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

Now instead of a force for how long in time we consider a force for how long in distance.

Work = Force x distance or W = F.d Units - Joules (J) or ft.lb BTU = 778 ft.lb (energy of one wooden

kitchen match) Pushing on a wall and wall doesn’t move

(no work done on the wall)

x

F

xFW x

xF

yF

Video ClipVideo Clip

Manpowered MachinesManpowered Machines

2. POWER

Power = Work/time or P = W/t

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

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

Demo - Dart gunDemo - Dart gun

Other examples - Springs, bow, sling shot,

chemical energy, and gravitational potential

energy

The latter is GPE = 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.

Video ClipsVideo Clips

InclineIncline

ScrewScrew

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 = KE

4. CONSERVATION OF ENERGY

Demo - Bowling ball pendulumDemo - Bowling ball pendulum

Demo - Loop the loopDemo - Loop the loop

Video - Pole VaultingVideo - Pole Vaulting

Galileo's inclines

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

ParabolaH

yperbola

Energy is conserved alongall of these paths.

Perigees

Apogees

Circle

Video ClipsVideo Clips

Driving NailsDriving Nails

Water WheelWater Wheel

Roller CoasterRoller Coaster

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

5. Machines

If no losses then

work input = work output

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

Examples - levers, block and tackle, etc. Demo - Block and tackle Demo - Block and tackle

Demo - Hydraulic liftDemo - Hydraulic lift

FD = FD

D D

6. 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.)

7. COMPARISON OF KINETIC ENERGY AND MOMENTUM

KE is a scalar and cannot be canceled.

ip

0fp

I

Thief absorbs all the kinetic energy.

if ppp

This is the impulse applied to the bullet.

This is the impulse applied to the man.

ip

Thief does not absorb all the kinetic energy.

fp ip

-

I

if ppp

This is the impulse applied to the bullet.

This is the impulse applied to the man.This is the impulse from previous slide.

Rubber bullets versus lead bullets Slow and fast football players with different

masses. Consider head-on with one having twice the mass but half the speed of the other.

0)2()2( vmvm

Twice the mass at half the speed. Momentum can cancel.

2mv21 )2(

21 vmKE

22 )2(

21 vmKE

Kinetic energy is not a vector and cannot cancel out. The kinetic energy of the big slow person is

The kinetic energy of the small fast person is

22mv

Punch is the same

but the energy delivered is not.

8. SOURCES OF ENERGY

Except for nuclear and geothermal power, the source of practically all our energy is the sun.

Nuclear power

Geothermal power

Solar power

Wind power

9. ENERGY FOR LIFE

Reading assignment

Chapter 7 Review Questions

A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work?

(a) 10 lb weight

(b) 20 lb weight

(c) same work for each lifting

(d) not enough information is given to work the problem

(c) same work for each lifting

Two cars, A and B, travel as fast as they can to the top of a hill. If their masses are equal and they start at the same time, which one does the most work if A gets to the top first?

(a) A

(b) B

(c) they do the same amount of work

(c) they do the same amount of work

An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest?

(a) 180 Joules

(b) 2700 Joules

(c) 36 Joules

(d) 5 Joules

(e) 180 N

(b) 2700 Joules

A 20 pound weight is lifted 4 feet. The change in potential energy of the weight inft.lb is(a) 20

(b) 24

(c) 16

(d) 80

(e) 5

(d) 80