Final answer:
A trailer that is bouncing up and down slowly on the highway is likely heavily loaded as the extra weight compresses the suspension making it less springy and causing slower oscillations.
Step-by-step explanation:
If you notice a freight truck with a trailer bouncing up and down slowly on the highway, it's more likely to be heavily loaded rather than empty. A fully loaded trailer has more mass, which means the suspension system has less tendency to oscillate, so it will absorb bumps and dips more effectively causing slower bounces. An empty trailer, with less mass, tends to bounce more rapidly because the suspension system is not as heavily compressed, and therefore, it has more springiness, leading to faster and more frequent oscillations.