We use two different systems of objects to do the energy calculations with diffe

Question

We use two different systems of objects to do the energy calculations with different energy formulas for each system.

Why do we include the gravitational potential energy in one system's energy formula but not the other?
Choose the one incorrect explanation.

A. Because with the Earth part of the system of objects, gravity is an internal force and has potential energy.

B. Because the vertical forces on the cart (the force of gravity and the normal force) cancel each other out and have no effect on the energy of either system.    

C. Because when the Earth is not part of the system of objects, gravity is an external force and does work on the system.

Explanation / Answer

Here option A is incorrect which is "Because with the Earth part of the system of objects, gravity is an internal force and has potential energy."

Here option C is correct which is "Because when the Earth is not part of the system of objects, gravity is an external force and does work on the system."

Explanation: We include the potential energies of external forces which are conservatives not the internal forces or the forces which are non-conservative.

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