- 天同Lv 71 十年前最愛解答
From definition of [work done], any work done must involve a force and a displacement. If the force and displacement are in the same direction, the force does positive work.
On the contrary, if the force and displacement are in opposite direction, the force is said to do negative work.
1. An object falling under gravity, the weight (which is a force) of the object and the displacement of the object (which is the same as the displacement of the force) are iin the same direction. Hence, the weight does positive work. The work done by the weight (gravity) is changed into kinetic energy of the object.
2. An object is raised slowly and steadily from the ground to a certain height by an external force. In this case, the weight (which acts downward) and the displacement (which is vertically upward) are in opposite direction. Hence, the weight does negative work. Neagtive work done implies that energy is "taken out" from the object. Thus, there is no increase of kinetic energy of the object, as contrast to the increase in the first example. Just imagine that in the absence of the weight, the object should have an acceleration, i.e. a gain of kinetic energy, when acted upon by an external force.