One of my favourite tricks in school is to bring in a box and place it on the table in front of the lads. After they all get used to its presence I move the box so that three quarters of its base is off the table. The box, because of its unusual properties does not fall. In most classes a teacher who goes to this kind of trouble would be rewarded with applause, exclamations of surprise, wonder or at least questioning looks. For myself, I live in hope that I will someday be asked a question or that someone will remark: "There's something wrong here". However I'm more likely to be greeted with a yawn and asked: "Is it break-time yet?"

Anyway, to understand how it's done, you need to know that "The centre of gravity of an object is the point at which all its weight appears to act." This is a particularly difficult idea to understand without demonstration, so I usually show an ordinary narrow box standing on a table and say that the weight of the box is really acting at some point in the centre of the box. If I then slide the box towards the edge of the table so that its centre of gravity is no longer over the table, the box will fall. Nothing unusual here.

The second box has a large weight- I use diver's lead- attached to one side. This causes the centreof gravity to shift to the side with the weight. The box is moved so that only the side with the extra weight remains on the table. It doesn't fall.

This trick can also be done with a rolled-up newspaper. Use a lead fishing weight taped to one end.

Wash your hands carefully after touching lead.



While watching an old Titanic film, I noticed that the Captain balanced a pencil on a plate in the dining hall to show how stable and level the ship was. Since a pencil standing on its end is not very stable it must be assumed that the centre of gravity of the pencil was directly over its point of suspension. Needless to say, this demonstration was performed early on in the voyage.

Stick the sharp end of a needle into a cork as shown in the second picture. You will find it impossible to balance the needle on any surface.

Now if you lower the centre of gravity of the object by sticking two forks into the cork as shown, you should be able to easily balance the needle on practically any surface.

The reason is that the extra weight of the forks is concentrated lower down than the point of suspension. This makes it much more stable.

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