Center of mass: What would make the truck tipover?

[okgo]

Homework Statement

http://screencast.com/t/ZTk3NDUxNjU

I need help with understanding number 18.

Homework Equations

Center of mase formula: m1x1+m2x2/(all masses)

The Attempt at a Solution

How do you tell if adding the weight will make the truck tipover.

I calculated that the center of mass would shift to 1.75m from where the load was added. Or .25 m from the axle. (axle 1)+(axle 2)/(the whole mass of the system)
2000*4.5+8000*1.5/12000=1.75m from where the new load was added.

 

[jack action]

Take the moment about the front axle to find resultant weight on the rear axle.

If the rear axle weight is greater than the total weight, then the truck will tip over. Because this will mean that the reaction at the front axle will be downward (like a chain pulling the axle down)

 

[okgo]

hmm I don't quite understand.
would the equation be
x*3/120000=2.5?

This would mean that the distance of the weight of the load would be ignored.

 

[jack action]

Moment about front axle:

F_{rear axle}*3m = W_truck * 2,5m + W_load * (3m + 1,5m)