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# STEP Maths Mechanics (Pulley Question) watch

1. This is question 11 from the STEP 1 2014 exam.

Although I have been able to answer this using my own method, I have a question about part (ii).

When looking at worked solutions, they have called the tension in the string between M and p1 T and the tension in the string between m1 and m2 T/2. However, if the tension was T/2 then surely the resultant force on the pulley p1 would = 0 as it would be T - 2(T/2). This would then mean the pulley is not accelerating. However this is not true as the pulley would have the same magnitude of acceleration as the particle M.

Perhaps it because the particles are light and so we can treat it 'as part of the string' and so the tension must become T/2 so that the overall tension is T?
2. (Original post by GCSEsThen)

This is question 11 from the STEP 1 2014 exam.

Although I have been able to answer this using my own method, I have a question about part (ii).

When looking at worked solutions, they have called the tension in the string between M and p1 T and the tension in the string between m1 and m2 T/2. However, if the tension was T/2 then surely the resultant force on the pulley p1 would = 0 as it would be T - 2(T/2). This would then mean the pulley is not accelerating. However this is not true as the pulley would have the same magnitude of acceleration as the particle M.

Perhaps it because the particles are light and so we can treat it 'as part of the string' and so the tension must become T/2 so that the overall tension is T?
If the net force wasn't 0, the pulley would have to have infinite acceleration (since acceleration = F/m and m = 0). Conversely, if the net force is zero, the same equation gives 0/0, which is indeterminant. So basically we go for the scenario which at least *allows the possibility* of the pulley having finite, non-zero acceleration, as opposed to the scenario that implies the certainty of infinite acceleration.

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