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# M2 centres of mass (SOLVED) watch

1. helpppppppp anyone pls?

for question a) isnt for x direction:
3 x M + 6 x 6m = x(8+M) ?????

how to find x?
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2. Part a) says STATE the distance; that usually, and in this case, means no working out is required.

Hint: consider symmetry.
3. (Original post by seniocs)
is the answer for part A, 2.4cm?? since i know that centre of mass must lie in the middle, and use tan a = 1/x....
Why do you think that? What's the line of symmetry; in terms of the weights? How far is it from AD?
4. isnt line of symmetry between AD and CB?
5. (Original post by seniocs)
isnt line of symmetry between AD and CB?
Yes, and it's midway.

Hence the distance from AD is just half AB
6. but... isnt that only true when the retangular metal plate is uniform? i mean... won't the centre of mass change if the plate has been loaded at points ABC and D?
7. (Original post by seniocs)
but... isnt that only true when the retangular metal plate is uniform? i mean... won't the centre of mass change if the plate has been loaded at points ABC and D?
The centre of mass will change yes, but the additional masses are arranged symmetrically about that line of symmetry, hence the centre of gravity will still be on that line of symmetry, albeit not in the dead centre.
8. oh... i hv just realized i was putting the masses at wrong points... that makes sense now... thx
9. so for the last question, do I take moments from A, and do:

1.25(M + 8m) = 1 x M + 2 x 6m...

then M = 8m???
10. (Original post by seniocs)
so for the last question, do I take moments from A, and do:

1.25(M + 8m) = 1 x M + 2 x 6m...

then M = 8m???
You're actually taking moments about the line AB, aside from that, yes, that's correct.
11. thx very much..., btw, im new here... so i am supposed to close this thread once my question been solved or shud i just leave it here?
12. (Original post by seniocs)
thx very much..., btw, im new here... so i am supposed to close this thread once my question been solved or shud i just leave it here?
Most people don't bother to do anything further.

However, if you can amend the title to append SOLVED or something, it would save anyone who was trying to help, wasting time; if you do, you'll be one of the few :-)
13. Welcome to the considerate few.

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