Chapter 6

Work, Energy, and Power

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.

WORK

For motion in a straight line the WORK done by a force is defined as the product of the component of the force in the direction of motion times the distance moved.

x

F

xFWx

xF

yF

x)cosF(

Work is a scalar quantity. Work can be negative. Work is the transfer of energy from one entity

to another by way of the action of a force applied over a distance. The point of application of the force must move if work is to be done.

Pushing on a wall and wall doesn’t move(no work done on the wall)

The Units of Work

N.m {Joules (J)} or ft.lb 1 erg = 10-7 J. 1 ft.lb = 1.355 J. 1 BTU = 778 ft.lb (energy of one

wooden kitchen match)

ENERGY

Energy is a measure of the change imparted to a system???

It can be mechanically transferred to an object when a force does work on that object.

Further, when an object does work, it gives up an amount of energy equal to the work it does.

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 EnergyKinetic Energy

Potential Energy

Kinetic Energy

It is the energy possessed by an object because of its motion.

2

21 mvKE

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

on mass m) = KE

Potential Energy

Energy of position or configuration

Demo – Dart GunDemo – Dart Gun

Other examples - Springs, bow, sling shot,

chemical energy, and gravitational potential

energy

The latter is PEG = mgh

Gravitational Potential Energy

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.

mghPEG

221 )2( ivm

Work to Stop KE

)(4 221

imv

xFmvmv xif 2212

21

xFmv xi 221

0

221 4 ivm xxF

Note

The Work-Energy Theorem

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

Net Work = KE

From text: when work is done on a point mass or a rigid body, and there is no change in PE, the energy imparted can only appear as KE. Insofar as a body is not totally rigid, however, energy can be transferred to its parts and the work done on it will not precisely equal its change in KE.

Energy cannot be created or destroyed.

It may be transformed from one form into another,

but the total amount of energy never changes. Energy lost due to friction is actually not a loss; it

is just a conversion. Energy Conservation in Satellite Motion

(Next slide)

CONSERVATION OF ENERGY

Ellipse

Ellipse

ParabolaH

yperbola

Energy is conserved alongall of these paths.

Perigees

Apogees

Circle

Condition for Conservation of Mechanical Energy

No work can be done on the object by a

nonconservative force.

A nonconservative force is a force that

converts mechanical energy into another

form.

Example: Friction

No work is required to maintain circular

motion at constant speed.

2mcE

POWER

workthisdototakentime

forceabydoneworkpowerAverage

tWP

or

tFdP vF

Units - J/s = W 550 ft.lb /s = 1 hp

1 hp = 746 J/s = 746 W

1 BTU/hr = 0.293 W

100 W bulb = 0.1341 hp

250 hp engine = 186,450 W

The Kilowatt-Hour

The kilowatt-hour is a unit of energy. If a force is doing work at a rate of

1 kilowatt (which is 1000J/s), then in 1hour it will do 1 kWh of work.

1 kWh = 3.6 x 106 J = 3.6 MJ

Machines

If no losses then

work input = work output

(F.d)input = (F.d)output

Examples - levers, block and tackle, etc.

FD = FD

D D

EFFICIENCY

Efficiency = work done/energy used Useful energy becomes wasted energy with

inefficiency. Heat is the graveyard of useful energy. EER = energy efficiency ratio

It is the output capacity (BTU/hr)/input energy (Watts)

(Output capacity represents energy moved.)