Suppose a fire truck, with a huge tank of water. If the tank is half-full, when you slow down the water's inertia is going to push the truck forward. When it's full, the water can be thought like a solid, so it doesn't have the same effect. The question is: what are the concepts involved in this problem? If the tank is, say, 90% filled, is it going to have more or less effect than when it's half-full (by "effect" I mean, how the water movement will affect the truck brake). Thanks!