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I am trying to understand what forces act on which objects, and which forces are pairs with regards to newtons 3rd law.

So imagine a rope attached to a bob is being spun round in a circle parallel to the ground (I know it can't be perfectly horizontal but just for ease lets say it does). I know the tension acts as the centripetal force on the bob. But does this directly act on the bob because the tension acts in the rope so surely it acts on the rope? And what would be the equal but opposite force be of the centripetal force?

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Let's say this whole setup lies on a smooth table so we can safely ignore gravity. Now answering your questions:

In reality, every segment of the rope is pulling on the segment attached to it (and vice versa according to the third law). But usually, we assume the mass of the rope to be zero so the tension in the rope is same throughout. You could think of the rope as a long chain of massless blocks each pulling on the adjacent blocks.

The equal and opposite to the centripetal force that the rope applies on the bob is the force which the bob applies on the rope (which equals the tension in the rope). That is the force you will feel when you swing your arm trying to spin a bob very fast.

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  • $\begingroup$ regarding the force that acts on the string which has been exerted by the bob, where abouts does this act? Ive read that "the string exerts a centripetal force on the bob (its Tension) towards the centre, so in accordance to Newton's Third Law, the bob must exert a force on the string away from the centre which acts at the support" the support being the centre of the circular motion where the string is attached. is this right? $\endgroup$ – DLB May 12 '17 at 18:46
  • $\begingroup$ Yup. The line joining the support to the bob (ie along the stretched rope) is the direction of the force. $\endgroup$ – Abhijeet Melkani May 12 '17 at 18:48

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