7.13: Work Done by Gravity
Gravitation is one of the four fundamental forces in nature. The force between objects on Earth and Earth itself is called gravity.
Like other forces, gravity does work on an object if it displaces it toward the Earth's center. In this case, the work done by gravity is said to be positive. If an external force acts on the object against the pull of gravity and manages to lift it away from the Earth's center, work is done against gravity. In this case, the net work done is said to be negative. If an object moves along the Earth and has no displacement toward or away from the Earth's center, then no work is done by or against gravity.
Of course, no object on Earth is a particle. The center of mass framework developed in studying composite objects bypasses this problem. A single particle of mass equal to the object's total mass can be considered to be present at the object's center of mass before considering the effect of external forces, including gravity.