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AQA A Level MM2B June 2016 Q8

Hello!
I'm having trouble understanding this question from AQA M2 June 2016 Q8.

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I understand that energy must be conserved:

Initial EPE due to PR + GPE at A = work done against friction + work done in extending PQ + work done extending PR

I don't understand why we must include that last term for the work done in extending PR. Surely PR stops being taut once P passes (R + 6) m, and P does not travel far enough past R for it to become taut again?

Can't think how I could explain this more clearly, sorry it's a bit hard to follow.
(edited 6 years ago)
Original post by etothexdx
Hello!
I'm having trouble understanding this question from AQA M2 June 2016 Q8.
I understand that energy must be conserved:

Initial EPE due to PR + GPE at A = work done against friction + work done in extending PQ + work done extending PR

I don't understand why we must include that last term for the work done in extending PR. Surely PR stops being taut once P passes (R + 6) m, and P does not travel far enough past R for it to become taut again?

Can't think how I could explain this more clearly, sorry it's a bit hard to follow.


You're right that it doesn't travel that far, but then again, it doesn't even travel far enough for PR to become slack in the first place.

By including the PR term, you are implicitly assuming that 0<x<50 < x < 5 so that PR is taut (note that xx is the measurement from A down the slope, so by this condition we get that PR always has EPE)
(edited 6 years ago)
Reply 2
Original post by RDKGames
You're right that it doesn't travel that far, but then again, it doesn't even travel far enough for PR to become slack in the first place.

By including the PR term, you are implicitly assuming that 0<x<50 < x < 5 so that PR is taut (note that xx is the measurement from A down the slope, so by this condition we get that PR always has EPE)


Thanks, that clears it up perfectly. Obviously had a mental block and didn't consider that PR doesn't stop being taut. Thanks very much for your reply!

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