Ever mixed up your eggs? Some simple physics will enable you to avoid that problem in the future…
- raw egg
- hard boiled egg
- Place the two eggs on a flat surface and set them both spinning.
- Gently and briefly place your finger on the top centre of each egg.
- Notice that the hard boiled egg is much easier to spin, but it stays still when you take your finger off. In contrast, the raw egg is difficult to start spinning but will keep spinning when you take your finger off.
How Does it Work?
Momentum is the key to this demonstration. A raw egg is filled with a liquid, whereas a hard boiled egg is effectively a solid. Firstly consider what happens when you stop the eggs: when you gently place your finger on the top, you stop the outer shell of both eggs from moving. Since the hard boiled egg is solid, all of the egg stops moving, and so the egg remains stationary when you remove your finger. However, the liquid inside the raw egg will keep spinning even though the outside shell is stationary. The drag of that liquid on the shell will start the raw egg spinning again. Similarly, a hard boiled egg is easier to spin since the entire egg starts spinning at the same time, whereas in the soft boiled egg only the outer shell is spinning at first, and gradually the liquid insides begin to spin as they are dragged around by the shell.
Tips for Success
Don’t set your eggs spinning too hard or they may roll off the table. Make sure you start them spinning at approximately the same rates or your audience may think you are trying to fool them!
This trick is of most interest to adult audiences as younger people generally haven't experienced the difficulty of distinguishing between raw and hard boiled eggs. You may want to demonstrate that the egg really is hard or soft boiled by breaking it in front of the audience.
Did You Know?
There is a classic physics trick that involves placing a lit match or piece of paper into an old milk bottle, and setting a peeled hard boiled egg on top of the bottle. The egg is pushed into the bottle in a very similar way to the water being sucked into the pint glass in the 'Lifting Lemon' trick. Unfortunately it is very difficult these days to locate old-style glass milk bottles so this trick seems to be going out of fashion, although it is possible to use a water balloon and a wide-necked bottle instead.