Spinning Eggs
Classical Mechanics
Level
1
Suppose we take a hard-boiled egg and an uncooked egg, and spin them on a flat tabletop using a hand. Assuming that we use the same hand motion (grip, spin, etc.) to spin each egg, what difference would we notice?
The hard boiled egg spins faster and shorter
The hard boiled egg spins slower and shorter
The hard boiled egg spins faster and longer
The hard boiled egg spins slower and longer
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tldr; The uncooked egg spins slower and shorter because it has liquid within.
A hard-boiled egg is a solid rigid object, which means that the position of any point in the egg relative to another point in the egg remains constant. On the other hand, an uncooked egg is liquid within, so its contents can move independently of its shell.
When the uncooked egg rotates, its contents move outwards towards the shell, and as a result its moment of inertia is greater than the moment of inertia of a hard-boiled egg. So when we give both eggs the same initial impulse, the hard-boiled egg spins faster than the uncooked egg. This is because both eggs have the same angular momentum, and since the uncooked egg has a greater moment of inertia, its angular velocity is smaller.
Since the uncooked egg contains liquid inside it, there is more friction inside it and it loses kinetic energy more quickly than the hard-boiled egg. As a consequence, the uncooked egg comes to a halt much more quickly than a hard-boiled egg.
We see that the hard-boiled egg spins faster and for a longer time than the uncooked egg. This is a good way to distinguish between an uncooked egg and a hard-boiled egg without breaking them open!