50 L of water falls 10.0 m into the bucket of a water wheel. How much energy is imparted to the wheel?
1 L of water has a mass of 1 kg, so the potential energy of 50 L of water 10 metres above the bucket is:
50*1*9.8*10 Joules = 4900 Joules
This is turned into the same amount of kinetic energy as it hits the bucket.