Chapter 8 Energy. Work = force x distance W = Fd - application of a force - movement of something by that force.

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 PEPE due to elevated positions

GPE = weight X height

PE = mgh

Kinetic Energy (KE)

Definition: energy of motionExamples: throwing a ball, moving car

KE = 1/2mv2

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.Lever2.Wheel and

Axle3.Pulley4. Inclined Plane5.Screw6.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 forceinput 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 advantagetheoretical mechanical advantage

OREfficiency =

output work input work

X 100 %

Even the best designed engines are about 35% efficient