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Work and Power• Power is the rate at which work is
donePower = Work time
• Remember that W = Fd• So, Power = Fd
t• Power is measured in Watts
– 1 horsepower = 745.5 Watts
Machines• Any device that makes work
easier• Change size & direction of
force• When machines do work
there are:–2 forces–2 distances–2 kinds of work
2 Forces• Effort force (EF or Fe) – force applied to the machine
• Resistance force (RF or Fr) – force applied by the machine to overcome resistance due to gravity or friction
2 Distances• Effort Distance (de) – distance through which the machine moves
• Resistance distance (dr) – distance through which the force applied moves the object
2 Kinds of Work• Work input (Win) – work done on the machine Win = Fe x de
• Work output (Wout) = work done by the machineWout = Fr x dr
Machines• Machines do NOT save work• You can never get more out of
the machine then you put inWout is never greater than Win
• You apply less force to a machine, but the force must be applied over a greater distance
Mechanical Advantage
• The # of times a machine multiplies your EF
MA = Fr
Fe • Machines that only change the
direction of your EF have a MA of 1• Machines with a MA of < 1
increase the distance an object is moved or the speed at which it moves
Mechanical Efficiency
• Compares Wout to Win & is expressed a percent
• The higher the efficiency the more Win is changed to Wout
• No machine is 100% efficient b/c of friction
Efficiency = Wout x 100 Win
Simple Machines• Do work with only ONE
movement• Work is made easier b/c the EF is moved over a greater distance
• There are 6 simple machines
Inclined Plane• Sloping surface
used to raise objects
• MA = length height
• MA is NEVER less than one b/c length is never shorter than height
• Ex. handicap ramp
Screw• An inclined plane
wrapped in a spiral around a cylindrical post
• The closer the threads on the screw the greater the MA
• Ex. spiral staircase
Wheel and Axle• Consists of 2 wheels of
different sizes that rotate together (Ex. Doorknob)
• Wheel – largest wheel• Axle – smaller wheel• Effort arm = radius of
the wheel• Resistance arm =
radius of the axle• MA = radius of the
wheel radius of the axle
Levers• A bar that moves
around a fixed point called a fulcrum
• There are 3 classes of levers– 1st Class– 2nd Class– 3rd Class
• Classified according to the location of the fulcrum
First Class Lever
• Fulcrum is between the EF & the RF
• Changes the size & direction of the EF
• Ex. seesaw, crowbar, scissors
Second Class Lever
• RF is between the fulcrum and the EF
• Multiplies the EF but does not change the direction
• Ex. Wheelbarrow, nutcrackers, door
Third Class Lever
• EF is between the fulcrum and the RF
• EF is greater than the RF• Does not change EF but multiplies
the distance the EF must travel• Ex. shovel, fishing pole, your
forearm
Pulley• A belt, rope, or
chain wrapped around a grooved wheel
• Types of pulleys:–Fixed–Moveable–Block and
Tackle
Fixed Pulley• Attached to a
stationary structure
• Does not multiply EF
• Only changes the direction of the EF
• MA is always equal to 1
Moveable Pulley• Hung on a rope
and hooked to a resistance
• Multiplies the EF but does not change the direction of the EF
• MA is greater than 1
Block and Tackle• When 2 or
more pulleys are used together
• MA can be determined by counting the sections of rope that give support upward