The work done on an object by an external agent in bringing it closer to a planet is given by mΔV. So, when the object is brought closer to a planet, the work done by the external agent is negative. But why is it so? What does it mean? The kinetic energy of the object increases, does that have anything to do with the explanation?
When an object is moved away from a planet, the work done by the gravitational force is negative, but of the external agent it is positive. I am confused on how we can determine when the work done by the gravitational force and external agent is negative and when it is positive.
Great thanks for your help!
Best Answer
This may be a source of confusion. As stated, it is not necessarily true. An external agent is not constrained to provide either positive or negative work in such a scenario.
The easiest to consider is that there is no external agent, so that there is no change in total energy. In that case as the object free-falls closer it will gain KE. Because the total energy is conserved the PE must decrease.
To determine the work of an external agent you need to specify what the agent does. Often the external agent acts to keep the KE constant. In that case, the PE decreases, but the KE does not change. This means that the earth-object system loses energy. Hence, the external agent does negative work on the system.
Always look at the energy. Positive work done on a system increases the energy of the system. Negative work done on a system decreases the energy of the system. Work done by a system has the opposite effect.