Eggs
Classical Mechanics
Level
2
There are two eggs. One boiled another raw. Both the eggs are given same torque so that they rotate about their standard axes. Which egg will rotate faster?
Boiled egg
Raw egg
Both of them will rotate at the same rate
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1 solution
When you spin a hard boiled egg all of the contents are spun together so all the force you apply is transferred to motion at the same rotational rate and support keeping it spinning. When you spin a raw egg, the shell is moving full speed at the start, but it has to drive the liquid (white) inside in shear and the closer you get to the middle, and the yolk, the more the liquid slips behind to slip in the viscous liquid, then that has to try to get the semi solid yolk turning and it has a different pattern of liquid flow. The result is that you don't get the insides moving up to speed and as soon as you let go of the shell, the still slower moving insides start to drag on the shell and slow it down. Interestingly, since it may be hard to judge speeds, the quickest way to tell if an egg is raw or cooked is to spin it and then tap it to bring it to a stop. If the egg is raw, the moving insides will start it moving again while the cooked egg will just stay stopped.