first thing, that you might wanna know is, that i am not from an english-speaking country, so please, excuse my grammar...;-) 1. The problem statement, all variables and given/known data hi i have a problem here and i already know the solution, but the thing is, that i do not understand it. let's see: the first question was: the moon rotates around the earth and now some evil person stops the rotation. so the moon is still that far away from the earth as he was before but now his velocity is 0. The question now was: how long does it take until the moon crashes into the earth? And the solution was, that you use kepler's third law and set: T²/(r/2)³=t²/r³ i should explain that " t " is the time the moon would ordinarily need to go one time around the earth and r is the distance between the earth and the moon and T/2 is the solution. That seems okay to me, because you only want to know how much time it takes to go one way to the earth and not back again. but why do they set r/2 in the denominator? i always thought that there you should use the aphelion, which is the farthest distance between the central object and the travelling object. but in this case, the starting position should be the farthest position, as the moon would not go farther from the earth than this initial distance, or am i wrong?