M2 - Centres of Mass question

Question (see image)
M2 June 2012 Q4b

So I understand taking moments etc and I've done questions similar to this before.
However,
I have always thought that the weight of the masses acts PARALLEL to the vertical.
In this question, from the mark scheme and the model answers, it shows that when taking moments about G, the weight is seen to continue acting downwards (not parallel to the vertical)

Why is this?

Weight always acts down (towards the centre of mass of the earth)


Posted from TSR Mobile

So I understand taking moments etc and I've done questions similar to this before.
However,
I have always thought that the weight of the masses acts PARALLEL to the vertical.
In this question, from the mark scheme and the model answers, it shows that when taking moments about G, the weight is seen to continue acting downwards (not parallel to the vertical)

I haven't seen the answers that you're referring to, but I wouldn't be taking moments to solve this. I'd just calculate the position of the combines centre of mass and say that when hung from S, SG will be vertical.

What's the difference between downwards and vertical?
How is 'parallel to the vertical' different to 'vertical'? I don't see the difference.

Yep so SG is vertical, wouldn't the directions of the weight be parallel to the vertical.
Diagram taken from the model answers
So in the diagram below, the dotted line is the vertical, but as you can see it shows the weight still acting "downwards" on the paper

So I think that the weights should be acting parallel to the vertical and I don't see why I'm wrong

If the dotted line is the vertical, the weight would act parallel to that


Posted from TSR Mobile

So the model answer's diagram is wrong, but it gives the correct answer.. its the same in the official mark scheme :/

So the model answer's diagram is wrong, but it gives the correct answer.. its the same in the official mark scheme :/

Yes the diagram is wrong

It's not wrong, but it isn't a diagram of suspending the object from S either.

What they've drawn is what happens when supporting the object at G. They're essentially using the given angle / position of G to calculate k.

I see,
So how does it make sense to take moments if the weights aren't perpendicular to the line OG?

From the angle they give, you calculate the position of G (combined for m and km).

Imagine supporting the combined object at G - it will balance. You know that moments about G will be 0 (actually about anywhere, but you eliminate the reaction at G this way).

When you hang the object by S, you know that SG will be vertical, hence how you calculate the position of G from the angle given.

You can draw the diagram with SG (and all the weights) vertical, but you'll have to add some trig (that cancels), so it's a little more work.