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