Chapter 7

Energy

Introduction

• Universe is made up of matter and energy.

• Energy is the mover of matter.• It has several forms. To

understand this concept we will begin with a closely related physical concept.

1. WORK

Work = Force x distance or W = F.d Units - Joules (J) Pushing on a wall and wall doesn’t move

(no work done on the wall)

x

F

xFW x

xF

yF

2. POWER

Power = Work/time or P = W/t

Units - J/s = W 1 hp

1 hp = 746 J/s = 746 W

100 W bulb = 0.1341 hp

250 hp engine = 186,450 W

3. MECHANICAL ENERGY

When work is done on an object, the object generally has acquired the ability to do work.

This is called energy and it has the same units as work.

Two Types of Mechanical EnergyPotential EnergyKinetic Energy

Potential Energy

Energy of position or configuration

Dart gun – BalestraDart gun – Balestra

Other examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy

The latter is E = mgh

The potential energy of an object depends on a reference position.

It represents the work done against gravity to put the mass m in its position h above some reference position.

It is an energy of position.

Kinetic Energy

It is an energy of motion.

KE mv 12

2

It is a square law. Total Work (work done by all forces acting

on mass m) = KE

221 )2( ivm

Work to Stop KE

)(4 221

imv

xFmvmv xif 2212

21

xFmv xi 221

0

221 4 ivm xxF

Note

Work-Energy Theorem

The net work done on an object is equal to the change in the kinetic energy of the object.

Net Work = Ek

4. CONSERVATION OF ENERGY

Energy is always conserved Energy lost due to friction is actually not a

loss; it is just a conversion.

Energy cannot be created or destroyed. It may be transformed from one form into

another, but the total amount of energy never changes.

Energy Conservation in Satellite Motion (Next slide)

Ellipse

Ellipse

ParabolaHyperbola

Energy is conserved alongall of these paths.

Perigees

Apogees

Circle