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the problem isn't written in English originally, so there might be some slight mixing up in the terminology

A cylinder with mass M = 2 kg is attached to a spring and lying on a horizontal plane. (Imagine looking at it from the side, with the wall where the spring is attached forming a 90 angle with the plane. The spring extends to the cylinder, which will look like a circle from our point of view (we're looking straight through the cylinder axis))

The spring has a stiffness of k = 30 N/m. The cylinder is pulled slightly out from the equilibrium position and is then let go. It's rolling back and forth without slipping. What's the period T?

(No friction involved here)

Seemed pretty straightforward to me: We have the angular velocity (I think it's called) which is ω = [itex]\sqrt{k/M}[/itex]. The period is given by T = [itex]\frac{2∏}{ω}[/itex]

Values go in and voila: [itex]\frac{2∏}{3.87}[/itex] = 1.62 seconds. Problem is the solution set says 1.99 seconds!

Any suggestions? Does it have something to do with the relevant object being a non-slipping cylinder?

Thanks, J