# Machines. Work and Power Power is the rate at which work is done Power = Work time Remember that W = Fd So, Power = Fd t Power is measured in Watts –1.

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Machines

Work and Power Power is the rate at which work is done Power = 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 F e ) – force applied to the machine Resistance force (RF or F r ) – force applied by the machine to overcome resistance due to gravity or friction

2 Distances Effort Distance (d e ) – distance through which the machine moves Resistance distance (d r ) – distance through which the force applied moves the object

2 Kinds of Work Work input (W in ) – work done on the machine W in = F e x d e Work output (W out ) = work done by the machine W out = F r x d r

Machines Machines do NOT save work You can never get more out of the machine then you put in W out is never greater than W in 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 = F r F e 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 W out to W in & is expressed a percent The higher the efficiency the more W in is changed to W out No machine is 100% efficient b/c of friction Efficiency = W out x 100 W in

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

Wedge Inclined plane that moves The sharper the wedge the greater the MA Ex. Axe

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 –1 st Class –2 nd Class –3 rd 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

Lever MA = length of effort arm length of resistance arm

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

Compound Machines A system of 2 or more simple machines The MA is the product of all the mechanical advantages of the simple machines

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