Chapter 8 Energy

Work = force x distance W = Fd - application of a force - movement of something by that force

Work is measured in Joules Joule (J) = N*m

Power – the rate at which work is done measured in watts

Power = work done time interval

Mechanical Energy Definition: the energy due to the position of something or the movement of something

Potential Energy (PE) Definition: energy that is stored and held in readiness Examples: stretched rubber band, fossil fuels, food

Gravitational PE PE due to elevated positions GPE = weight X height PE = mgh

Kinetic Energy (KE) Definition: energy of motion Examples: throwing a ball, moving car

KE = 1/2mv 2

Work-Energy Theorem: Work =  E

Law of Conservation of Energy Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

Machines definition: device used to multiply forces or simply to change the direction of forces

Input force: the force you exert on the machine Output force: the force exerted by the machine

Types of Simple Machines (there are 6 of them)

1. Lever 2. Wheel and Axle 3. Pulley 4. Inclined Plane 5. Screw 6. Wedge

lever - a bar that turns about a fixed point; Ex. crowbar

Fulcrum: the fixed point on which a lever turns or pivots fulcrum

The lever exerts a large force over a short distance while you exert a small force over a long distance

TYPES OF LEVERS: - refer to drawings and explanations of the three different types of levers

Wheel and Axle: two circular objects fastened together and that rotate on a common axis

- always rotate together - Ex. doorknob, steering wheel

Pulley: a wheel that has a rope or chain passing over it; used to change the direction of the force that’s applied to the object Example: flagpole, window blinds

Inclined Plane: a ramp or slope that reduces the force you need to lift something

- Inclined planes decrease the effort force Ex. ramps

Screw: an inclined plane wrapped around a cylinder to make a spiral  Threads: the ridges spiraling around the screw  Ex. Screw, jar lid

Wedge: an inclined plane that moves Ex. Axes, chisels, knives, hatchets

Simple Machines in Your Body…  Your tendons and muscles pull on your bones and make them act as levers  incisors (front teeth) are wedges

Compound Machines: a combination of simple machines that makes it possible to do something that one simple machine alone cannot do

Ex. Can opener The handles are levers; the crank is a wheel and axle; a gear is then turned, which turns another gear which moves the blade which is a wedge

Mechanical Advantage= output force input force  Tells you how much force is multiplied

The larger the mechanical advantage, the more help the machine provides

When we calculate mechanical advantage we look at ideal situations  An “ideal” machine would be 100% efficient  100% efficiency NEVER happens in practice

Whenever work is done; some energy will be lost as heat Efficiency looks at this heat/energy loss

Efficiency = actual mechanical advantage theoretical mechanical advantage

OR Efficiency = output work input work X 100 %

Even the best designed engines are about 35% efficient