# BMX Tricks On A Driving Truck? – Physics Is On Your Side!

During the last weeks I spent a large amount of time at the university and, consequently, I visited the local canteen on a regular basis. Not only does this place have windows and food, but there is also the possibility to take a break from the daily university routine. It was quite a surprise when I happened to stumble upon the foundation of Albert Einstein’s special theory of relativity at this location!

Well, I certainly have to admit that all this probably happened due to the fact that my head is normally crammed full of physics stuff when being at university and many others would have rather preferred to concentrate on the plate of goulash if they were in the same situation. Anyway, I’d like to say a word or two on the famous theory of relativity and explain why you are currently able to see it in the canteen.

In this video, you can see a BMX rider performing some tricks in a halfpipe (or is this called a ramp or something? …I’m no BMX expert!), while a truck cruises around the city – along with the whole halfpipe. “Pretty impressive”, was my first thought.

But when you ignore the psychological overcoming, which is certainly required by a biker jumping in a moving halfpipe, and also the fact that the tricks and the sporting performance are really impressive, the whole stunt doesn’t really seem that spectacular anymore – at least it doesn’t from a physical point of view.

As it turned out, the physical laws are the same in every uniformly moving frame of reference. It doesn’t make any difference at all, if you (as a BMX rider) perform tricks in a normal (“stationary”) halfpipe or if you do it in a halfpipe which is in uniform motion. Physics is the same in every case – you won’t feel any additional forces or accelerations when doing stunts on a moving truck. In fact, you wouldn’t even be able to determine your state of motion, if the halfpipe would be placed in a big and windowless container.
The physical laws have the exact same form in all admissive reference frames! In saying this words, one could pretty much describe this so-called relativity principle.

So the biker in the video is a demonstrative example of the validity of the principle of relativity!

All I said so far is of course only true for reference frames which don’t experience any acceleration. If the truck driver steps on the gas, brakes, or changes his direction of travel while the biker is just up in the air, the latter will certainly notice the change. At the latest when the truck driver pulls the rug/truck out from the biker’s feet, the sportsman will realize that the physical laws of uniformly moving reference frames don’t work as usual anymore.
(Additionally, the unrestrained passenger in the halfpipe will feel the more air resistance the faster the truck goes. In this case, too, he would be able to draw conclusions about his state of motion, even if his eyes were bandaged. But as one can easily see in the video, the truck is moving quite slowly. Hence, any influence from air resistance can be confidently ignored.)

Interestringly enough, it is indeed possible to deduce fundamental principles of the special theory of relativity from the relativity principle only – e.g., the prominent gamma factor (or, for physicists: the $\gamma$ factor). This factor is responsible for the time actually slowing down in relatively moving frames of reference or for moving object’s lengths contracting in the direction of movement.

I don’t want to ramble on the $\gamma$ factor and associated phenomena like the time dilatation and the Lorentz contraction, but instead refer to some videos of the excellent and outstanding YouTube channel Sixty Symbols, like this one, for example.

So with a little bit of curiosity, interest, and practice, there’s virtually no place where you won’t find some fascinating piece of physics. This doesn’t simultaneously mean that you won’t be able to enjoy your goulash anymore – instead quite the opposite is true: You realize that the goulash will taste equally good, even if the whole canteen would drive through the city in uniform motion. (Nevertheless, I would be very sceptical and check a moving canteen for its physical content! 😉 )