Energy

ENERGY

Energy Defined

• Energy is the ability of an object to produce a change in itself or the environment.

Energy Transfer

• Work is done when energy is transferred from one form of energy to another.

The Units for Measuring Energy

•Are the same as the units for measuring WORK

•JOULEs in the SI system

•FOOT-POUNDS in the British System

TYPES OF ENERGY

Mechanical Energy

• Energy is the ability to do work

Mechanical Energy(ME) - enables an object to do work

A. Kinetic Energy (Ek) - energy of motion

B. Potential Energy(Ep) - stored energy

Gravitational Potential Energy

• An object lifted against the force of gravity has stored energy.

• UNIT= JOULE

Gravitational Potential Energy

• Only changes in potential energy can be measured

Ep = mghm= mass g= gravity h=

height

• As an object, such as a boulder, begins to move, it gains velocity.

• Gaining velocity increases the object’s kinetic energy.

Example ProblemLegend has it that Isaac Newton

“discovered” gravity when an apple fell from a tree and hit him in the head. If a 0.20 kg apple fell 7.0 m before hitting Newton, what was its change in Ep during the fall?

Kinetic Energy

Unit: Joules

2

2

1mvEk

Kinetic Energy Examples

Pitched baseball 150 J (148 g @45 m/s)

Football linebacker 4500 J (110 kg @ 9.0 m/s)

Example Problem

A greyhound at a race track can run at a speed of 16.0 m/s. What is the KE of the 20.0 kg greyhound as it crosses the finish line?

Work

• Work is the transfer of energy from one object to another, or . . .

• The change in KINETIC ENERGY!

KEKEKEW if

Ek and Ep are

constantly changing.

Law of Conservation of Energy

• Energy cannot be created or destroyed.

• ENERGY is merely transformed .

APPARENT LOSS OF ENERGY

• FRICTION WASTES ENERGY BY CHANGING USEFUL ENERGY INTO HEAT

Total Energy• At any point, the total energy

of an object

Et = Ek + Ep + HEAT

Potential Energy to Kinetic Energy

Law of Conservation of Energy

ffii PEKEPEKE

ELASTIC POTENTIAL ENERGY

Elastic Potential Energy

•Sling shots•Trampolines

•Rubber bands•Rubber• balls

A Jack in the box has stored energy in a compressed spring.

k =

force to stretch divided by distance stretched

The Elastic Constant

ELASTIC POTENTIAL ENERGY

•EEP=

1/2 kd2

Elastic Potential Energy• If a spring of stiffness

constant (k) is stretched an amount (d), the elastic potential energy stored in the spring is equal to 1/2 the the spring constant times the square of the distance stretched.

An automobile stores chemical energy in the gasoline.

Thermal Energy

• The total energy of vibrating atoms and molecules that make up a substance.

• Temperature is the average KINETIC ENERGY of a substance.