Physics

Physics

Problem

State the mathematical expression for the work done (or equivalently the change in gravitational potential energy) when a body of mass mm is moved from a distance r1r_1 to a distance r2r_2 from the centre of a planet of mass MM.
Intuitive explanation

Gravitational potential energy depends on the inverse distance between masses, not linear distance—this is the key difference from everyday potential energy like mghmgh. When you move a mass in a gravitational field, you're not just multiplying force by displacement; you need to integrate the varying gravitational force F=GMmr2F = \frac{GMm}{r^2} over the path. The work done equals the change in potential energy, which you find by evaluating the potential energy function at both positions and taking the difference.

Intuitive explanation

Gravitational potential energy depends on the inverse distance between masses, not linear distance—this is the key difference from everyday potential energy like mghmgh. When you move a mass in a gravitational field, you're not just multiplying force by displacement; you need to integrate the varying gravitational force F=GMmr2F = \frac{GMm}{r^2} over the path. The work done equals the change in potential energy, which you find by evaluating the potential energy function at both positions and taking the difference.

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