25 November 2008

If only I knew more physics...



The other day, cycling in the rain down a long, quiet road, I noticed that the water made a 'line' that, when going in a straight direction, would be fully aligned with the centre of the wheel (see picture, Fig.A). Then I noticed that when I made small adjustments, little turns of the handle bar, the water line didn't follow the direction of the wheel - it stayed in its previous position, like a gyroscopic compass pointing to the true North.

I tried asking my more scientifically trained school friends of all for their input. All I got were recommendations to watch the road and comments about age finally getting to me. How disappointed Galileo would be...

2 comments:

David Hembrow said...

Are you sure it was actually going straight ? It must turn as well because if you go around 180 degrees you're not going to end up with the spray from your tyre going in the opposite direction.

However, it will be delayed, or appear to be delayed. I think what you are seeing is the effect of what is generally called "centrifugal force."

More accurately the water simply is carrying on in the same direction it was already travelling in, while you are turning, if only slightly. Once the water leaves your tyre there is no longer a force to push it sideways, while you are being pushed due to your tyre travelling sideways across the ground.

So, as long as you only turn the handlebars a little, as it seems you were, it will appear that the water somehow "knows" the direction you are travelling in.

How much you can get the directions of the water and yourself to part depend on your speed and the rate of your turn.

Can't be bothered to do any maths, but I think that's a fair explanation.

coco said...

It makes sense - it's probably one of those optical illusions to do with movement (have you noticed when you are driving on the motorway on a windy day, the trees don't seem to sway?). And my friends are probably right: I should concentrate on the road!

Now, I've been observing traffic patterns...