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Why the object actually rotates ?

When I researched about this topic , I have gotten satisfying answer from here: Why do object rotate at all?

All the atoms and molecules of the object are connected with each other by intermolecular force. So when we apply force on the object other than its centre of mass these force oppose the motion and result in the rotation.

My question is : if all the atoms and molecules are bind by intermolecular forces , then why the half of the portion of the object moves in opposite direction (i.e , why the half the part of object rotates in the direction of force and other half rotates in the opposite direction of force)

So what am I missing here?

1 Answers1

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All the individual molecules are connected by strong bonds so each molecule wants to stay at a particular distance with respect to its neighbours. If you were to change this distance by a little bit it would cost a lot of energy.

So a natural question to ask is, which transformations can I apply to a set of points such that the distances between each pair of point stays the same? The answer is translations and rotations. If I apply an arbitrary force to a solid object you can expect the result to be a combination of translation and rotation. (or you would produce a sound wave which is basically an excitation of the bonds in the solid but that's not really relevant here).

To answer your question more specifically you can image a molecule of just two atoms connected by a bond. If you give one of the atoms a push at 90 degrees to the bond it will start rotating (and translating, i.e. moving). To rotate an object around a circle it needs a centripetal force and this force is provided by the bond between the particles. The momentum of each particle tends to move each particle further away from its center of mass but the centripetal force keeps them together, resulting in rotation.

AccidentalTaylorExpansion
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  • Imagine you push a rod from its right end , then it start to rotate. Why the part of the rod in the left side of COM rotates in opposite direction ?. Due to the intermolecular attraction it should move in the same direction . –  May 19 '22 at 09:24
  • @VaibhavTiwari If you're imagining pushing one end of a rod in free space, both ends of the rod will move in the direction you've pushed, one will just move more slowly, which causes a rotation on top of the translational motion. If you're imagining a rod that's fixed to pivot at its center, then the pivot exerts an opposing force to your push. – Jahan Claes May 19 '22 at 16:52
  • It mean that , the part of rod also moving I same direction , but due to slow speed it looks like it is moving in opposite direction. Am I right? –  May 20 '22 at 06:06
  • @VaibhavTiwari "Due to the intermolecular attraction it should move in the same direction" This is not exactly correct, the intermolecular forces can do whatever they want as long momentum is conserved, in other words as long as the center of mass keeps moving with the same velocity. Like I mentioned, a force that is directed at the center can cause a particle to rotate. – AccidentalTaylorExpansion May 20 '22 at 10:44
  • @VaibhavTiwari If you hit a rod in the right way you can have parts of it go "backwards", so in that case that part is not just slower but also moving in the opposite direction – AccidentalTaylorExpansion May 20 '22 at 10:46
  • @AccidentalTaylorExpansion , why it is moving in opposite direction? –  May 20 '22 at 18:20