Power- is the rate at which work is done over time▪ P= W/t or P= Fd/t or P=(mg)d/t

Power is expressed in Watts

1. A man does 120J of work in lifting a box from the floor to the table. If it took 2.0s to perform the task, what is the power?

2. How much power is developed by a 60 kg person running up a 4.5m high flight of stairs in 4.0s.

Power practice questions on handout (correct together)

Power handout- hand inPower lab

1. P= ?W= 120J P=W/tt=2.0s = 120J/2.0s

= 60W2. P=?

m= 60kg P=(mg)d/tg= 9.8 N = (60kg)(9.8m/s2)

(4.5m)/ 4.0sd= 4.5m = 2646 N*m / 4.0st= 4.0s = 661.5 W

= (6.6x102 W)

the joule is a very small unit. A fan on a furnace does several millions of joules of work a day. To avoid using such large numbers we resort to the kilowatt- hour

A kilowatt hour is the amount of work done by a machine with a power of 1 KW in 1 hour.

1 KW*h = 3,600,000 Joules or 3.6x106J

(1000W)(3600s)

How much work (in kilowatt hours) is done by a 25 kW water pump running steadily for a week?

P=25kW P=W/tt=7days W= P*t =(7days*24hrs) W= (25kW)

(168h) =168hrs = 4200

kW*h