During the 1999 X Games in San Francisco, a wildly enthusiastic crowd edged on professional skateboarder Tony Hawk as he attempted to do the impossible. No one had ever done this trick before and every attempt that Hawk tried to do it was a potential trip to the emergency rom. Standing on the right side of the half-pipe, Hawk plunged down to gain momentum and flew up and out on the left side into the skies. Airborne and high above the ground, he quickly tucked his entire body, clutched his skateboard, and twisted his body for two and a half revolutions, equivalent to 900 degrees. As soon as he finished his revolutions, he extended his legs and pushed the board out beneath him before sliding back onto the left wall of the half-pipe. Tony Hawk had just completed the first-ever “900”, a trick that had eluded him for almost a decade and was considered the Holy Grail in skateboarding up until that day in 1999.
The first question that comes to mind is what made the 900 possible? Although many professional skateboarders have never even attempted tricks as life-threatening as the 900, they still possess the ability to fly through the air in a fusion of unique twists and turns. This raises another question; what makes any of this possible? While the easiest explanation may be that skateboarders are defying the laws of physics, they are actually exploiting the forces of nature and using them to their advantage. Every trick a skateboarder performs utilizes a group of fundamental principles that control the forces of motion in nearly every sport: momentum, rotation, gravity, and a person’s own strength.
Before the legendary 900 was ever dreamt of, there was the Ollie. It was invented in the late 1970s by Alan “Ollie” Gelfand and has become the foundation for more complicated tricks. The Ollie is a jump where the skateboard appears to be attached to the person’s feet and it allows the skater to hop over obstacles without getting off the board. Many onlookers assume that the board is stuck to the skater’s feet when he is performing an Ollie however, it’s not. In order for a skateboarder to propel into the air, he must first push down on the tail. A detailed explanation reveals that the secret behind this old-school maneuver is skillful control of the rotation around multiple axes.
The Ollie begins in an explosive manner as the skater rolls across flat ground placing his front foot in the middle of the board behind the front trucks and his back foot on the tail of the board. Dropping into a crouch, he lowers his center of mass for maximum pop. Before a skater performs an Ollie, there are always three main forces acting on the board and the skater. They are the weight of the rider himself, the force of gravity on the board, and the normal force of the ground pushing up on the skateboard. These three forces balance out to zero and without any net force the skateboard does not accelerate. Assuming that there is no friction, it rolls at a constant speed. In the presence of friction, it would obviously stop at one point.
Once the skater wants to perform an Ollie, he thrusts himself up with his arms, bolting his body in an upward motion before his feet rise. Coming from a crouching position, the skater has a greater distance to accelerate and thus a higher jump. During the pop, or jump, the skater’s back foot applies a much greater force on the tail of the board than the front foot does on the nose or front of the board. This counter force causes the board to rotate counterclockwise about the rear wheel. As the tail of the board strikes the ground with great force the board begins to pivot clockwise around the center of its mass. The tail then bounces back up and now the board is completely in the air, rotating forward with the nose of the board beginning to seep as the tail soars up. If left alone without any outside force, in this case the skater, the board would eventually flip tail over nose. However, the skater has no intention of abandoning the trick and while he is airborne he uses his feet to control the rotation of the board, sliding his front foot in an upward sweeping motion to drag the board higher into the air along with his rising leap. With the help of the grip tape on the skateboard’s surface, the dragging allows the skater’s foot to stay in contact with the skateboard because of its sand paper like features. Meanwhile, the skater rises up his rear foot to get out of the way of the popping tail. Done and timed correctly, the skater’s feet and board will rise in perfect unison, level at its maximum height. At this point, the board along with its skater will begin to fall down back to earth due to gravity. The skater then bends his knees to absorb the impact of the skateboard striking the ground.
Once a skater masters the fundamental Ollie, they will usually move on to more advanced techniques such as aerial maneuvers. In a real life scenario, a skater launches high into the air from the top of a ramp. Hanging in the air for a split second, as if to tease the laws of physics, he suddenly rotates at the top of flight and flies back down the ramp. Skateboarders call this trick a front side 180. Physicists may call it extremely sneaky.
At first glance, the front side 180 breaks the law of conservation of angular momentum completely. The law states that “If you’re rotating, you’ll keep rotating unless a twisting force or torque acts to stop you”. It seems like a very basic law. However, one thing to keep in mind is that the only force that can act on you while you are airborne is gravity. One more thing to remember is that on Earth, gravity does not have the ability to make you rotate; it can only make you fall down. So, how is it possible for a skater to go from a non-rotating state of motion to a rotating state of motion without any external forces acting on them?
The answer to this riddle is very simple. Borrowed from a housecat, this trick allows skaters to rotate in midair while keeping their angular momentum constant at zero. A cat extends its hind legs and twists the front portion of its body toward the ground when falling. The rear half of its body also rotates in the opposite direction, but not as hard. The cat then pushes out its front legs and turns its rear legs down toward the floor. By repeating these motions, the cat gains a significant rotation that allows it to land with its paws on the ground.
In order to understand this more in depth, an experiment can be conducted to show exactly what allows you to rotate in midair. First find a clear area in which you can stand and jump. Have an assistant at hand to help you out. Launch into the air and have your assistant point either left or right indicating which direction they want you to attempt to turn. Whichever direction they chose, quickly turn your body that way. By pulling this off, you have just mimicked what a skater does in the air; use his upper body to rotate. As your legs rotate beneath you, your arms and chest rotate in the opposite direction. By sticking your arms out you increase the rotational inertia of your upper body. The idea is that the large rotation of your legs is cancelled by a slight rotation by your arms. Because the two rotations effectively cancel, your angular momentum stays at zero and the law of conservation of angular momentum is not broken.
A skateboarder does exactly the same thing when he is airborne. While in midair, skaters may twist their arms and legs in opposite directions. When they fall back to the ground, they manipulate the friction between their feet and the deck of the board to twist their torso back to being parallel with the board and skate along.
One of the most apparent but overlooked application of physics in skateboarding is in the vert ramp. A vert ramp is essentially a half-pipe where the steepest section of the ramp is straight down and is usually over eight feet in height. (note, book) The goal in a vert ramp for the skater is to gain enough momentum to go over the top of one wall or side of the ramp, rotate in midair or basically perform a frontside 180, and go back down repeating the process on the other side. At the top of each ascent, they are airborne for a short period of time. The skaters are not just skating back and forth. They are working to get their momentum.
Physics says that when you are at a certain point above the ground, for example a vert ramp in skateboarding; you have potential energy proportional to this height. By rolling down, you convert this potential energy into kinetic energy and convert it back to potential energy when you roll up the other side of the ramp. However a skater must take into consideration the loss of energy due to air resistance and friction between the wheels and concrete. If the skater wants to rise above the ramp they must add more energy.
On a flat surface a skater would add more energy is by pushing off the ground. On a ramp however, a much more intricate method called pumping is used. Centripetal force keeps a body moving in a circular path, in this case a half-pipe, and makes it hard to rise up. Pumping overcomes the centripetal force and gives you a net energy gain.
To pump in a half-pipe, the skater pulls down into a crouching position while rolling along the bottom of the half-pipe. When they approach the sloped part of the ramp, known as the transition, they straighten up their legs and pull their body back up. The skater raises their center of mass at the beginning of the ramp’s arc, and thus gains energy and creates an increase in speed. By repeating this process, a skater gives himself consecutive speed boosts every time he passes through the transition. This kind of pumping is identical to the kind that you do on a swing. You lift your ankles and feet as they pass through the bottom of the arc and then drop them at the top of the arc. Paul Doherty, a physicist at the Exploratorium in San Francisco explains; “When you lift your legs at the bottom, your muscles have to work extra hard against the gravity force and the centrifugal force. The energy you exert by lifting your legs against these forces makes you go higher and faster.”
Most skaters know that in order to get big air, you need a big ramp. Larger ramps are much easier to master for the daredevil skater, because their bigger transitions ease the shift from a vertical motion to a horizontal motion. Also, since greater speeds mean a greater centrifugal force to push against, large ramps make pumping much more effective. However, eventually the energy added with each successful pump cannot make up for the energy lost due to wind resistance. The point? Height records will continue to be soar, but with each consecutive inch that is added on, a steeper price will come for the skater.
It should now probably be clear how Tony Hawk was able to pull of the 900. He had to create a strong enough pump to propel himself high above the vert ramp and have time to rotate 900 degrees. Plus he had to create the necessary torque to twist his body two and a half times. The couple of seconds he was airborne was not sufficient enough for him to fabricate the 900 degrees of spin. So in order for Hawk to complete the mega trick, he had to leave the ramp already spinning. Then he had to manipulate this rotational energy into an even faster spin with a technique borrowed from ice-skaters.
In order for ice skaters to accomplish their midair spins, they start with a wide spin, arms and legs extended. Once airborne, they retract their limbs inward, thus decreasing their rotational inertia, causing them to spin faster. In a similar fashion, before Hawk launches from the top of the ramp, he generates a significant amount of angular momentum. Approaching the top of the ramp with outstretched arms, he tucks and begins to spin his body, slamming down hard on the board to create an angular force. The momentum gained in this vital moment is all he will have throughout his flight. Once he leaves the ramp for good, he cannot get any more.
Once he is airborne, he accelerates his spin by stretching one arm over his head to add rotational torque. He also drops his other arm in order to cradle the skateboard since there is not enough friction between his feet and the board in order to keep the board throughout his mind-blowing trick. Since his arms are now in alignment with his entire body, it speeds his spin giving him the ability to perform two and a half rotations in a split two seconds. Hawk has almost on control of what he’s doing. Turning at incredible speeds, his body is nearly parallel to the ground and he loses sight of the ramp from which he has launched his unbelievable trick and on which he must land. However Hawk maintains some control by lunging his arms wide after the second full revolution. This allows him to spot his landing by slowing down his rotation and as his skateboard touches back onto the ramp, he absorbs the impact by shifting into a crouching position, preparing for a controlled landing at the very bottom.
Moments after Tony Hawk had just landed the first ever 900, people were already beginning to wonder whether he could do three full spins. An unheard of, absolutely ridiculous 1080, that could be a definite but delayed possibility. Hawk put rest to the speculation in an interview stating that he would leave the 1080 to younger, more robust skaters.
Starting with the Ollie in the 1970s, skateboarding has advanced beyond its original boundaries and has extended to much more than the original “Just get down the hill” philosophy. In an Ollie, the main thing to remember is rotation around multiple axes and how keeping a low center of mass is crucial in order to get high into the air. The frontside 180 is performed by manipulating the law of conservation of angular momentum. The skateboarder uses his arms as a torque in order to turn himself around in mid-air. Centripetal force keeps objects moving in a circular path. When you skate in a half-pipe this force keeps you in a crouching position and by pumping you not only overcome the centripetal force but you boost yourself up and over the side of the ramp. Tony Hawk did what was considered the impossible back in 1999. Editor of Thrasher magazine, also known as the “skateboarder’s bible” enlightens us with his opinion: “The greatest thing about skateboarding is it changes every day. The first time I saw somebody Ollie on the street I was like, ‘No way!’ But now every kid can get on a board and make an Ollie. Today’s impossible trick is just cannon fodder for the future.” By understanding the laws of motion and the basic principles of physics, every skater can develop their skills and everyone who is amazed by these street performers can finally understand that it’s not magic; it’s simply science.