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.

Machines

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

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– 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

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