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# M2 2015 help! watch

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1. I'm doing this question from the 2015 M2 paper& I have some small problems

for part (i), why do you take the gain in GPE at P (8*g*h/2) from the loss at Q (10*g*h) instead of the other way round?

and fot part (ii) (A), the mark scheme gives: Loss in GPE=Gain in KE to find the speed of Q, my question is why dont you need to consider the downward force at P?

2. (Original post by wamble)
for part (i), why do you take the gain in GPE at P (8*g*h/2) from the loss at Q (10*g*h) instead of the other way round?
Well, P gains GPE and Q loses GPE. You can either

(i) see that it's obvious that P gains less energy that Q loses. So the overall change is a loss and you can say the loss has magnitude P-Q (since the loss has to be a positive amount).

(ii) Do P-Q and notice it's positive so the loss is P-Q. Do Q-P and see it's negative (meaning it's a loss) so the loss is the + version of that.

and for part (ii) (A), the mark scheme gives: Loss in GPE=Gain in KE to find the speed of Q, my question is why dont you need to consider the downward force at P?
Because there is no force down the slope acting on P since it's smooth ramp/slope. That is, there is no resistance to friction or motion, the only two energies in play here are K.E and G.P.E.

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