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Chelsea12345
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http://pmt.physicsandmathstutor.com/...20M1%20OCR.pdf
Question 1 : part iii
Working out :
Down the plane
28 - P + 24 = 0
52 - P =0
P = 52N.

Up the plane :
-28 - F + P =0
-28 - 24 + P = 0
-52 = P

Mark scheme attached below:
http://www.physicsandmathstutor.com/...20M1%20OCR.pdf

Why does the mark scheme does not seem to have included friction despite it saying in part ii that friction is present?
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ThomH97
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The mark scheme has included it, it just didn't explicitly say it. Stating things more explicitly as you have is better if you're unsure, as it shows you have the understanding.

I think you're fine with part a, but your answer to part b looks like you've put a minus sign in front of everything, then missed one when taking the P across the equals. With part b, you still have the same forces acting, but it is only friction that has its direction reversed (weight still down, P still up) - since the box is on the point of moving up the plane, friction must be acting downwards, in the same direction as the weight component.
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