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A graph plots energy versus orbital radius for a body in circular planetary (gravitational) motion, showing three curves A, B and C. Which option correctly identifies them?

1. A is kinetic energy, B is total energy and C is potential energy.
2. C is total energy, B is kinetic energy and A is potential energy.
3. C and A are kinetic and potential energy respectively, and B is total energy.
4. A and B are kinetic and potential energy respectively, and C is total energy.

Correct answer: A is kinetic energy, B is total energy and C is potential energy.

Solution

For a gravitational circular orbit, kinetic energy K = +G*M*m/(2r) lies above the axis (positive), potential energy U = -G*M*m/r is the most negative, and total energy E = K + U = -G*M*m/(2r) sits between them. So the topmost curve is kinetic (A), the lowest is potential (C), and the middle is total (B).

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