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# M2 centre of mass Watch

1. Where am I going wrong?
Q4i

http://m.imgur.com/80f1TRr,9qg3DOH
2. (Original post by Super199)
Where am I going wrong?
Q4i

http://m.imgur.com/80f1TRr,9qg3DOH
What is the general formula for the CoM of a semi circle?
3. The centre of mass of the circle is 4r/3pi from the base. That's (r- 4r/3pi) + 3 from the side CD.
4. (Original post by Marxist)
What is the general formula for the CoM of a semi circle?
I thought it was 2rsinalpha/3alpha. Where alpha = pi/2?
5. (Original post by Super199)
I thought it was 2rsinalpha/3alpha. Where alpha = pi/2?
That is the formula for a sector of a circle - the general formula for the CoM of a semi-circle is 4r/3pi
6. (Original post by Marxist)
That is the formula for a sector of a circle - the general formula for the CoM of a semi-circle is 4r/3pi
That's what he's said. "where "
7. (Original post by Zacken)
That's what he's said. "where "
Oops yes. I was up late to help this guy for some odd reason - I barely read the reply properly. Sorry OP.
8. (Original post by Zacken)
That's what he's said. "where "
I don't see the point of using 2rsinaplha/3alpha though for this question when you can see that it's a semi-circle and therefore 4r/3pi.
9. (Original post by Super199)
I thought it was 2rsinalpha/3alpha. Where alpha = pi/2?
What is the distance from the CoM of the semi-circle to CD? That's where you're going wrong.
10. (Original post by Marxist)
What is the distance from the CoM of the semi-circle to CD? That's where you're going wrong.
I thought it would be 3+ com of semi
11. (Original post by Super199)
I thought it would be 3+ com of semi
Are you sure? That's not correct. What's the distance of the CoM from AB?
12. (Original post by Marxist)
Are you sure? That's not correct. What's the distance of the CoM from AB?
dw I've done it now. 6-com of semi circle is what you could have said at the start
13. (Original post by Super199)
dw I've done it now. 6-com of semi circle is what you could have said at the start
Good and nah, it's better to get the answer out of someone than to tell them the answer.

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