If you havent seen this video before then go watch it :D Question: Answer: It is pretty hard to imagine it as a spring and here is why: 1) At (o) the spring should move by its inertia not by any force. If we look at the planet we can summarize that there are 2 points (As I think) that could have this applied to it. First, At the center of the planet, Everything is balanced out and nothing is affecting on it ( Note: I think he mentioned that the gravity is acting from the closest point to r = 0 So the gravity should be at its maximum?). The second point, is really far off the planet (There is still a little bit of gravity) 2) At any of (A) or (B), The force that is acting on the spring should be as big as possible. Well here isnt.. Lets assume that A and B are on the planet's surface( and you have to assume that if you want to calculate the time that the particle will take to cross the planet to the other side) Well as we can see from newton law that: F = G m M/ r^2 Whenever we go away from an object the force becomes less. Also when the particles goes deeper in the planet the force will become bigger and bigger because you are getting close to the center of (Gravity) force. And that is not simple harmonic motion.. (From what I studied, Maybe I haven't encountered a lot of states of simple harmonic so I just want to understand this problem).. So is there is any other explanation or something I missed out in the video?