![]() ![]() This is backed up by the fact that this portion of the ollie makes the largest displacement.įlex of the tail (mainly the J-shaped ones) could cause a reasonable upthrust. Out of all these obviously the lever mechanism causes the most upthrust. Let us first examine all possible mechanisms for forces that have an upward component. Overall this stunt involves a carefully practiced choreograph of pushes and pulls on each leg to make the board leap into the air. At this point the rider's other foot stops the rotation with a slight nudge, leaving only the vertical motion allowing for the board to lift off completely.įrom this point on the rider has to actively maintain the orientation of the board as it comes down so that it lands in a way where they can continue riding on it. At this point, if the board was allowed to continue to rotate it would pivot about its end launching itself between the rider's legs (not pleasant). Once enough vertical velocity (yellow arrow) is accumulated by the center of mass and the end of the board strikes the ground, this reaction causes the wheel to lift off. To accelerate upwards more, a lift force is needed. This is important as kinematically the rotation of Stage-I can only lift the center of mass so much, and then it stops. The back foot causes rotation which is resisted by the front foot, while the foot is sliding upwards causing the sliding friction to add to the lift on the skateboard. For the whole time before the riders foot touches the ground, the center of mass of the board is accelerating upwards (acquiring vertical momentum) At least the initial part that sets things up for the stunt.Īt this stage, the foot pushes downwards, but the reaction from the ground is greater because the center of mass (yellow dot) must move upwards, due to the kinematics of where the pivot is. ![]() The answer by is correct, but I wanted to illustrate the process a bit. Updated Answer based on comments by that actually know more than me about this. ![]() This won't get the board off the ground in the first place (since friction is always parallel to the board), but once the board is oriented somewhat upright, the board can be pulled further upward by the front foot. As the board leaves the ground and rotates up into the front foot, it produces a normal force, which allows the front foot to impart a frictional force parallel to the surface of the board. In particular, the front foot can add some lift to the board as it slides forward to the nose. There is nothing related to aerodynamics at play here, this trick could be performed exactly the same way in a vacuum.ĮDIT: There seem to be some other factors at play that I've missed here. If they just stomped on the kicktail without doing anything else, the board would arc upwards and backward, flipping end over end through the air. Because the skateboarder is tens of times as massive as the board, they are easily able to manipulate its momentum with their body, while changing their own momentum relatively little (if you look closely, you can see that both the skateboard and skateboarder do, in fact, land slightly behind the point of liftoff). The skateboarder then uses their front foot to stop this horizontal/rotational motion of the board and keep it under their feet, which is possible because the skateboarder has much more mass/inertia than the board. The impulse imparts both vertical and horizontal momentum, which you can see in the first part of the skateboard clip as the center of the board moves both upward and backward. You may notice that the object not only flies up but also across the room toward the end you hit. To set up a similar experiment, lay a ruler or pencil so it hangs over the edge of a table a small amount, hit down on the free end, and watch it fly up into the air. If you do this fast enough, the skateboard's center of mass gets enough upward momentum to lift the entire skateboard off the ground. The skateboard acts as a lever around the rear wheels, so when the kicktail is pushed down, the center of mass of the skateboard rises up. The skateboard is able to lift off the ground because of the momentum imparted to it by the skateboarder pushing down on the kicktail. ![]()
0 Comments
Leave a Reply. |