✔ 最佳答案
你錯了。應該是轉緊的satellite要返地球,首先是要decrease gravitational potential energy。再因為r的減少,反而speed,因此k.e.會增加。
GMm / r2 = mv2 / r
Kinetic energy = GMm / 2r
而gravitational potential energy = -GMm / r
所以,total mechanical energy
= -GMm / r + GMm / 2r
= -GMm / 2r
是一個負數
如果satellite要返地球,可能是有work done against resistance,所以total mechanical energy decrease,r便會減少(total mechanical energy的value變得愈來愈負)
所以,gravitational potential energy便會減少。
但由於r的減少,kinetic energy (GMm / 2r)會增加。可是它增加的速率小於gravitational potential energy減少的速率,所以能量守恆並沒有違反。
By the law of conservation of energy,
Loss in G.P.E. = Gain in K.E. + work done against frictional force
所以,是由於satellite要do work against friction,而令total mechanical energy減少。繼而令r減少(即orbital radius減少),因此它的速度(K.E.)會變高。