Final answer:
A trailer bouncing slowly on a highway is likely nearly empty, as a heavy load would dampen such oscillations due to greater mass and inertia.
Step-by-step explanation:
When you pass a freight truck with a trailer on a highway, and notice that its trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty.
The key factor here is the damping effect provided by the mass of the load. A heavily loaded trailer would have more mass, which means that once set into motion, it would tend to keep moving due to its inertia, resulting in slower, less noticeable bouncing because the mass would dampen the oscillations from road irregularities. On the other hand, an empty trailer would have less mass and less damping effect, so any bumps from the road surface could cause it to bounce more readily and with greater amplitude, leading to the slow bouncing motion you observed.