Today’s post is really funny. To play we need a toy or imagination, and today we will use both. The chosen toy is the slinky spring, I’m sure we all understand which one it is. The fact is that in addition to the fun of throwing it downstairs or playing with it to pass the time, when we let it fall from a certain height something amazing happens.
Before starting the fun we recommend to reflect and try to guess what will happen, In advance we tell you it will not fall without further ado. In fact, if you have any of these springs at home we would encourage you to try this, there is no magic, so anyone can do it despite being some clumsy. And now, tried it or not, we will show you the video so you can see what happens and then the explanation.
As you can see the spring seems to levitate in the air while contracting, as if by magic. But it is not magic, of course, it is physics. What happens is a very simple and easy to understand effect that can be seen in two ways: the center of mass and the double force. We start with the center of mass which is the theoretical one and then the other, more simple.
The center of mass is a mathematical contraption that can treat an object as if it were a point. This point is much easier to treat than an extended object and is what always follows the laws of physics as we know them. The center of mass does not care about the length of the object or the internal forces. In the spring, the center of mass falls as they had taught us in class. This does not mean that all parts do, only that on average, the spring falls with constant acceleration of gravity.
The second way of looking at it is to think of the internal forces. In the spring we have the force of gravity pulling down and the spring itself trying to shrink. These forces are in the same direction at the top while in bottom they go in opposite directions in balance and cancel each other. Thus the top of the spring begins to fall faster and the bottom does not start falling until the end, when the spring is fully compressed and no longer on the upward force of the spring.
So you see, sometimes things are more surprising than they seem. We hope you to try if you can or at least that you have fully understood.