有關於作負功的問題

When the force exerted on the cord equals to the weight (mg) of the object, there is no net force acting, the object is moving down at a constant speed. There is no increase in kinetic energy of the object.

Since you need to exert a force in order to make the object falling at constant speed, the production of such force increases the metabolic rate of your body , which leads to an increase of heart beat and blood flow...etc. The energy required for such increase of metabolism comes from the loss of potential energy of the falling object.