Stopping For a Cup of Tea

I’m constantly amazed by modern cars. Take for example, the brakes on my Honda Accord. It’s not a performance saloon, yet every time you bring it to rest the brakes have to work incredibly hard to stop it without catching fire. It’s amazing the brakes last so long.

Some numbers: when empty the car weighs about 1575kg. When we’re driving any distance, you can add to that the following:

  • a full tank of diesel (about 45kg in itself)
  • me
  • Katrina
  • Lucy and her car seat
  • luggage

In total, I think this will be around 1850kg. Now here comes the science bit: concentrate! 70mph is 31.3m/s, which means to stop the car requires the removal of around 882kJ of energy. Obviously some of that will disappear with drag and engine braking, but we’ll ignore that for now.

So in the case of an emergency where you can’t just let the engine do a lot of the braking for you, this means that in the time it takes to stop (just a handful of seconds), the brakes need to get rid of enough energy to boil 2.34 litres of water. Even taking a lot of air resistance into account, that’s enough water to make 5 or 6 cups of tea.