A student of mass m=50kg runs an experiment. He throws a ball of mass m = 400g from a height h = 20m. What will be the speed of the ball he records just before it touches the ground?

First, we need to see what the important information in this problem is: the mass of the student is irrelevant as the problem focusses on the ball. So we now know that the important information is: m = 400g and h = 20m. We also need to recognise that the mass is not in the standard unit, so we need to transform it into kg: 400g = 0.4kg (divide by 1000 or move the point 3 steps).Now, we need to remember that the ball has potential energy at the start and that that potential energy turns into kinetic energy as it falls. The two formulas we need are: potential energy = mgh - will be maximal before it starts falling; kinetic energy = 0.5mv^2 - will be maximal (and equal to starting potential energy) when the ball reaches the ground. So, we can now calculate:potential energy = 0.4kg10m/s^220m = 80J. The kinetic energy when it reaches the ground = 80J -> v = sqrt(2*80/m) = sqrt(160/0.4) = 20m/s