Post on 09-Feb-2016
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WORK, POWER, AND MACHINES
Physical ScienceChapter 12
Machines devices that change the direction of
a force or the size of a force that help us to do work
machines will multiply your force OR change the direction of your force
BUT CANNOT DO BOTH (can’t do work for you)
2 forces involved in the use of a machine:effort force (input)- force applied
to the machineresistance force (output) force
opposing the effort force often equal to weight of object
work input is ALWAYS > work outputWHY?friction
simple machine- machine that has only 1 or 2 parts
6 simple machines:
1. lever- bar that pivots, or turns, on a fixed point3 parts:a. effort arm- (input)- end of the bar you push onb. resistance arm (output)- end of bar that pushes on object you want to movec. fulcrum- point at which it pivots
3 classes of levers:A. first class lever-effort arm on one side,
resistance arm on other, fulcrum in middle
ex: see saw
B. Second class lever: fulcrum is at one end, EA at the other, RA
in the middle
ex: wheelbarrow
C. Third class lever fulcrum at one end, RA at the other, and
EA in the middle
ex: tweezers, forearm
2. Wheel and Axle consists of a lever connected to
a shaft lever usually shaped like a knob
or wheeleffort arm- radius of wheelresistance arm- radius of axleto be considered W&A, BOTH wheel and axle MUST move
3. Pulley consists of a wheel FREE TO SPIN on
its axle lever rotates around a FIXED POINT 2 FXNS:
changes direction of forceprovides mechanical advantage
(multiplies your force)
2 TYPES:
a. fixed pulley- attached to something that doesn’t moveused to change the direction of forcedoesn’t multiply force at all
B. movable pulley- attached to an object being movedmultiplies force
BLOCK & TACKLE- when fixed & movable pulleys are used together
multiplies A LOT of force
used to lift car engines, etc
4. Inclined Plane consists of a flat, sloping surface simplest of all machines used to move objects from 1 level to
another
5. Wedge used to push objects apart inclined plane that moves multiplies force and brings it to
a single point
6. Screw inclined plane wrapped around a
cylinder multiplies force
how much it increases force depends on PITCH
pitch- how far apart the threads are
smaller the pitch, the more force it multiplies
Compound Machines 2 or more simple machines put
together to do work
ex: ax, scissors
Mechanical Advantage # of times a machine multiplies force
MA = resistance forceeffort force
the greater the number, the less work you have to do
all machines have a MA
MA of pulley is equal to the number of rope segments pulling up the resistance force