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M3 Circular Motion HELP!

Question 7 from this paper: http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202009/6679_01_que_20090129.pdf

Showing that cos (theta) = 3/4, letting speed at C = v.

I have found using Energy, from A to C that v^2 = 2.25ga - 2gacos(theta)

But in the markscheme, it uses mgcos(theta) + (R) = (mv^2)/a

I thought the motion at C would be R - mgcos(theta) = ma?

Can someone explain to me why this is please? (mark scheme is here: http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202009/6679_01_rms_20090312.pdf )

Thanks!

Reply 1

Yeezy2k

I don't get why the mark scheme has that +R in brackets. Just apply N2L to the situation resolving towards the centre of the sphere.

mx-double dot = mgcos(theta) - R
Then rearrange for R and x-double dot = v^2 / r

R = mgcos(theta) - mv^2 / a

Using conservation of energy to find an expression for v in terms of u , g , theta and a will get you your answer

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(edited 9 years ago)

Reply 2

the bear

it looks like they put +R when it should be -R... a misprint

Reply 3

BeardedCow

They add the normal reaction because it doesn't matter. These are simplified mark schemes from the ones they actually use, in the proper one they would have said that it didn't matter if it was + or -.