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# M2 CoM watch

1. Some custom homework our teacher set us.

Not very good at this topic but here's my horrendous working which is definitely wrong. Please help. It's part B i need help with.

2. (Original post by will'o'wisp)
Some custom homework our teacher set us.
You have the distances of the CofM for the lamina the wrong way round; should be 5 from AD and 15 from DC.
3. (Original post by will'o'wisp)
Some custom homework our teacher set us.

Not very good at this topic but here's my horrendous working which is definitely wrong. Please help. It's part B i need help with.
I haven't checked all your working but the mass of ABCD is M, not 300. And I can't see M anywhere in your working

You're getting mixed up with finding the COM of compound shapes where the area of each shape is proportional to its mass.

Try again using M and post your working if you get stuck.
4. @OP

If you put the items in my previous, and notnek's (PRSOM) posts toegether, it should work out.
5. (Original post by ghostwalker)
You have the distances of the CofM for the lamina the wrong way round; should be 5 from AD and 15 from DC.
(Original post by notnek)
I haven't checked all your working but the mass of ABCD is M, not 300. And I can't see M anywhere in your working

You're getting mixed up with finding the COM of compound shapes where the area of each shape is proportional to its mass.

Try again using M and post your working if you get stuck.
But doesn't the term "uniform" mean area=mass?
(Original post by ghostwalker)
@OP

If you put the items in my previous, and notnek's (PRSOM) posts toegether, it should work out.
Tried again
6. (Original post by will'o'wisp)
But doesn't the term "uniform" mean area=mass?

No. It means the density of the object is the same throughout the entire object.
7. (Original post by RDKGames)
No. It means the density of the object is the same throughout the entire object.
So what tells me when you to use the area of the shape as its mass?
8. (Original post by will'o'wisp)
So what tells me when you to use the area of the shape as its mass?
What you're saying doesn't make sense though. If I have perfect uniform sphere, the area of it does not need to strictly equal its mass.
9. (Original post by RDKGames)
What you're saying doesn't make sense though. If I have perfect uniform sphere, the area of it does not need to strictly equal its mass.
But when it does, what piece of info do i use to tell me to use its area as its mass?

question 1 of this homework i used the area as its mass

here
10. (Original post by will'o'wisp)
Tried again
Yep, I got M=4m, hence m=M/4
11. (Original post by ghostwalker)
Yep, I got M=4m, hence m=M/4
Thanks!
12. (Original post by will'o'wisp)
But doesn't the term "uniform" mean area=mass?
Uniform, in the case of a thin lamina, means mass per unit area is constant.

In the first question you are told the mass M. But if you didn't know it, you would have to use 300k, where 300 is the area and k is the mass per unit area, hence the total mass is 300k.
13. (Original post by ghostwalker)
Uniform, in the case of a thin lamina, means mass per unit area is constant.

In the first question you are told the mass M. But if you didn't know it, you would have to use 300k, where 300 is the area and k is the mass per unit area, hence the total mass is 300k.
ok

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