Thursday, April 9, 2009

Raindrops falling on my windshield

Son (in the car, watching the raindrops roll up our windshield as we travel on the freeway):

"Why do the rain drops roll up the windshield instead of down?"


Because of the 70 miles per hour winds blowing over the windshield put more upward force on the drops than gravity does.

We can calculate the force this fluid (air) has of the object on the windshield (raindrop). I'm not going to bother doing the calculation right now, but it can be done. We can also calculate the down-ward force applied by the acceleration due to gravity.

The force applied to the rain drop by the wind moving past is larger than the force applied by the acceleration of gravity on the mass of the rain drop. Therefore, the raindrop moves in the direction the wind-force is being applied (there may be other forces, but the are so minor that I'm going to ignore them).

Son's follow-up: "Why do some raindrops go further up than others?"

Because some are larger than others (and therefore catch more wind and do not dry out as quickly), some find a cleaner path than others, some have more wind-force applied to them because of their position on the windshield (not blocked by the wipers, for example).

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