Law of Conservation Law of Conservation of Energy of Energy

Energy can neither be created nor destroyedIt can change forms

– Sound becomes mechanical in your ear– Chemical becomes electrical in a battery– Chemical becomes mechanical and heat in your car

engine– A chemical energy in a firecracker becomes sound,

light, heat, and shock wave

Conservation of EnergyConservation of Energy

I lift a 2 kg book 2 meters into the airAs the book is held in the air at 2 meters,

it has 40 J of potential energyIf I drop the book, it will become 40 J of

kinetic energy on the way to the floor

You try it . . . You try it . . .

How much energy does 70 kg person have when an elevator lifts her 20 m into the air?– PE = mgh– PE = (70 kg)(9.8 m/s2)(20 m)– PE = 13720 J

Continue the problem . . .Continue the problem . . .

What happens to the energy of the same lady when she descends on the elevator to a height of 12 m?– PE = mgh– PE = (70 kg)(9.8 m/s2)(12m)– PE = 8232 J– Therefore 5488 J of energy was “lost” – it went into

KE as she descended because energy can not be lost!!

How much PE?How much PE?

If the 70 kg person is now 20 m up, how much PE?– PE = mgh– PE = (70 kg)(9.8 m/s2)(20 m)– PE = 13720 J

What is a machineWhat is a machineA machine is something that

changes the magnitude (amount) or direction of a force

It does not make for less work, only makes the work easier!!

Machines Transform Machines Transform EnergyEnergy

Machines transform energy.Because energy is not created or

destroyed– work input = work output

Sometimes some of the input is converted to sound or heat due to friction

6 Simple Machines6 Simple MachinesIncline PlaneWedgeScrewLeverPulleyWheel & Axle

This is a rampThe lower the slope, the less

force neededThe lower the slope, the more

distance needed

Incline PlaneIncline Plane

Wedge Wedge This is two incline planes put

togetherAn axe, a knife, and the cutting

part of scissors are all wedgesA wedge will separate material

as it pushes through

Screw Screw A screw is an incline plane that

is wrapped around a cylinderScrews, bolts, and spiral

staircases are all examples

LeverLeverA lever has two parts

– The lever (the part that moves)– The fulcrum (the part or location that

the lever moves around)

Classes of LeversClasses of LeversThree classes of levers

– 1st class – a seesaw– 2nd class – a wheelbarrow– 3rd class – a tennis racket

PulleyPulleyA single pulley will only change

the direction of the forcePulleys may be fixed or

moveableA set of pulleys working together

is known as block and tackle

Wheel & AxleWheel & AxleAs the wheel is turned by the

effort, the axle will turn with more force

A winch and a rear bike tire are examples of wheel and axle

Mechanical AdvantageMechanical AdvantageMechanical Advantage (MA) is a

measure of how much the force is multiplied– = force output/ force input