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# M2: Centre of Mass - Uniform Laminae watch

1. Sorry for the crappy diagram

Find the centre of mass

Which way round it is?

Do you take into account the negatives or do you ignore them?

ANS (3 2/3, 0)
2. The mass of B can't be negative, so it's 4.5 not -4.5.

Seeing as you've set your axis up the way you have, the CoM of A would be positive in the y-direction (above the x-axis) and the CoM of B would be negative in the y-direction (below the x-axis). In your table you should put the positive values as positive and the negatives as negative (apart from weight as you've wrongly done, an object can't have a negative weight).

So for both A and B, the "x-axis" value will be the same (it is one of the two you've written) and the "y-axis" values will be opposites of eachother which is why they cancel out in the answer.
3. (Original post by primaverasnap)
The mass of B can't be negative, so it's 4.5 not -4.5.

Seeing as you've set your axis up the way you have, the CoM of A would be positive in the y-direction (above the x-axis) and the CoM of B would be negative in the y-direction (below the x-axis). In your table you should put the positive values as positive and the negatives as negative (apart from weight as you've wrongly done, an object can't have a negative weight).

So for both A and B, the "x-axis" value will be the same (it is one of the two you've written) and the "y-axis" values will be opposites of eachother which is why they cancel out in the answer.
ok so are x and y the same? or different?

I know it either x2/3 or x1/3, by I get confused by which one you use when?
4. (Original post by emmaxoxo)
ok so are x and y the same? or different?

I know it either x2/3 or x1/3, by I get confused by which one you use when?
The position of the centre of gravity of a right angled triangle is 1/3 of the way up or across from the right angled corner (assuming the two edges are along the axes).

So for A. The right angle corner is at (3,0).

The length of the triangle in the x direction is 3, so one-third of that is 1, and the direction from the right angled corner is towards the origin, so the x-coordinate of the centre of gravity for A is at 3-1 = 2.

The length of the side in the y direction is again 3. One-third is 1, in the direction from the right angled corner is that of y positive, or "upwards". So the y-coordinate of the centre of gravity for A is 0+1 = 1

Hope that helped.

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Updated: February 8, 2010
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