Quarter pipe ramps and skateboard half-pipes are often seen at skateparks. Easy to transport, they consist of a bank or slope at a straight or curved angle to perform jumps. The most commonly used are launch ramps and kickers. Photography of Lisa Fahncke, Justin Philip Nash, and others skateboarding by Kate Stone, SOMA Skatepark in San Francisco, May 2017.Skate ramps, also known as skateboard ramps, are used by skateboarders to perform tricks and jumps in the air. For this reason, she can’t go up and down the half-pipe forever skateboarding over time, she will lose energy to friction, so she can’t maintain the same amount of kinetic energy, which in turn can’t be converted to the same amount of potential energy to reach such high heights on her jumps. Jonathan: In addition to potential and kinetic energy, some energy is lost due to friction between the wheels and the ground during a skateboarder’s run. Kate: What other physics principles do we see in skateboarding? So no, skateboarding doesn’t actually defy gravity. But gravity is always acting on them and is the only reason that skateboarders come back to earth after a good jump. Skateboarders can just take advantage of other physics principles-principles of energy conservation and angular momentum to help fly through the air and seemingly defy gravity. It can seem like skateboarders defy gravity, but the force is always acting on them. The only reason we feel the gravitational attraction to the Earth is because the planet is so massive! That gravitational force is just so weak that we don’t notice it. A stationary observer would observe this force acting on the skateboarder. In the same way, as a skateboarder moves in a circular motion up the half-pipe, she is now in an accelerating reference frame, with a centripetal ( not centrifugal) force pulling her toward her axis of rotation. The rabbit motion would look different if you were running away from it, for example, which would be a moving reference frame.
If you are standing at your window watching a rabbit run across your lawn, then your frame of reference measuring the rabbit motion would be stationary-you at the window. All motion must be considered from some reference frame. Instead, the centrifugal force arises when considering dynamics from a rotating reference frame. Jonathan: Centrifugal force is actually a “fictitious force,” in that it’s actually not a physical force such as gravity or electromagnetism that is affecting you. Kate: What is centrifugal force and how can someone use it skate up a wall?
Jonathan: The short answer is that skateboarders can change their speed coming off the half-pipe by first crouching and then shooting straight up. How can understanding physics help a skater successfully execute a half-pipe? Kate: That was a lot of good information. RELATED: Waves of Physics: The Science of Surfing Thus, she has used the principles of momentum conservation to get extra “air” above the half-pipe. Since angular momentum must be conserved, her angular velocity must increase, giving her even more speed as she hits the top of the half-pipe and flies into the air, doing as many tricks as she can. This moves her center of mass up and closer to the axis of rotation, decreasing her moment of inertia. Now, just as she’s flying up the half-pipe, she extends her body straight as far as she can and shoots her hands in the air. Therefore, if the skateboarder crouches during her approach to the half-pipe, her center of mass moves farther away from the axis of rotation, increasing her moment of inertia. Now, remember that the moment of inertia increases with distance from the axis of rotation. If we consider the half-pipe as one-quarter of a circle, then the skateboarder is rotating in a circular motion as she moves along the half-pipe, with an axis of rotation somewhere in space far from the half-pipe itself. First, consider her axis of rotation as she’s curving up the half-pipe. Jonathan: A skater can take advantage of angular momentum conservation to get especially high on her half-pipe jump.
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