One cubic meter of water is poured off a 100-m high bridge. If the change in gravitational potential energy is converted into electricity with an efficiency of 90%, how long can this energy illuminate a standard 60-W light bulb?

the light bulb can be illuminated 14700 sec ( = 245 minutes or 4 hours and 5 minutes)

Step-by-step explanation:

the change in gravitational potential energy ΔE is

ΔE = m*g*h = ρ*V*g*h = 1000 kg/m³ * 1 m³ * 9.8 m/s² * 100 m = 980000 J

where

ρ = density of water

V = volume of water

g = gravity

h = height

the energy required to illuminate a light bulb is

e*ΔE = P*t

where

P = power required by the lightbulb

t = time

e = efficiency

then

t = e*ΔE/P = 0.9 * 980000 J / 60 W = 14700 sec ( = 245 minutes or 4 hours and 5 minutes)