# Quick question solid mechanics

1. Dec 13, 2012

### Marchese_alex

Ok, so the formula I know to find weight is W=mg. Why is it that when doing a sum of forces in z they multiply (distance)(mass)(gravity) and not simply sum the gravity force that is mg?

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2. Dec 13, 2012

### SteamKing

Staff Emeritus
Because the pipe has a mass of 12 kg per meter of length. Read the problem statement carefully.

3. Dec 13, 2012

### Marchese_alex

ooohh... so if it said only 12 kg, then I would only use mg?

4. Dec 13, 2012

### Marchese_alex

why when calculating sum Mx=0 isnt (60)(.4)(.2)? isnt moment=force time the distance to where the moment is being taken?

Last edited: Dec 13, 2012
5. Dec 13, 2012

### SteamKing

Staff Emeritus
The 60 N forces are applied 400 mm from the x-axis. There are two 60 N forces acting in opposite directions. The moments produced by these forces about the x-axis cancel out.