I'm sure there's an obvious answer to this, but this problem has been confusing me for some time. Imagine there were a massive object attached to the end of a crankshaft. A force is applied to accelerate the crankshaft, causing the mass to oscillate. Assuming there is no friction, what would happen to the system when the force is no longer applied? It shouldn't be possible for it to slow down if there's no energy loss, but wouldn't it require a force to continue to accelerate the mass back and forth?