Sputnik , according to Newton, him of the apple fame, before it weas hijacked for a computer, stated that I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
Meaning that if the timber is traveling at say 30mph, and the vehicle stops fast,the timber may well carry on at the same speed.
An idiot in a pickup decided to drive down a 1 in 7 hill round here with timber sloping ontothe ladder rack, un secured, he braked and according to the driver behind, the timber launched itself over the cab into a car coming into the opposite direction, result a massive fine and points, and had the van taken off him by his boss.