Dizzy Eggs
How can you tell the difference between a raw and hard-boiled egg without breaking them?
This article demonstrates how to exploit the effects of rotation to find the difference between a hard-boiled and raw egg. We'll discuss three methods and why they work. The first is simple. Watch the video in Figure 1. The hard-boiled egg is colored blue and the raw egg is colored red. Notice that when spinning the hard-boiled egg, it turns quickly without much other movements.
On the other hand, the raw egg wobbles around and doesn't spin as fast as the hard-boiled egg. This happens because of a concept called the center of mass. It is the average position of the mass of an object. If you take a perfect stick, the center of the mass is right in the middle of it. However, if we take something like a pair of scissors, the center of mass is near the handle because that's where most of the mass is. The inside of the hard-boiled egg is mostly solidified. It has a single center of mass which the egg rotates around. On the other hand, the raw egg has a liquid inside which gets pulled around as the egg spins. Since the center of mass of the egg is the average position of its inside liquid and outside shell, while the inside moves around, the center of mass also moves around. An object will usually try to spin around its center of mass. Because the raw egg spins around the center of mass while the center of mass spins, it takes on a "wobbly" motion.
Our first method of finding the difference between hard-boiled and raw eggs is spinning them. If the motion is wobbly, it is a raw egg and if it spins like a ball, it is hard-boiled.
Figure 1
Figure 2
Our second method of determining the type of egg will be using inertia. Inertia is the tendency of an object to resist a change in state. For our purposes, we can use the inertia of motion. For example, if a box is in a moving truck and the truck stops abruptly, the box will continue to move. Watch the video in Figure 2. When you spin the hard-boiled egg and stop it, it stops as expected.
However, when we stop the raw egg, it begins to spin again. This happens because when we spin the raw egg, the liquid on the inside also spins. When I stop the egg with my hand, the liquid in the egg still continues to spin. After I let go, the liquid spinning in the egg pushes on the outer shell and makes the whole egg spin again.
This is our second method of finding the difference between hard-boiled and raw eggs. If we stop the spinning egg and it stays stopped, it is hard-boiled. If we stop it and it continues to spin after, it is a raw egg.
The final method of determining the type of egg is rolling them both down a ramp. Watch the video in Figure 3. Both eggs start on top of the ramp and have the same amount of energy given to them by gravity. This energy will be split into kinetic (moving down the ramp) and rotational (spinning on its axis). If one egg has more rotational energy than another, it also must have less kinetic energy. The egg with less kinetic energy has the least velocity and will reach the bottom of the ramp last.
To summarize, the more rotational energy an egg has, the longer it will take to reach the bottom. We can imagine the hard-boiled egg as being turned from its surface and all the solid parts inside also have to be rotated. The raw egg has liquid on the inside, which has less of a resistance to being turned. The hard-boiled egg uses more rotational energy to be turned which makes it reach the bottom last. This is our third method of finding the difference between hard-boiled and raw eggs. When both are rolled down a ramp, the raw egg will always reach the bottom first.
Figure 3