Power Questions

PFY p.120

f) Power is the rate of doing work
Power (in Watts) = work done (in joules) / time (in seconds)

g) 1 watt is a rate of working of one joule per second

13) A girl does 1000 joules of work in 5 seconds. What power does she develop?
P = Wd/t
P = 1000/5
P = 200 W

14) A man lifts a weight of 300N through a vertical height of 2 m in 6 seconds. What power does he develop?
Wd = F x d
Wd = 300 x 2
Wd = 600J

P = Wd/t
P = 600/6
P = 100 W

16) A crane lifts a load of 3000N through a vertical height of 10 m in 4 seconds. What is its rate of working ina) Watts
Wd = F x d
Wd = 3000 x 10
Wd = 30,000J

P = Wd/t
P = 30,000 / 4
P = 7,500 W

b) Kilowatts
7,500 / 1000 = 7.5 kW