Skateboarding is physics! Find out what principles of physics a skateboarder uses to appear to defy gravity on the one-half-piping.

By Kate Rock and Dr. Jonathan Trinastic

People skateboarding might seem to defy gravity equally they soar loftier higher up a one-half-pipage, only they are actually taking reward of specific physics principles that help them reach such heights. Physicist Dr. Jonathan Trinastic explains.

Kate: As a physicist, what practice you lot see happening in these photos?

Science of Skateboarding: Half-Pipe Physics

Jonathan: People skateboarding on a half-pipage take advantage of two master physics conservation principles: conservation of energy and angular momentum. Permit's offset with conservation of energy. Energy can be cleaved down into two master categories: kinetic free energy and potential energy. Kinetic energy is associated with movement and is proportional to the square of the velocity of an object—in this case the skateboarder. Potential energy is different depending on what force the skateboarder is feeling, but in this case that forcefulness is gravity. With gravity, the potential free energy is proportional to how high from footing level the skateboarder is: the college the skateboarder, the more potential free energy she has. The key idea backside conservation of energy is that the total amount of kinetic plus potential free energy can never change, but each can be transformed into the other.

Kate: So, how practise these principles apply to the half-piping?

Jonathan: Commonly, a person skateboarding will start from a high platform near the height of the half-pipe. This gives her a large amount of potential energy since she is high up, but no kinetic free energy since she starts at residue. This also means her full amount of free energy is equal to her potential energy. Now, the skateboarder jumps down on her board and begins rolling downwards the one-half-pipage. She loses potential free energy but picks up speed—this indicates that her potential energy has converted to kinetic free energy. When she reaches the bottom of the half-pipe, she has lost all her potential energy just has a lot of kinetic energy in the form of a high speed. The skateboarder can then use this speed to zoom upward the other side of the half-pipage and launch into the air to practice tricks.

Kate: In that case, how practice skateboarders go so high on their jump, seemingly higher than their starting points on the other side of the half-pipe?

Science of Skateboarding: Half-Pipe Physics

Jonathan: The answer lies in the second physics principle: conservation of angular momentum. Merely similar traditional momentum being equal to mass times velocity, athwart momentum is a rotational analogue, equal to an object's moment of inertia times its athwart velocity. Moment of inertia can be idea of as a "rotational mass" and depends on the mass of an object and its altitude from its centrality of rotation. The farther an object is from its axis of rotation, the larger its moment of inertia, and therefore the larger its athwart momentum. Angular momentum must always be conserved, so if the object's angular velocity increases, then its moment of inertia must decrease, or vice versa.

Kate: What does this accept to do with the skater on the one-half-pipe?

Jonathan: A skater tin take advantage of angular momentum conservation to get especially high on her half-pipage leap. Get-go, consider her axis of rotation every bit she'south curving up the one-half-pipe. If we consider the one-half-pipage as one-quarter of a circumvolve, and then the skateboarder is rotating in a circular move every bit she moves forth the one-half-pipe, with an axis of rotation somewhere in space far from the half-pipe itself. At present, remember that the moment of inertia increases with altitude from the axis of rotation. Therefore, if the skateboarder crouches during her arroyo to the half-pipe, her center of mass moves further away from the axis of rotation, increasing her moment of inertia. Now, merely as she's flying up the half-piping, she extends her body straight as far as she tin can and shoots her easily in the air. This moves her center of mass up and closer to the axis of rotation, decreasing her moment of inertia. Since athwart momentum must be conserved, her angular velocity must increase, giving her even more than speed as she hits the meridian of the one-half-pipe and flies into the air, doing equally many tricks as she can. Thus, she has used the principles of momentum conservation to get extra "air" above the half-pipe.

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Kate: That was a lot of proficient information. How can agreement physics assist a skater successfully execute a half-piping?

Jonathan: The short answer is that skateboarders can change their speed coming off the half-pipe by first crouching and then shooting straight upwardly.

Kate: What is centrifugal strength and how can someone use it skate up a wall?

Jonathan: Centrifugal force is really a "fictitious force," in that it's really not a physical force such as gravity or electromagnetism that is affecting you. Instead, the centrifugal force arises when considering dynamics from a rotating reference frame. All motion must be considered from some reference frame. If you are standing at your window watching a rabbit run beyond your lawn, then your frame of reference measuring the rabbit motility would be stationary—yous at the window. The rabbit motion would look different if you were running away from information technology, for example, which would exist a moving reference frame.

In the aforementioned way, as a skateboarder moves in a circular move up the half-pipe, she is now in an accelerating reference frame, with a centripetal ( not centrifugal) forcefulness pulling her toward her centrality of rotation. A stationary observer would observe this force acting on the skateboarder. Withal, from the

Science of Skateboarding: Half-Pipe Physics

skateboarder'southward frame of reference, she feels a fictitious centrifugal forcefulness pulling her against the half-pipe wall, due to her inertia taking her in a direction tangential to her circular move.

This centrifugal fictitious force is felt past anyone moving in circular motility. A common example is on spinning entertainment park rides, which plaster u.s.a. confronting the walls with nothing supporting usa below. Although it feels like a force is pushing us into the wall, it's really but the inertial motion from the round path we take.

Kate: Are skateboarders defying gravity? Also, what is gravity?

Jonathan: Gravity is the force between whatever 2 objects with mass. Normally, we remember of gravity betwixt u.s. and the Earth, merely we really have a gravitational allure betwixt us and any other affair around usa: a tree, a water glass, a friend.

Science of Skateboarding: Half-Pipe Physics

That gravitational strength is just so weak that we don't notice it. The simply reason we feel the gravitational attraction to the Globe is because the planet is and then massive!

Information technology can seem like skateboarders defy gravity, just the force is always acting on them. Skateboarders can just have advantage of other physics principles—principles of energy conservation and angular momentum to help fly through the air and seemingly defy gravity. Simply gravity is e'er acting on them and is the only reason that skateboarders come back to globe after a skillful jump. So no, skateboarding doesn't really defy gravity.

Kate: What other physics principles do we run into in skateboarding?

Jonathan: In add-on to potential and kinetic energy, some energy is lost due to friction between the wheels and the ground during a skateboarder'south run. For this reason, she tin can't go upward and down the half-pipe forever skateboarding; over time, she will lose energy to friction, then she can't maintain the aforementioned corporeality of kinetic energy, which in plough tin can't be converted to the same corporeality of potential free energy to reach such high heights on her jumps.

Photography of Lisa Fahncke, Justin Philip Nash, and others skateboarding by Kate Stone, SOMA Skatepark in San Francisco, May 2017.