Hi. I have a question about the best way to explain why you can't solve a physics problem the way one of my students tried to. Here's the problem: Lauren is running at 3.2 m s-1 when the road starts to go downhill. When she reaches the bottom of the hill she has dropped a vertical distance of 15 m. If all the gravitational potential energy she loses on the way down had been changed to kinetic energy, what would Lauren’s speed have been at the bottom of the hill? Instead of calculating the total kinetic energy at the bottom = Ek(top) + Ep(lost) and finding v from that (= 17.6 ms-1), my student calculated the "added speed" from the gravitational potential energy lost (17.3 ms-1) and then added the initial speed (3.2 ms-1) to get 20.5 ms-1. What's the best way of explaining why this doesn't work out? Thanks for any help.