New Scientist - Last Word
contributions by Dr Hugh Hunt

06 May 2006

Q: In the TV programme Knight Rider, lead actor David Hasselhoff drives his talking car, Kit, up and onto a moving lorry via a ramp projecting from the back onto the road, often at high speed. The same thing happened in the original The Italian Job movie starring Michael Caine. But is this possible? As soon as the car hits the ramp, it would be moving relative to the lorry and therefore would only have the length of the inside of the lorry in which to brake, probably ending up crashing into the cab. Am I correct?

(there were several contributions - see the full New Scientist article at 6 May 2006)

This was my contribution to the discussion:

A: At a relative speed of about 10 km/h - about jogging pace - a car can easily stop in a short distance, although there will be plenty of screeching as the fast-spinning wheels come to a halt, which is possibly where the confusion lies. Perhaps it is easier to imagine instead a small aircraft landing on the back of the truck: as long as the difference in speed between the aircraft and the truck was small, there would be no problem landing in the limited space.

“The car can stop in a short distance, although there will be screeching as it comes to a halt”Moving travelators in airports offer another perspective. In order to avoid being jerked when we roll our luggage trolley onto the moving walkway, we make sure that we're walking at about the same speed as the travelator. Then as soon as we're on board, we stop pushing. Having made sure that the trolley is moving at just the right speed, there's no more to do once the wheels of the trolley have stopped turning, and friction takes care of that.

Hugh Hunt, Cambridge, UK

Back to the TV, Radio and Popular Press page: TV and Radio
For fun stuff on spinning things go to my Dynamics movies page