I have experienced that when we go on a bike at a high speed, keeping our hands close to the surface of the bike is not much a problem and we feel negligible force which the air applies on us. But as we move our hands away from the surface, we feel more and more force on our hands.

A possible explanation I could think of was involving the viscous nature of fluids and how the relative velocity affects the force of impact, and wanted to know whether it is indeed because of this or something I am unaware of.

My explanation was since the air is also a fluid and has some viscosity, the column of air closer to the bike will have less velocity relative to us than the column of air which is away from the bike. Because of this, the force of impact of collision of our hands and the air will be more as we more our hands away.

As on the internet, I found one example stating:

Imagine a car travelling at 70 mph runs into the back of a car doing 69 mph on a motorway/highway. The closing speed (relative velocity) is 1 mph and the damage caused by the initial impact will be minimal. This is a 1mph collision not a 70mph collision. If one of the cars becomes out of control and collides with a barrier/tree then the closing speed between the car and the barrier will be close to 70mph and will cause somewhat more damage.

ref: https://qr.ae/pGXBls

So is my explanation correct or is there some other factors because of which this happens?

I am a high school student so I am sorry if there errors of any sort


1 Answer 1


When you arms are close to your body, they are surrounded by air which has been already stirred up and set into motion by your passage through the air. as long as your arms are inside this boundary layer, they will not experience the full force of the moving air outside the boundary layer.

Then lifting your arms up and away from your body makes them stick out of the boundary layer and now they are exposed to the full force of the wind.

The faster you ride your bike, the thinner the boundary layer gets.

  • $\begingroup$ can this "boundary layer" be explained by viscosity of air? $\endgroup$
    – qubitybit
    Mar 23, 2021 at 16:41
  • $\begingroup$ It depends on several things- have a look at "boundary layer" on wikipedia. $\endgroup$ Mar 23, 2021 at 21:09

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