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# M3 - ring in circular motion watch

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1. Just did M3 January 2007 which was really easy apart from one question, Q5.

can someone explain for 5. a) why there is a tension acting horizontally towards the centre of the circle, and how we are expected to know that it is the same magnitude as the tension in the string?
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2. (Original post by 雷尼克)
Just did M3 January 2007 which was really easy apart from one question, Q5.

can someone explain for 5. a) why there is a tension acting horizontally towards the centre of the circle, and how we are expected to know that it is the same magnitude as the tension in the string?
Look at the diagram, the string isn't going just from A to the particle, it's also going from the particle to B - as I've highlighted below - the red bit is part of the string. It has simply been threaded through the ring.

The tension is the same because it's a smooth ring. (or because it's an inextensible string).

This question is what I like to call the "circular motion equivalent of M1 pulley".

Edit: let me guess, you're "not going to understand any of this".

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