First slide
Gravitaional potential energy
Question

A satellite is moving in a circular orbit at a height R above the surface of earth with kinetic energy K. If the satellite is suddenly stopped, with what kinetic energy will it strike the surface of earth? 

Easy
Solution

Initially, K=GMm2(2R)K=GMm4R

when the satellite is falling on to the surface of earth, conserving energy, 

K1+(GMmR)=0+(GMm2R)

K1=GMm2R=2K

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