Mechanics Tutorial 5  Moments and Bridges
At its simplest, a bridge is a plank of wood placed between two supports. As the plank of wood has a mass, it must also have a centre of mass. At this level, we will assume that the plank is totally regular, and its centre of mass is exactly in the middle.
Notice that at each end there is a pivot, around which we will need to take moments. Even if there were no actual pivot mechanism, we treat the bridge as if there were. If the plank broke in the middle, the right hand half would turn about the right pivot, and the left half around the left pivot.
So let us have a look at the moments around each
pivot:

Worked Example

Now let's do an example with two vehicles on. The principles are the same.
Worked Example
On the bridge shown in the diagram above, what are the forces on the abutments A and B? 
To work out the force on A, we need to take moments about B
To work out the force on B, we need to take moments about A:
Note that we have given our answer to 3 significant figures. No more than this is needed. 

Ladders
A ladder uses the ideas of turning moments.