Power

Chapter 5.6

WOD is underlined

QuestionA 2400 kg car can slow from 10 m/s to rest in 3.2

meters.If the car has traditional friction brakes, what force do they apply while decelerating?

(Show how to get vf^2 = vi^2 + 2av from w = Delta KE)

Question

If this same car is traveling at 15 m/s, what will the stopping distance be?

At 20 m/s?

25 m/s?

30 m/s?

Show that

Vel vs distance is a

Quadratic function.

Can also solver eqn.

Relate to a student

Speeding at 95 vs 65.

Speed increase is 50%.

Distance increase is 100%

PowerPower is work / time

P = W / t

Power is Capital P

Do not confuse this with lowercase p (p = rho = momentum)

Power Add units to WOD.Power is work / time

P = W / t

Units are Joules/seconds or “Watts”1 Watt = 1 kg m2/s3

1 W = 1 J / s

The concept of POWER focuses on TIME.

English units is horsepower. Based upon actual power 1 horse can generate.

Power Add units to WOD.Power is work / time

P = W / t

Do Power companies sell us power?

Power Add units to WOD.Power is work / time

P = W / t

Do Power companies sell us power? No, Power is kWatts. They sell us kW-Hours. P * time = (Work / time) * time = Work = Energy

PowerQuestion: If someone uses 1000 watts of power for one hour, how much energy do they use?

(shown next slide)

PowerQuestion: If someone uses 1000 watts of power for one hour, how much energy do they use?

(Work out here)

P = 1000 watts = 1000 J /sProblem is 1000 J /s * 1 h = 1000 J/s * 3600 s/hr * 1 h

PowerQuestion: If someone uses 1000 watts of power for one hour, how much energy do they use?

3.6 * 10 6 Joules

Different Units:American Unit is “Horse Power” or “Power supplied by one horse”

1hp = 550 foot*lbs/second 1 hp = 746 Watts

Also, energy can be measured in KiloWatt Hours1 kwh = 3.6 * 10 6 JoulesKwh = power * time = energyYour electric bill probably uses kwh instead of Joules

If you run your hair dryer for 20 minutes at 1500 watts at 12 cents a kWh. How much did you spend?Cost = kW * hr * $ /kWh * 30 days = 1.5 * 20/60 * .12 * 30 = $1.80How about switching 15 light bulbs from 100 watt to 13 watt?

Watt savings is 15*(100-13) =1305W= 1.3 kW

Cost = kW * hr * $ /kWh * 30 days = 1.3 * 8 hr/day * .12 * 30 = $37.44

How about switching off your computer and monitor? How much do you save? Those use about 250w/hr each. Assume you turn it off 20 hours a day instead of leaving it on all the time.

Know for test: Cost = kW * hr * $ /kWh Like test question, calculate for 1 day.

Cost = kW * hr * $ /kWh = .500 * 20 hr/day * .12 = $1.2/day

Savings on utility bill by turning it off for the

20 hours a day that the computer sits idle

= $1.2/day * 30 = $36 a month

Problem An elevator must lift 1000 kg mass a

distance of 100 m at a velocity of 4 m/s. What is the average power the elevator exerts during this trip?

Problem An elevator must lift 1000 kg mass a distance of 100 m at a

velocity of 4 m/s. What is the average power the elevator exerts during this trip?

Power = work / time = mgh/t = mg (h/t) = mg*v = 1000 kg (9.8m/s2) (4m/s) = 39200 watts