Spins, Flips and the Laws of Inertia



      When it comes to riding, there’s nothing like mastering that new trick. A common area of interest between professional and novice riders alike is new arial flips and spins. Well my friends, let the laws of physics ease your mind as you conquer new spins and flips. The topic of this page is revolved around the physics of rotational motion, more specifically, a little something called the moment of inertia.


Moment of inertia is defined as the characteristic dispersion of mass troughout a spinning object. As it turns out, controlling ones’ moment of Inertia is a great way to control motion during rotational motion similar to the kind experienced during a spin or flip, let's explore... All objects that are moving and possess mass also possess momentum this fact is modeled in the equation p=mv for linear motion, but it also applies to objects in rotational motion. Just like linear motion, a spinning objects' momentum is conserved and is modeled by the equation L=Iw . The law of angular momentum conservation states that the angular velocity of an object is dependent on the moment of inertia (granted constant mass). As a rule of thumb, the farther from the center of mass an objects’ weight lies, the larger its’ moment of inertia. The Law of conservation of momentum is portrayed by the equation Iw(intial)=Iw(final), where Iw=L (angular momentum).    This law of motion has valuable implications to the world of a snowboarder. The consequence is that for angular momentum to be conserved, the variables on either side must even out to give a steady number. This means that as the moment of inertia changes, angular velocity must also fluxuate to maintain constant momentum. So in effect, as a moment of inertia decreases, an object gains angular velocity to compensate likewise, when inertia increase angular velocity must decrease accordingly. Knowing this law, we are able to form a few well founded guidelines for approaching and excecuting high caliber spins.

·      Steps to pulling off a high grade spin

1.)   At the last moments prior to leaving the end of the ramp, dig your edge in to direct your board parallel to the jump in a curved motion, the direction of your spin. Simultaneously pre-winding your torso and arms in the opposite direction of spin to supply the torque in the following step.

2.)  The instant your board leaves the end of the ramp, release your pre-wound torso like a spring, supplying as much force from this motion as possible. Your body should be at about 2/3 height at this position. At this instant your body has a relatively large moment of inertia, so the angular velocity is not optimized at this point in flight.

3.)  Tuck up your legs and suck in your arms, this motion will lower your moment of inertia and cause an increase in rotational velocity.

4.)  Drop the landing gear. With only a second or two to spare, let your arms come out and legs drop down while keeping an eye out for your landing. This will slow your rotational motion and have a stabilizing effect on your course of flight. Remember that a second or so is all you need to drastically reduce rotational motion. Be sure not to unwind too early, as it is difficult to maneuver in the air once you have. Slightly tucked legs at the end of a landing can also act as a cushioning spring, a nice thing to have at the end of a long fall. The low friction forces allow riders to quickly transfer the energy towards motion instead of say, the riders’ bone structure.




Where's the prove you ask, let's see an example...

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        This example reveals a remarkable increase in rotation. Indeed it may even seem excessive, remember however that the equation does not account for air drag and mass dispersion may not be true to life (i.e. 15 kg arms are too heavy). We must also keep in mind that rotational motion requires energy. His kinetic rotational energy will thus diminish over time, since there is no permanant acceleration supplying determining rotational velocity. The principle however, remains true to life and accounting for this law can dramatically improve your spin game, keep it in mind.                                  


Friction: Friend or Foe?


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