Final answer:
A freight truck's trailer bouncing slowly on the highway is likely nearly empty, because an empty trailer's suspension system has more room to bounce than a loaded trailer, which would have less bounce due to its weight.
Step-by-step explanation:
When you notice a freight truck with a trailer bouncing up and down slowly on the highway, it is more likely that the trailer is nearly empty rather than heavily loaded. A heavily loaded trailer would have less bounce due to the additional weight compressing the trailer's suspension system. On the other hand, an empty or lightly loaded trailer has more room to bounce because the springs in the suspension have less weight compressing them down, leading to a greater range of motion, and as a result, more noticeable bouncing.