Who freewheels downhill faster, a fat person or a skinny person?

21 January 2008
Presented by Diana O'Carroll.

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This week, we find out if a fatter cyclist free-wheels faster than a thinner cyclist, and ask if an aeroplane runway is replaced with a treadmill, can the plane still take off? Also, how is oxygen recycled on the international space station?

In this episode

Police officer on a bicycle

00:00 - Fatter Cyclists Faster Downhill?

Who would go faster freewheeling downhill, a fat cyclist of a thin cyclist?

Fatter Cyclists Faster Downhill?

This question was answered by Dr Jos Darling, Dept Engineering, University of Bath.

That's a tricky one. It's been thought about for a long time really because Aristotle was the first bloke that thought about objects falling due to gravity. At that time he decided that heavy object fell more quickly than light objects. Later on, people like Newton decided that, with gravity, objects fall at the same rate. Strictly speaking that's only true if you're in a vacuum. On a bike you're far from it. The big issue with the bike is the aerodynamic drag.

If there were no aerodynamic drag then a fat person or a thin person would end up accelerating down a hill at the same rate. The point is that, with the fat person, assuming that they're not incredibly wide, the aerodynamic drag is going to be less significant in terms of their falling down the hill than for the thin person. Ultimately, a thin person's going to end up going slower than a fat person.

So, if you're in a race, you want to minimise that aerodynamic drag! And of course the downside to being fat is that there's always going to be a hill on the other side of the downhill, meaning that you've got to put a lot of work in to get up the other side. There's always a catch!

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