Gravitational Field

Question 1

A student wants to calculate the change in KE of a satellite launched from Earth to its orbit.

The student calculates the initial and final KE using KE=GMm/2r.

Suggest and explain whether the student is correct.

[Extension: state another formula that can be used for satellites in orbit but not on Earth]

Answer 1

KE=GMm/2r is only applicable for masses in orbit, where the only force on a satellite is gravitational force which provides centripetal force.

On the surface, centripetal F is provided by the difference between height and normal contact force from the surface on the satellite.

Hence, satellite is not in orbit, so the expression is not applicable

[Extension: Total Energy = -GMm/2r
Note: PE = -GMm/r all can use]