Chapter 6Chapter 6

EnergyEnergy

EnergyEnergy Universe is made up of matter

and energy.

Energy is the mover of matter.

Energy has several forms: – Kinetic, Potential, Electrical, Heat,

etc.

WorkNow instead of a force for how long in

time we consider a force for how long in distance.

Work = Force Distance W = F d

The unit for work is the Newton-meter which is also called a Joule.

Questions:

How much work is done when a weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters above the ground?

How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 Newtons for 15 seconds?

Power

Power is equal to the amount of work done per unit time.

interval time

donework Power

The unit for power is the Joule/second which is also called a Watt.

Light Bulbs and Appliances

How much energy does a 100 Watt light bulb use in one hour?

How about a 40 Watt light bulb? Which bulbs shines brighter?

interval time

usedenergy electricalRatingPower

Mechanical EnergyWhen work is done on an object, the

object generally has acquired the ability to do work.

This "ability to do work" is called energy and it has the same units as work….Joules.

Two Types of Mechanical Energy– Potential Energy and Kinetic Energy

Potential Energy

The energy that is stored is called potential energy.

Examples: – Rubber bands– Springs– Bows– Batteries– Gravity?

Gravitational Potential Energy

PE = Weight height

PE = m g h

Question:– How much potential energy does a

10kg mass have relative to the ground if it is 5 meter above the ground?

Kinetic EnergyKinetic Energy is the energy of

motion.

Kinetic Energy = ½ mass speed2

2mv2

1KE

Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second?

Work/Energy RelationshipIf you want to move something, you

have to do work.

The work done is equal to the change in kinetic energy.

Work = KE

Example Question

When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h?

• Answer: 9 times

Conservation of EnergyEnergy cannot be created or

destroyed; it may be transformed from one form into another, but the total amount of energy never changes.

Demos– Galileo's incline– Bowling ball pendulum– Loop the loop

Example ProblemA 100 kg mass is dropped from rest

from a height of 1 meter.How much potential energy does it

have when it is released?How much kinetic energy does it have

just before it hits the ground?What is its speed just before impact?How much work could it do if it were to

strike a nail before hitting the ground?

100 kg

100 kg

100 kg

1 meter

0mvKE 221

J980)m1)(s/m8.9)(kg100(mghPE 2

Joules 980mvKE 221

Joules 0mghPE

Joules 980DistaceForceDone Work nail

Machines - An Application of Energy Conservation

If there is no mechanical energy losses then for a simple machine...

work input = work output

(F d)input = (F d)output

Examples - levers and tire jacks

Efficiency

Useful energy becomes wasted energy with inefficiency.

Heat is the graveyard of useful energy.

usedenergy

donework fficiencyE

Comparison of Kinetic Energy and Momentum

Kinetic energy is a scalar quantity.

Momentum is a vector quantity.

Discuss rubber bullets as compared to lead bullets.

Example 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(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(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(b) 2700 Joules

A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton.meters is

(a) 20

(b) 24

(c) 16

(d) 80

(e) 5

(d) 80(d) 80

End of Chapter 6

Your Mission:• Print out the study guide and define

each term.• Read all of the “blue questions” and

answers in Chapter 6.• Make sure that you have read this

chapter.