Alevel Maths Mechanics Questoon

Would anyone be able to help me with this question? I have been stuck on it for ages and keep getting the coefficient of friction as 0.52 and the answer says it’s wrong.

The answer uses sin when working out moments but I thought it should be cos.

Any help would be appreciated !
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Reply 1

Teribblestudent

I think there might be a mistake in the question, I can vaguely remember my teacher saying something about it

Reply 2

mqb2766

If you extend the weght mg upwards, then the perpendicular distance from B (to the left of B) is
a sin(alpha)
for example

(edited 10 months ago)

Reply 3

zoe2404

Original post by mqb2766

If you extend the weght mg upwards, then the perpendicular distance from B (to the left of B) is
a sin(alpha)
for example

ohhh I’ve just realised alpha is from B not from A ! I need to read the question properly. Thank you

Reply 4

mqb2766

Original post by zoe2404

ohhh I’ve just realised alpha is from B not from A ! I need to read the question properly. Thank you

Lesson 0, read the question :-).

If you were happier about using the angle with the horizontal, you could have done it in terms of that and used the complementary
sin(90-alpha) = 4/5
cos(90-alpha) = 3/5
but a diagram with stuff marked on properly is the main thing.