# How to solve for moment with uniform distributed load

How do we get the moment when there's a distributed load?

For example, the picture below has 9[(60)(18)]. I get that 60 is the distributed load and 18 is the total length of the load, my question is how did it get the 9?

The resultant of distributed loads always acts on the centroid of the distributed load geometry, here the distributed load is uniform so its centroid lies half the way. If the distributed load varies linearly from zero at one end to a maximum value at the other end, then its centroid would lie at $$\frac{1}{3} L$$ from the "max load" end and $$\frac{2}{3}L$$ from the "zero load" end, with $$L$$ the side length.