Notes from Catherine Rowett, former Green Party MEP for East of England and deputy coordinator of the Eastern Region Green Party*(UK). Biographical reflections on life as an MEP. Longer reflections and discussions on issues relating to policy, the good life, justice, equality, anti-austerity economics and the future of the planet. This is also a forum for exchanging ideas on how to tread lightly on the planet and avoid supporting exploitation and corrupt practices. Here we go...

Tuesday, 29 May 2007

Cycling and energy

There's a long running dispute in our household as to whether it uses more energy to cycle fast than to cycle more slowly.

I don't mean does it take more energy for the same amount of time. Obviously cycling faster is harder work, and uses more energy in a short time.

The question is this: if you need to go from here to the Sidgwick site, and you could go gently at, say, 10 miles an hour, or very energetically at about 15 miles an hour, you'd get there quicker by the latter method, but would you have burnt up all your weetabix more effectively?

Well here's the answer. Suppose the Sidgwick site is 2 miles away. Going at 10 miles an hour it will take you 12 minutes. Going at 15 miles an hour it will take you 8 minutes.

If you weigh 10 stone (which I don't, but never mind: we're trying to compare the results for the same person, not different people), riding at 10 miles an hour uses about 381 calories an hour, so you'll use 76.2 calories in 12 minutes.

The same 10 stone individual riding at 15 miles an hour uses about 636 calories an hour. So you'll use 84.8 calories in 8 minutes.

A serving of two weetabix with milk provides 190 calories, so by riding to the Sidgwick site you'll have used less than half your calories either way. But the difference between riding fast and riding slowly is 8.6 calories, which is about 1/10th of a weetabix. So if you are going to ride energetically, you'll need to eat a bit more breakfast if you're not to get hungry before elevenses (but on the other hand you gain an extra 4 minutes in which to go to the Buttery and get a coffee and a doughnut to keep you going).


Catherine Rowett said...

I remain a little puzzled as to why riding half as fast again uses up twice as much energy. Why not twice as much energy for twice as fast?
It makes me wonder whether the calculations assume that the slower rider does more freewheeling, taking it easy on the downward runs, rather than assuming someone who goes consistently less fast. Then I can see that you'd be using energy only part of the time. If that's the case it doesn't really show that it uses more energy to go faster, but only that it uses more energy to ride the whole time for 2 miles minutes, as opposed to riding part of the way and free-wheeling part of the way for 2 miles.

Tiger said...

Why not twice as much energy for twice as fast?

Probably because the kinetic energy of motion is proportional to the square of the velocity, and the energy losses due to friction, wind resistance and so on more or less follow suit. So riding one and a half times as fast should use two and a quarter times the energy.

For the same reason, if you're driving your car at 40mph your brakes will take four times the distance to stop the vehicle in an emergency, compared with one travelling at 20mph. Also while your brain is getting round to applying the brake the car will travel twice as fast. So 20mph zones in towns can lead to an darmatic reduction in traffic accidents and a much reduced casualty rate in the collisions that do occur, particularly for pedestrians and cyclists.