Chances are you’ve seen the above video of legendary skater Tony Hawk riding the first horizontal loop on a skateboard. It took some painful trial and error, but the successful feat has been viewed over 1.5 million times in just five days. For those of you old enough to travel down memory lane to the first incarnation of Jackass, you’ll remember that boarders (including Hawk himself) have been making vertical loops for some time. So what makes this so much harder? As usual, it all comes down to physics.
The vertical and horizontal loop attempts were performed here on Earth, meaning Hawk had a very pesky force to contend with in both situations: weight. Hawk is relatively close to the same size in both videos, so the force of his weight (mass “m” times the acceleration due to gravity “g”) arguably didn’t changed much from one to the next. But if the same force is pulling him down in either situation, how can one be harder to combat than the other? We’re missing an important factor: what did change was the direction of gravity’s pull relative to the ramp. Once you pass the apex in a vertical loop, gravity is pulling you down, but that’s exactly where you need to go in order to complete the circle. In this situation, Hawk’s weight is actually helping to set him on the right trajectory.
When you rotate the apparatus on its side, however, weight becomes much more of a hindrance. Gravity is still pulling you down of course, but that’s no longer where you want to go. It’s forcing you away from the desired path.
To that effect, you’ll notice that Hawk never actually completes the loop, but rather spirals out at the bottom. In order to make a full horizontal loop, you’d need a lot of centripetal force. The easiest way to achieve this is by making your rotation time faster – in this case, by building a tighter ramp. Remember spiral wishing wells? Those devilish devices that tricked you into feeding them your hard-earned coins as a child? The same forces are at work here.
As you drop a coin into a spiral wishing well, it makes large, lazy loops. It spirals down because the force of gravity acting on the coin is greater than the centripetal and friction forces holding it against the wall. But as the coin moves deeper into the well, the distance it has to travel to make a single rotation becomes smaller and smaller. Ergo, its rotation time has gone down. If you look closely, you’ll see that the coin actually drops less during the second rotation than the first. This pattern continues, until eventually the loop becomes so small that the coin is able to make several full rotations before dropping at all.
Without any help, say, from a rope to hold him up, or a jetpack to propel him around the circle faster (cheating, we know … but, jetpacks) Hawk’s loop would have to be impossibly tight to complete the full thing without dropping at the end. We’ll give him an “A” for effort. Beating physics isn’t easy.