# Q & A: Bicycle Experiment

Q:
In my science project I put people of different weights on the same bike. They drifted down the hill, and I timed them over the same distance. Over all, the heavier a person was the faster they went down the hill. Why did heavier people go faster rather than the lighter ones?
- Andy Alger (age 12)
Brunswick, MD USA
A:
Andy -

If there were no air resistance or friction acting, and if the wheels of the bike were really light, the mass of the rider wouldn't matter and the riders would be going the same speed at the bottom no matter what their weight was.

The reason this is the case is the same reason that causes two objects with different weight (for example a book and a paper-clip) to hit the ground at the same time if you drop them from the same height: Gravity pulls harder of the heavier object, but because its heavier its also harder to "speed up", so in the end it all comes out the same (if you ignore air resistance).

In real life, the wheels of the bike do have some mass, and there is also some friction and some air resistance, and the problem is a bit more complicated. What is true, however, is that all of these "extra" forces tend to slow the bike down a little bit. The only force that acts to make the bike go faster is gravity, so making the rider heavier (and thus making the force of gravity bigger) is really just making the other forces small in comparison, and therefore making the bike go a bit faster.

-Mats

(published on 10/22/2007)