Final answer:
The correct height for a trailer to be hooked to a tractor is indicated by the proper alignment of the kingpin with the fifth wheel. On the highway, a trailer that bounces up and down slowly is likely nearly empty because less weight means less damping of road irregularities.
Step-by-step explanation:
When hooking a tractor to a trailer, you will know the trailer is at the correct height when the kingpin on the trailer aligns properly with the fifth wheel on the tractor. Precision is crucial in this process to ensure a safe and secure connection. If the kingpin and fifth wheel do not align properly, adjustments to the trailer height must be made before attempted coupling.
If on the highway, you notice that a trailer is bouncing up and down slowly, it is more likely that the trailer is nearly empty. An empty trailer has less weight to dampen the movement from the road's surface, causing it to bounce more visibly than a heavily loaded trailer. A loaded trailer would have more mass, leading to less bounce, as the weight provides stability and absorbs the irregularities of the road surface better.