# Centre of Gravity and Centroids

1. Dec 4, 2015

### DiamondV

1. The problem statement, all variables and given/known data
http://puu.sh/lJgzB/5b877de108.jpg [Broken]

http://puu.sh/lJgAR/c28393f051.jpg [Broken]

Can someone please explain these two slides to me? Especially the first one, It seems to be summing the Moments in y and Moments in x. The sum of the moments in y is xW. I understand that but then it says that xW is equal to Sum of X times Change in W. Thats what I dont get. This is like the first page of the notes, theres no other info given. This is all.
EDIT: Ive realised that we are assuming that the rigid body is made up of millions of tiny particles each at different distances(x) and different weights(W). So we sum it. But I still dont get why we use Delta W instead of W itself?
2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 7, 2017
2. Dec 4, 2015

### SteamKing

Staff Emeritus
Because W is the weight of the entire plate or piece of wire.

Last edited by a moderator: May 7, 2017
3. Dec 4, 2015

### DiamondV

Why is Change in W used then?(delta W)

4. Dec 4, 2015

### DiamondV

Or is the Delta W just signifying the weight of the individual tiny particle that when summed together(sigma) results in the weight of the the rigid body?

5. Dec 4, 2015

### SteamKing

Staff Emeritus
Right. $W = \int dW$