Puzzle #25: Car crash maths
12 October 2019
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From New Scientist #3251, 12th October 2019 [link] [link]
Two cars of the same model, one blue, one yellow, are on a motorway in the UK. The blue car is in the inside lane travelling at 70 miles per hour*, which is the speed limit. The yellow one is speeding in the outside lane at 100 mph.
At the instant when they are neck and neck, both drivers see a fallen tree across the road some distance ahead. Both immediately brake, each applying the same constant braking force. The blue car manages to stop centimetres short of the tree. To the nearest 10mph, at what speed does the yellow car hit the tree?
a) 10 mph
b) 30 mph
c) 50 mph
d) 70 mph
Use your intuition (particularly if you are a driver) to have a guess. Then work out the answer to see if you were right.
* Alternatively, call the starting speeds 70 kilometres per hour and 100 km/h, the number in the solution will be the same.