Work-Energy Theorem – key concepts


Answer to question:

Work done by external forces (excluding gravitational force) is negative.

Since E_k=\dfrac{GMm}{2r} for satellites in orbit (where G is the gravitational constant, r is the radius of the orbit, M is the mass of the planet and m is the mass of the satellite), kinetic energy increases. Hence \Delta {E_k}=GMm(\dfrac{1}{2r_2}-\dfrac{1}{2r_1}) is positive.

Work done by gravity is positive as the radial displacement is in the same direction as that of the force. Work done by gravity = -\Delta{E_p}=-[-GMm(\dfrac{1}{r_2}-\dfrac{1}{r_1})]=2\times\Delta{E_k}

According to the Work-Energy Theorem, net work = work done by gravity + work done by external force = \Delta {E_k}. Hence, work done by external force = -\Delta{E_k}.

Seng Kwang

Leave a Reply