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@thelawnet
Picked my son up from school today. He got looked up and down by a father who takes his son home in a Mercedes ML, a sort of 'you neglectful bastard, how dare you take your children home by bike' look.
I asked him if he had been quizzed about it. 'The school secretary asked me what happened, I said I fell off my bike, and she asked me why, so I said I got hit by a car' 'She said I should wear a helmet'. I said to him 'that won't do much to protect your chin will it'.
He then asked me 'how do bike helmets work'. I said 'do you see that car? What will happen if it hits another car?' 'It will crush', he said, 'so bike helmets work the same way'. 'Yes I said, but do you know how much a car weighs?' 'Two or three tonnes' 'And how much does a bike helmet weigh?' 'About 300 grams'. 'And do you know what the formula for force is?' 'No' 'It's force = mass * acceleration. What do you think the acceleration would be if a car hits a wall at 60mph?' 'About 5mph per second?' 'No, it wouldn't take 12 seconds to stop would it, it would be much quicker than that.'
Anyway he had to go to his climbing lesson at this point, so I didn't have time to explain the rest of the relative physics of deceleration between a car bonnet and airbags and a polystyrene helmet.