How much energy does a car have a 20 mph? 30 mph? 70 mph? Watch out, here comes a maths lesson…

Ek = mv2
Energy = mass X speed X speed

The faster you drive, the more kinetic energy your car has. When you crash into something (or someone), the more kinetic energy you have, the more damage you will do: this damage ranges from low speed bumps and bruises, through medium speed broken bones, to high speed serious life-changing injury and death.

Let’s keep the maths simple by ignoring the fact that we should convert miles per hour to metres per second. And let’s also ignore that energy is measured in Joules: we will call them “killing units”. Finally, we will estimate the weight of a car at 1000kg (a Fiat 500 is much less: a Nissan Qashqai is much more)

The formula to work out killing energy is weight X speed X speed (yes speed is there twice).

At 20 mph we have 1000 X 20 X 20 = 400,000KU

What happens if we double the speed to 40mph? Heres a clue: it doesnt double to 80,000KU. Get your calculator out and plug the numbers in…

At 40 mph we have 1000 X 40 X 40 = 1,600,000KU

Double the speed equals four times the killing power. Tempted to drive at 80 mph? Go on, do the maths (6,400,000KU)

This huge disparity between speed and energy gives rise to the statistic of “hit a child at 30 mph and theres an 80% chance they will live, hit a child at 40 mph and theres an 80% they will die.”

And at 20 mph the likely outcome is minor injuries.

P is for Physics
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