Chapter 5Chapter 5

Work, Energy, PowerWork, Energy, Power

WorkWorkThe work done by force is defined

as the product of that force times the parallel distance over which it acts.

cosFsW The unit of work is the newton-

meter, called a joule (J)

EnergyEnergyThe amount of energy transferred to the object is equal

to the work done.

Types of Energy

• Kinetic Energy = “Motion Energy”

• Potential Energy = “Stored Energy”

Kinetic EnergyKinetic EnergyKinetic Energy is the

energy possessed by an object because it is in motion.

221 mvKE

Work-Kinetic Energy Theorem

When work is done by a net force on an object and the only change in the object is its speed, the work done is equal to the change in the object’s kinetic energy

– Speed will increase if work is positive– Speed will decrease if work is negative

net fiW KE KE KE

Ex: Work and Kinetic EnergyThe hammer head

has a mass of 0.4 kg and speed of 40 m/s when it drives the nail. If the nail is driven 3.0 cm into the wood and all of the kinetic energy is transferred to the work one on the nail, What is the average force exerted on the nail.

Example: Block w/ frictionA block is sliding on a surface with an initial

speed of 5 m/s. If the coefficient of kinetic friction between the block and table is 0.4, how far does the block travel before stopping?

5 m/sx

y

Gravitational Gravitational Potential EnergyPotential Energy

Gravitational Potential Energy is the energy possessed by an object because of a gravitational interaction.

mghPEg

Work and Gravitational Potential EnergyPE = mgy Units of Potential

Energy are the same as those of Work and Kinetic Energy

figravity PEPEW

Potential Energy in a Spring

Elastic Potential Energy– related to the work required to

compress a spring from its equilibrium position to some final, arbitrary, position x

– 2

s kx2

1PE

Force

Distance

F=kx

PowerPower

Power is the time rate of doing work or how fast you get work done.

SpeedForceFvt

Fd

erAveragePow

t

W P

work thisdo taken totime

force aby donework

PowerPowerThe unit of power is a joule per

second, called a Watt (W).

W746s

lbft550hp1

Power - Examples

Watt is the work output if you perform 100 J of work in 1 s?

Run upstairs! If you raise your body (70 kg or 700 N) 3 m in 3 seconds, how powerful are you?

Shuttle puts out a few GW (gigawatts, or 109 W) of power!

Conservation of EnergyConservation of Energy

Energy can neither be created nor destroyed, but only transformed from one kind to another.

finalinital )PEKE(W)PEKE(

Energy is ConservedEnergy is “Conserved” meaning it

can not be created nor destroyed – Can change form – Can be transferred

– PE into KE, KE into PE, KE into HEAT

Total Mechanical Energy does not change with time.– ΔPE + ΔKE = 0

– PE + KE = constant

Energy Conservation ExampleDrop 1 kg ball dropped from 10 m.

– starts out with mgh = (1 kg)(9.8 m/s2)(10 m) = 98 J of gravitational potential energy

– halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy

• ½mv2 = 49 J v2 = 98 m2/s2 v 10 m/s

– at floor (0 m), all potential energy is given up to kinetic energy

• ½mv2 = 98 J v2 = 196 m2/s2 v = 14 m/s

10 m

8 m

6 m

4 m

2 m

0 m

P.E. = 98 JK.E. = 0 J

P.E. = 73.5 JK.E. = 24.5 J

P.E. = 49 JK.E. = 49 J

P.E. = 24.5 JK.E. = 73.5 J

P.E. = 0 JK.E. = 98 J

Roller Coasters

Since

PE + KE = Etotal ,

The shape of a potential energy curve is exactly the same as the shape of the track!

Roller Coasters - Example

If the height of the coaster at A is 60 m from the ground, how fast will you be moving at B, C, D?

Assume no friction and the height of B is 10 m and C is 20 m

On to problems...

Mission for Next Time:

•Homework•Page 33: 1, 2, 3, 6, 24•Page 34: 18, 19, 23•Page 35: Q:1,2; P: 2,7•Page 36: 1, 3, 6, 8.

Mission for Next Time:

•Homework Packet- SET II•Page 21: 4, 5, 8, 11•Page 22: 12, 13, 17, 20, 22•Page 24: Q: 5, 6; P: 3, 8•Page 25: 2, 5, 7, 10.

•School Island•Web Assign