Energy

Ch. 5

Energy

I. Energy – the ability to do work (or cause change)

Most energy can be classified into 2 general categories:

Energy

1. Potential energy (PE) – stored energy or energy of position

PE = mghmass (kg)

9.8 m/s2 height (m)

Energy

2. Kinetic Energy (KE) – energy of motion

KE = ½ mv2

mass (kg)velocity (m/s)

Energy

3. The unit for energy is Joules (J).

James Prescott Joule1818-1889

Energy Types

Type KE

vs

PE

Description Examples

MechanicalEnergy

Both Energy associated withmotion and positionof everyday things

Speeding train,Bouncing ball,

Sprinting athletes

Thermal(Heat)Energy

KEEnergy is the motion ofparticles (faster = more

heat)

Candles, electricheaters

Energy Types

Type KE

vs

PE

Description Examples

ChemicalEnergy

PEStored in chemical

bonds in compounds(broken bonds – PE

Increases)

Gasoline,food, battery

RadiantEnergy

KEEnergy of

electromagneticwaves, including light

Solar energyX-rays

Radio waves

Energy Types

Type KE

vs

PE

Description Examples

SoundEnergy

KE Energy that movesthrough substances

(compression waves)

Music,Pencil hits floor,

etc

ElectricalEnergy KE

Movement of electricalcharges

Electricity inwires,

lightning

Energy Types

Type KE

vs

PE

Description Examples

NuclearEnergy

PE Energy that is stored in the nucleus (holds p+ &

neutrons together)

Sun (fusion), Powerplants

(fission)

Law of Conservation of Energy

A. Energy may change forms but it cannot be created or destroyed

B. If energy is conserved, then PE = KE ( is “change in”)

C. Conservation of Mechanical Energy:(KE + PE)beginning = (KE + PE)end

D. Energy flowchart shows how energy changes form.

Law of Conservation of Energy

a. The life of Stretchy the Rubber Band:

Stretchy is pulled – KE PE

Stretchy is released – PE KE

Stretchy is broken –

KE heat & sound

Energy Problems

Examples:

What is the potential energy at points A through E?

Energy ProblemsTo demonstrate a cheerleading stunt, Miss

Rokosik has to lift Miss Kalousek over her head (to a height of 2 m). Miss Kalousek has a mass of 50 kg. What is the potential energy of Miss Kalousek?

Energy Problems

What is Miss Kalousek’s kinetic energy while she is held above the ground?

Energy Problems

Miss Rokosik gets tired and drops Miss Kalousek from the 2 m height. What is her kinetic energy just before she hits the ground? How do you know?