Simple Machines

Making Life Easier Through Physics

Work and Energy• Energy is conserved – you

can’t create it or destroy it

• Work is transferring energy from one object to another

• Simple machines can’t increase or decrease the amount of work you do

Calculating Work

• Work doesn’t change when you use a machine• Most simple machines make our lives easier

by DECREASING the amount of force we need to apply to finish a job – If you decrease the force, the distance must

increase

• Some DSECREASE the distance instead– If you decrease the distance, the force must

increase

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Types of Simple Machines

•Lever Family–Lever–Pulley–Wheel and axle

•Inclined Plane Family–Inclined plane–Wedge–screw

Lever• Rigid arm with a turning

point• Turning point is called the

fulcrum• Moves force to a new

location– Resistance force is the force

the lever applies to an object– Effort force is the force a

person applies to the lever• Three classes of levers

1st class levers• Effort force and

resistance force are on opposite sides of the fulcrum

• Seesaws are a first class lever

fulcrum

Effort force Resistance force

2nd class levers• Resistance force is

between the fulcrum and the effort force

• wheelbarrows are a second class lever

fulcrum

Effort forceResistance force

3rd class levers• Effort force is

between the resistance force and the fulcrum

• Your biceps are a third class lever

fulcrum

Effort force Resistance force

Wrap-Up #1• Classify the following levers by

class1. Scissors2. Fishing rod3. Stapler

Wrap-Up #1: Answers• Classify the following levers by class1. Scissors

1st class – handle has effort force, the joint is the fulcrum and the blades apply the resistance force

2. Fishing rod3rd class – fulcrum by your body, handle has effort force and the line has the resistance force

3. Stapler 2nd class – the pivot is the fulcrum, you push in the middle and the staple is the resistance force

Pulleys• The middle point is

like the fulcrum of a lever

• Mechanical advantage is equal to number of ropes

• Three kinds– Fixed pulley– Movable pulley– Block and tackle

(multiple pulleys at the same time)

Wheel and Axle•Pulley (wheel)

connected to a shaft (axle)

•Small force on the large wheel is turned into a bigger force on the smaller axle

•Screwdrivers are also wheel and axles

Inclined Plane• Allow you to apply

a horizontal force to move things up

• You use less force than if you lift straight up

• You apply the force over a longer distance, so the work doesn’t change

Wedge• Changes the direction

of the force and splits it into two– If you push down the

wedge pushes to the left and right

• Useful for splitting things like firewood and your food

• Nails are wedges used to hold things together instead

Screw• An inclined plane

wrapped around a cylinder

• Allows you to twist it into place instead of pushing straight down

• Allows a longer plane than if it was straight (spiral staircase)

Wrap-Up #2• Classify each of these images as a type

of simple machine

#1

Wrap-Up #2• Classify each of these images as a type of simple machine

SCREW

• Classify each of these images as a type of simple machine

#1

Wrap-Up #2• Classify each of these images as a type

of simple machine

#2

Wrap-Up #2• Classify each of these images as a type of simple machine

INCLINED PLANE

• Classify each of these images as a type of simple machine

#2

Wrap-Up #2• Classify each of these images as a type

of simple machine

#3

Wrap-Up #2• Classify each of these images as a type of simple machine

PULLEY

• Classify each of these images as a type of simple machine

#3

Wrap-Up #2• Classify each of these images as a type

of simple machine

#4

Wrap-Up #2• Classify each of these images as a type of simple machine

LEVER

• Classify each of these images as a type of simple machine

#4

Mechanical Advantage• A measure of how

much the machine helps you– MA < 1, you apply a

stronger force over a smaller distance

– MA = 1, the force’s direction is changed

– MA > 1, you apply a lower force over a greater distance R

E

E

R

d

d

F

FIMA

Efficiency• Some energy

(work) is always converted into heat by friction

• The percentage of work transferred by the machine is its efficiency

• Higher efficiency means less friction

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