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# M2 centre of mass/moments watch

1. Attachment 702158
Stuck on past paper question would really appreicate some help, ive done the first part but am stuck on the second bit, the mark scheme is saying to take moments about A but in the mark scheme the perpendicular distance is root2xa which is the distance to O not A so im confused
2. it would help if you gave a link to which past paper and question you are actually stuck on?
3. (Original post by _RobbieL_)
it would help if you gave a link to which past paper and question you are actually stuck on?
sorry the pic didnt send first time
4. (Original post by uchihaitachi0023)
...
When it is hanged from A, you have two forces acting directly downwards; the CoM of the lamina, and the particle at D. You are told that OA is vertical, so I here extended the direction of the force backwards for you and so when you are taking the moments about A, you want the length of the red line as it's perpendicular to the direction of the force. Upon basic inspection, you should find that the red line is the same length as OD. And OD is just length . Get it?

5. (Original post by RDKGames)
When it is hanged from A, you have two forces acting directly downwards; the CoM of the lamina, and the particle at D. You are told that OA is vertical, so I here extended the direction of the force backwards for you and so when you are taking the moments about A, you want the length of the red line as it's perpendicular to the direction of the force. Upon basic inspection, you should find that the red line is the same length as OD. And OD is just length . Get it?

goddamn cant believe i didnt see that, nice one mate saved me a lot of time
6. (Original post by RDKGames)
When it is hanged from A, you have two forces acting directly downwards; the CoM of the lamina, and the particle at D. You are told that OA is vertical, so I here extended the direction of the force backwards for you and so when you are taking the moments about A, you want the length of the red line as it's perpendicular to the direction of the force. Upon basic inspection, you should find that the red line is the same length as OD. And OD is just length . Get it?

i know its a big ask but could u draw on diagram again to show how the centre of mass would be perpendicular to A,im having trouble visualising it
7. (Original post by uchihaitachi0023)
i know its a big ask but could u draw on diagram again to show how the centre of mass would be perpendicular to A,im having trouble visualising it
You want to find the red line. You know the pink line's length. The angle between the pink and the black line is...?? Hence then use cosine to work out the red line.

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