This is a really neat trick. And explaining the physics is at least half the fun! The glass tube is filled with water, and there is a large air bubble. Although there are two ‘constrictions’ along the length of the tube, it is just a single tube inside.
The challenge – to get the air bubble into the centre segment. People try and tip the tube, but find that the bubble overshoots, and although some might end up in the middle segment, they just can’t get all the bubble in the middle.The solution is very simple. Place the tube on a flat surface, spin it, and when it stops the bubble will be in the middle.
How does Bubble Trouble work?
If you have a marble on a flat surface in a stationary car, and the car moves forwards, which way does the marble roll? Correct – backwards. But what if you are holding a helium balloon in the car when the car moves off? Well, what happens is that the helium balloon moves forwards…(in a sense, the heavier air moves to the back of the car).
In the same way, with our glass tube. If we were to start off with the bubble at one end of the tube, and call this end the back of the tube; if we then accelerate the tube in a forwards direction, the air bubble moves to the ‘front end’ of the glass tube, i.e. in the direction of the movement, and the heavier water moves to the ‘back’ of the tube.
However we want to end up with the bubble in the middle of the tube. So… we spin the tube. The water moves towards the ends of the tube, and leaves the air bubble in the centre. (Remember, an object moving in a circle is actually accelerating…)
Easy to demo. Everyone is impressed. But a bigger challenge to explain the physics. We like this one!!
