Torque Shop

On an MRT train, I feel wind when the train accelerates. Why is it I don't feel wind in a car's cabin when it accelerates?

Air is invisible to the naked eye, but that does not mean it is weightless. Every cubic metre of air weighs approximately 1.2 kg.

A train carriage has a relatively large volume of air, so you will feel the movement of air when the train accelerates. It would feel like wind blowing from the outside. This, however, is only obvious when the train carriage you are in is relatively empty and not many passengers are standing inside to interrupt the movement of air.

The train's brisk acceleration (it is electric, after all) also plays a part in this phenomenon. If it were to move off slowly, there would be no sensation of wind.

In theory, the same relative movement of air does occur inside a car when it accelerates. But it is not obvious because of the small volume within the passenger compartment.

Even in a car which can accelerate quicker than any train, you are unlikely to feel "wind". It is, however, possible to feel the air from the air-conditioner vents changing directions when you take a U-turn quickly and this is because of the combination of high lateral acceleration and significant volume of air from the vents.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on November 09, 2019, with the headline 'Torque Shop'. Print Edition | Subscribe