Re: Thought experiment
"The logs might be rolling back relative to the lorry, but relative to the lamp post next to it, it'll still be moving forwards."
Not necessarily. Last year I was driving up a steep hill near home. Previously, after descending the hill I'd passed a big trailer loaded with hay bales making its way to the hill. Near the top the trailer had had a partial load-shed. These bales are large cylinders, about 2 metres dia & about the same length packed into wrappers. They're heavy. Fortunately the bales had come off askew and run into the site of the road otherwise they'd have been able to roll downhill for a few hundred metres to confront whatever was following.