1. The problem statement, all variables and given/known data A person exercises daily by lifting weights. When he lifts a weight, he is exerting a force on it to displace it. This means he is doing work and therefore burning calories. But when he lowers the same weight, does he do any work and burn calories? 2. Relevant equations W = F x d W is -ve when F and d are in opposite direction W is +ve when F and d are in same direction 3. The attempt at a solution When weight is lowered When weight is lowered, he still applies some force on the weight and therefore does work. But not sure if he burns calories. The person exerts an upward force on the weight while it's displacement is in a downward direction, which means that work done by this force is negative. I cannot understand how negative would mean he had to supply energy. When weight is raised When he raised the weight then force applied by him was in same direction as displacement and therefore work done on weight by him was positive which meant he had to supply the energy i.e. calories to do this work.