Have you ever seen a trick on TV or in a movie where there is a fully laid table and someone pulls the tablecloth out without disturbing the objects on top? This week, we are going to look at how they do it.
You will need
A cup or glass. Try to avoid plastic ones, you want one with a bit of weight.
A playing card
A coin
A friend
What to do
Put the glass on a table, with the open end facing up.
Put the card on top of it. Make sure one end of the card is sticking out over the edge of the glass.
Put the coin on top of the card.
Challenge your friend to get the coin into the glass, following these rules:
You can't touch the coin.
You can't touch the glass
You can't touch the card for more than one second.
When they give up, flick the end of the card that is sticking out with your finger nail. The idea is to shoot it back under the coin so the card shoots out horizontally. The coin should drop into the glass. (If they find another way to do it, congratulate them on their good thinking, then show them this way.)
Important: Practice this one beforehand. You need to flick the end of the card horizontally, so it goes straight out from under the coin. If it tries to go up or down at all, it will tend to bounce the coin up and it won't work.
What's happening?
This is a demonstration of inertia. Inertia is the way that, if something is staying still or moving in a straight line, then it will keep doing it unless some outside force pushes it. The heavier something is, the more inertia it has.
When the coin is sitting on top of the card, it will stay there unless something pushes it. When you flicked the card, the card started to slide underneath the coin. If we had a perfectly smooth card, the card sliding around underneath the coin wouldn't push on the coin at all, so the coin would just stay there until the card was gone, then drop into the glass.
Since we are using a real card, there is a little bit of friction between the card and the coin. When the card was sliding underneath the coin, the coin felt a slight force in the same direction the card was moving. But since the coin has inertia, it takes time to get it moving. Since the card only rubbed against the coin for a fraction of a second, the coin didn't have enough time to build up much speed and it dropped into the glass anyway.
The tablecloth trick works the same way. When the tablecloth is jerked out quickly, the inertia of the plates, cups, knives and forks stops them moving much for the fraction of a second the cloth is moving under them. Then, instead of falling onto the floor, they just fall a millimetre or so onto the table.
I have met people who can do the tablecloth trick. I don't recommend trying it at home, but if you are silly enough to try, there are a few things to keep in mind:
CAUTION: Only do this with adult permission and supervision.
One thing they never show in the movies is that the tablecloths have to be prepared in advance to make the trick work. The trick needs a smooth cloth with no hem, otherwise objects on the table will be bounced around as the hem goes under them.
Even with the right tablecloth, it takes a lot of practice to pull the cloth out quickly, evenly and horizontally.
Make sure you start with a soft surface on the floor under the table to reduce the chance of objects breaking. Also start with things that won't break if they take a trip to the floor, and won't hurt if they flick up in the air and land on you.
Practice getting your hand at just the right angle.
