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Possible Duplicate:
Rolling (without slipping) ball on a moving surface

Apparently I didn't log in properly when I asked a question this morning: Rolling (without slipping) ball on a moving surface and now I couldn't go back to leave comments. I am starting over here, so apologies for the confusion and inconvenience.

The user Raindrop left an answer to the above-mentioned question and I think he pointed out where I got confused. In the case where the surface (I am imagining more of a 2D arc swinging from side to side) is moving as the ball (appear as a simple circle) is rolling on it, the sum of force is not simply $F=ma=mgsinθ−f$, where $f$ is the frictional force acting on the ball. It was pointed out that I am missing the normal force. So here's my question: what is this normal force? I thought the normal force is what the surface exerts on the ball due to the gravitational force the ball is exerting on the surface. I thought the normal component of that force would be cancelled, leaving the $mgsinθ$ component only. Could someone explain that to me please? Much appreciated.

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    $\begingroup$ I've merged your old (unregistered) account with your new one and you should have control of the original version of this question again. $\endgroup$ – dmckee --- ex-moderator kitten Dec 20 '12 at 20:44