Final answer:
A freight truck trailer bouncing up and down slowly on a highway is likely heavily loaded because of greater inertia resisting rapid movement changes.
Step-by-step explanation:
When you pass a freight truck with a trailer on a highway, noticing that its trailer is bouncing up and down slowly could indicate that the trailer is heavily loaded. This slow bouncing is due to the greater mass inside the trailer, which has a greater inertia, thereby resisting rapid acceleration and deceleration. Inertia is a property of matter that causes it to resist changes in velocity. As such, a heavily loaded trailer will not respond as quickly to bumps and dips in the road compared to an empty one, which can bounce around more easily due to its lower mass and inertia.