GQC - it all depends on how fast you are travelling, how sudden the stop is, etc etc. F=MA Force = Mass (KG) x Acceleration (derivitive of velocity m/s) - when you apply your brakes you are accelerating in the opposite direction to which you are travelling. You know how when you brake, any bottles etc on the passenger seat go forward, well they are accelerating in relation to the seat. so if you crash, lets say headon, then you have a massive acceleration in relation to your seat, and thus the tank experiences the same acceleration, or 'g' in relation to the van. for example, when someone talks about experiencing 5g in a crash, that is a force exerted on them which is 5x their own mass (not weight, different thing) - my mass is 100kg (fatty!) but my weight is 9.81(m/s^2)x(100kg) = 981N
9.81m/s^2 is gravity.
sorry I'm going off on one.
more mass x more acceleration in opposite direction = more resultant force.
even simpler, 400kg decelerating from 30mph to 0mph in 0.8s = 32tonne/s