You need to lift a 100 lbmrock 300ft with an electric motor that is 60% efficient, how much will it cost if the electricity is 10 cents per kWh?

0.066 cent

Explanation:

Efficiency (E) = Energy output (Eo) / Energy input (Ei)

Eo = E * Ei

E = 0.6

Ei = mass * acceleration due to gravity * height

mass = 100lbm = 100*0.45359kg = 45.359kg, height = 300ft = 300/3.2808 = 91.44m

Ei = 45.359*9.8*91.44 = 40646.7Ws = 40.6467kWs = 40.6467kWs * 1h/3600s = 0.011kWh

Eo = 0.6 * 0.011kWh = 0.0066kWh

1kWh = 10 cents

0.0066kWh = 0.0066*10 = 0.066 cent