# Tennis racket theorem | Intermediate axis theorem | Dzhanibekov effect

As you can see in the above gif it is evident that physics behind rotating bodies is quite weird and odd. This incident happened in ISS which blew off the internet with full of posts and made our minds to go crazy. I myself surprised when I first saw this video. I don’t know what’s happening there and I came to the conclusion that maybe it’s due to the effect of external forces acting on that rotating body.

But after some googling, I found that this isn’t new and it is been there since the 18th century when Eular first described this in his thesis. Enough history lets dive into the real physics behind this weird behaviour.

Every rotating body has at least two rotating frames of references. But in the case of this particular body, it has three rotational axes. The first and second principal axes and an intermediate axis. This rotating body while rotating on the first and second principal axes it is quite stable and it tends to rotate on the same axis. But it’s intermediate axis is unstable and it tends to alter it’s direction frequently.

Now comes the moment of inertia. At the principal axes, the moment of inertia is low. So when we rotate on these axes we are not going to see this spin. This will happen only when we rotate on an intermediate axis.