINE NOTES, page 2
Efficiency
 shows how much of the work you put into the machine (input work) is
actually used to do work on the object by the machine (output work)
 equation:
Efficiency = Wo/Wi x 100%
where,
Wo = output work(J)
Wi = input work (J)
 machines cannot multiply your work or energy—they can only multiply
force, In other words – a machine cannot do more work than the work
you put into it
Example Calculations
1.
What is the mechanical advantage of a pulley system in which you
apply a force of 58 N to lift an object whose weight is 194 N?
Fe = 58 N
Fr = 194 N
MA = ?
2.
What is the mechanical advantage of a ramp that is used to raise a
sofa whose mass is 155 kg if the person applies a force of 273 N to
push the sofa up the ramp?
Fe = 273 N
Fr = 155 kg x 9.8
= 1519 N
MA = ?
3.
MA = Fr/Fe
= 1519/273
MA = 5.56
What is the ideal mechanical advantage of a ramp that is 1.8 m
high and has a length of 6.0 m?
de = 6.0 m
dr = 1.8 m
IMA = ?
4.
MA = Fr/Fe
= 194/58
MA = 3.34
IMA = de/dr
= 6.0/1.8
IMA = 3.33
What is the efficiency of a machine that provides an output work of
1125 J, if you put 1380 J of work into the machine?
Wo = 1125 J
Wi = 1380 J
Efficiency = ?
Efficiency = Wo/Wi x 100%
= 1125/1380 x 100%
Efficiency = 82%
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