Final answer:
A trailer that is bouncing slowly on the highway is likely nearly empty as it lacks the mass to absorb road imperfections, unlike a heavily loaded one that would bounce less.
Step-by-step explanation:
As you pass a freight truck with a trailer on a highway, if you notice that its trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty. A heavily loaded trailer would have less bounce because the weight of the cargo would provide a more damping effect, reducing the trailer's oscillations after hitting a bump in the road. When the trailer is nearly empty, there is less mass to absorb the energy from road imperfections, causing it to bounce more noticeably.