Sunday, August 10, 2014

19th Century Physics Brainteasers

Ars Technica: "The never-ending conundrums of classical physics".

For example:
If you force air out of the sprinkler depicted here by squeezing the bulb or connect it to a hose to water your lawn, there is little doubt that, if the top part is allowed to rotate freely, it will spin in the direction indicated by the large arrow. That's opposite to the direction of the expelled fluid.

But what happens when you release the bulb and suck air back in? Or, say, immerse the contraption in a container and force water into the arms through pressure?

Since the sprinkler was first discussed by the great physicist and philosopher of science Ernst Mach in the late nineteenth century, it has periodically reappeared to challenge the intuition of physics students and Nobel prize winners alike. It is now usually referred to as “Feynman’s sprinkler,” partly as a result of the popularity of Feynman’s memoirs, where the young physicist exploded a tank full of water in Princeton’s cyclotron facility while attempting to settle the question experimentally.

When encountering this puzzle for the first time, physicists tend to think the answer is clear. About half think it’s obvious that the sprinkler will rotate counterclockwise, while the other half think it’s obvious that it won’t move at all...
(Click here for the answer.)