Why is it, that if you lower your hand flat into, say, a pool there is no or hardly any resistance but if you were to raise your hand and slap it down hard into the water, there is so much resistance?
Asked by: J.J. True


For much the same reason you can stick your hand out of the window of a car moving at 5 mph with very little difficulty, but risk injury if you try the same thing at 100 mph. The resistance that a fluid (air, in this example) offers to an object (your hand) moving through it is determined by, among other things, the speed of the object. That resistance increases with the SQUARE of the speed, so just doubling the speed increases fluid resistance FOUR times.

In the moving car example above, increasing speed from 5 mph to 100 mph increases air resistance by a factor of 100/5 SQUARED, or 400 times. So a force on your hand of 1 pound at 5 mph becomes 400 pounds at 100 mph. Going from a gentle motion to a slap on the surface of a pool is another illustration of the effect speed has on fluid resistance.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

Science Quote

'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '

Michio Kaku

All rights reserved. © Copyright '1995-'2018   Privacy Statement | Cookie Policy