Final answer:
An empty trailer on a highway bounces up and down more than a heavily loaded one because it has less mass and inertia to stabilize it against the road's imperfections.
Step-by-step explanation:
If you pass a freight truck with a trailer on the highway and notice that its trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty. When a trailer is heavily loaded, the weight of the cargo tends to stabilize the vehicle, dampening the oscillations caused by irregularities in the road surface. Conversely, an empty trailer has less mass and therefore less inertia, which allows for greater movement and more pronounced bouncing when it encounters bumps or dips on the highway. This means that the suspension system has less weight to buffer against the road's imperfections, causing it to react more significantly to them.