If I keep a rotating disc along the x-y plane and apply a force along the same plane in line with the disc's centre of mass, assuming no friction and no gravity, will the disc move along the direction of the force or in some other direction?


closed as unclear what you're asking by sammy gerbil, Gert, user36790, Wolpertinger, Neuneck Aug 19 '16 at 10:06

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  • $\begingroup$ It's ambiguously phrased. Would "If I keep rotating a disc about the $z$-axis and apply a force in the $xy$-plane through the disc's centre of mass, [...]" not be a better way of putting it? Rotation is always about an axis. $\endgroup$ – Gert Aug 18 '16 at 16:28
  • $\begingroup$ Please can you explain more clearly what you mean, and what you think will or should happen? What is the significance of the disk? Don't you expect the same effect using a stone or a point particle? Is your question the same as asking what will happen to a football if you roll it horizontally off a cliff? $\endgroup$ – sammy gerbil Aug 18 '16 at 16:33
  • $\begingroup$ Possible duplicate of physics.stackexchange.com/q/35932 $\endgroup$ – user108787 Aug 18 '16 at 16:52

The disk rotates only in the direction of applied force, not in any other direction. You can answer this with very basic concepts of physics like conservation of momentum, energy, etc. But, if the friction is taken into consideration, two cases arise.

Case 1): The applied force is due to a field:

Even in this case, the answer is same.

Case 2) : The applied force is mechanical:

This case includes examples like pushing the disk with a stick, pulling it, etc.

In this case, due to the friction, the disk moves in other direction. (Daily life experience)

  • $\begingroup$ This does not really answer the question, it's more a comment than an answer. For example "You can answer this with very basic concepts of physics like conservation of momentum, energy, etc." suggests the OP answers his own question! The question is also about no friction. It would also be better to wait until the OP has cleared up the ambiguity in his question. $\endgroup$ – Gert Aug 18 '16 at 17:29

You applied force to the center of the disk, thus you did it in its center of mass.

Thus, you didn't apply any torque. Thus, you don't change its angular momentum. Thus, the disc will rotate to the same direction.


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