In this demonstration, students discover a practical way of determining whether or not an egg is raw or hard-boiled using Newton's First Law of Motion regarding an egg's centre of gravity.
Inside its shell, a hard-boiled egg is solid whereas a raw egg is fluid.
- When you spin the raw egg, its centre of gravity (or balance point) changes as the fluid inside the egg moves around. This results in a wobbling motion in order to achieve balance.
- When the hardboiled egg is spun, the solid centre has a fixed centre of gravity, causing it to move at the same time as the shell. This results in a smooth, balanced spin.
Trying to stop the egg from spinning is an excellent application of the principle of momentum. The raw egg keeps spinning after you touch it briefly because the liquid egg inside remains in motion, it's inertia is greater than that of the solid interior of the hardboiled egg. An object's momentum is its resistance to changing its state of motion; roughly "how hard it is to stop".
This is related to Newton's First Law of Motion: "An object in motion remains in motion, unless acted upon by an external force." Put more simply: if something is moving, it'll keep moving unless a force stops it. The friction between the table and egg will slow the egg and eventually stop the spinning. Though the liquid inside the egg keeps moving, it too slows down and stops because of the friction between the hard shell and the table.