2.  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

3. 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.

4.  Power practice questions on handout (correct together)  Power handout- hand in  Power lab

5. 1. P= ? W= 120JP=W/t t=2.0s= 120J/2.0s = 60W 2. P=? m= 60kgP=(mg)d/t g= 9.8 N= (60kg)(9.8m/s 2 )(4.5m)/ 4.0s d= 4.5m= 2646 N*m / 4.0s t= 4.0s= 661.5 W = (6.6x10 2 W)

6.  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 3.6x10 6 J (1000W)(3600s)

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

8. P=25kWP=W/t t=7daysW= P*t =(7days*24hrs)W= (25kW)(168h) =168hrs = 4200 kW*h