A diver of mass 50 kg leaves a platform moving vertically downwards at 2.0 m/s. The diver then falls a further 4.0 m vertically. During this fall, 80 J is transferred to thermal stores of the surroundings. Use g=10 N/kgg=10\ \mathrm{N/kg}g=10 N/kg.
Calculate the diver's speed after falling the 4.0 m.