Final answer:
If the trailer brakes away and the air lines are pulled apart on a semi, the trailer brakes are expected to engage due to the emergency braking system. The smoking brakes from Figure 14.5 show the mechanical equivalent of heat from brake friction. A bouncing trailer is likely to be nearly empty, as heavy loads usually cause less bouncing.
Step-by-step explanation:
You are driving a semi and if the trailer breaks away pulling apart the air lines, you would expect the trailer brakes to come on because of the emergency braking system used in semi-trucks. When the air lines are disconnected, air pressure is lost and the emergency system activates the brakes. The brakes are designed to engage automatically in such situations to prevent the trailer from rolling away uncontrollably, which could lead to a dangerous highway incident.
In the context provided, the reference to Figure 14.5 about the smoking brakes being evidence of the mechanical equivalent of heat demonstrates that when brakes are used excessively or fail, they can generate a lot of heat due to friction, which may become visible as smoke.
In a different scenario, if you are observing a trailer on the highway and it is bouncing up and down slowly, it is more likely that the trailer is nearly empty. A heavily loaded trailer would have less suspension travel and therefore bounce less due to the weight stabilizing it.