Hello, I have a question about structural dynamics. The question is about a forced oscilliation. My question is about this picture, with the very short text: http://img17.imageshack.us/img17/5025/sd001.jpg [Broken] This is a forced oscilliation with the ground moving, and a mass inside a structure moving. Here they obtain the equation for the oscilliation by using Newton's in a particular way. But I would like to obtain the equation by looking at the force that the ground excerts on the frame instead. Can you guys please help me? It might be a little hard to really understand my question, so I will try to explain it with a very similar example. Please look at this rotating mass: http://img24.imageshack.us/img24/6738/sd002.jpg [Broken] You can here obtain the equation for the oscilliation allmost the same way as they did in the first picture. But you can also obtain it by looking at the sentripetal force on the rotating mass. By deriving the equation this way I really got more understanding about the problem. And I would like to understand the problem in the first picture better aswell. Help would be greatly appreciated.