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# M2 centre of mass, quick angle question. watch

1. Hi,

I'm a bit confused about the triangle that they've used to work out the angle. To me it looks like the angle between BD and the horizontal but the question wants to the vertical. Please could someone explain why this triangle is used.

Thanks
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2. (Original post by VioletPhillippo)
Hi,

I'm a bit confused about the triangle that they've used to work out the angle. To me it looks like the angle between BD and the horizontal but the question wants to the vertical. Please could someone explain why this triangle is used.

Thanks
For a lamina suspended freely from a point P under gravity and in equilibirum, the centre of mass always lies on the vertical through P.

So asking for the angle between BD and the vertical is the same as asking for the angle between BD and BG since G is the centre of mass and the lamina is suspended from B.

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