Newton does an ollie
The helmet-clad skateboarder travels fast along a flat path approaching a five-metre wide urban chasm. He then achieves a seemingly impossible feat.
Without a ramp he leaps across the gap between pavements, taking his board with him, lands smoothly on the other side and continues on his way.
A skilled skateboarder is able to jump up kerbs, over obstacles and across chasms, all the time keeping the board with him. The board appears to be attached to his feet, but it's not. This acrobatic manoeuvre is called the ollie, after the American skater Alan "Ollie" Gelfand who first demonstrated it in the 1970s. The ollie was one of the earliest skateboard tricks and it's the one on which most other tricks are based.
Skateboard tricks happen very fast and are difficult to follow, and even more difficult to understand. But if you watch in slow-motion you can probably work it out. The skater approaches the obstacle with his front foot on the centre of the board and his back foot on the tail of the board.
He crouches down to lower his centre of gravity.
Then, just before he reaches the obstacle, he moves rapidly upwards by straightening his body and throwing his arms in the air. This exerts a lot of force on the board and the skater focuses most of this force on his back foot, which pushes the rear of the board down so that it rotates about the back wheel. When the tail of the board hits the ground, the board bounces upwards.
If there were no skater in the way at this point, the board would leap into the air rotating backwards. In practice, the board is now pressed hard against the feet of the skater who is rising into the air carried by his leap.
At the top of the jump, the skater pushes down with his front foot and raises his back foot to bring the board to a level position. He then falls under gravity and hits the ground with his knees bent to absorb the force of the impact. An impressive feat.
In another trick, the skater races to the top of a ramp and launches himself into the air. Airborne, he hangs for a moment, turns round and then crashes back onto the surface of the ramp. Skaters call this the frontside 180 because he turns 180 degrees.
But how does the skater turn when he has nothing to push against? It seems to defy the laws of physics; in fact, the skater is making very clever use of those laws. One practical consequence of the so-called law of conservation of angular momentum is that if you are not rotating then you can't start to rotate unless some force acts on you. The skater achieves rotation in the following manner. As he leaves the ramp he performs an ollie so that the board is pressed hard against his feet.
Then, when he is hanging in the air, he throws his arms out wide and rotates the upper part of his body. His lower body will automatically rotate in the opposite direction. The rotation of his arms is exactly cancelled by the rotation of his feet so the law of conservation of momentum is not broken. Overall he has not rotated.
Since his arms are outstretched his upper body has more momentum than his lower body because their centre of mass is further from the skater. This means that a small rotation of his upper body gives a large rotation of the lower body in the opposite direction. So, with a bit of practice, the skater is able to turn his feet and skateboard 180 degrees and go back down the ramp. Once he has landed, and is in contact with the ramp, he can turn his body back to face the direction of travel.
In some ways the stunts of a surfboarder are even more spectacular than those of a skateboarder. Surfers balance on a small fibreglass board and travel very fast along the front surface of a wave containing hundreds of tons of flowing water.
In fact, gravity is making the surfer slide down the front of the wave but he never reaches the bottom because the wave is constantly rising up beneath him. The board provides some buoyancy, but not enough, and the surfer is actually skimming over the surface of the water in the same way as a boat does when travelling at high speed.
If the wave is travelling directly towards the beach then all parts of the wave would break at the same time. What a surfer wants is a wave that is approaching the beach at a shallow angle so that the point at which the wave breaks travels along the wave and the surfer then travels with it.
Things can become very dramatic when the wind is blowing inshore and the leading edge of the wave becomes markedly concave. At high wind speeds the surfer finds herself travelling through a huge tube of flowing water.
Surfers steer their boards by putting more weight on the back foot to push the back of the board under the water. By pushing down on the right-hand side it goes right, or the left-hand side it goes left. This works because it increases the resistance on one side of the board which then turns in that direction.
If the surfer wants to turn faster, he borrows a trick from the skateboarders. He crouches down on the board and then suddenly straightens up, pushing more of the board under the water to increase the drag; he then tilts in the chosen direction. Pushing the back of the board under the water acts as a brake which the surfer will use to bring himself to a gentle stop at the end of the ride. Only, of course, in those cases where he has performed all these tricks and still managed somehow to remain upright.
So, the next time you come across skateboarders in the street, stand and admire the skill with which they employ and enjoy the laws of physics.
Dr Chris Holt is a freelance science writer based in Bedfordwww.720ice.com, and surfing at www.surfline.comvideoPictures of a falling cat righting itself can be seen at www.exploratorium.eduskateboardingtrick_midair_activity.html
TRICKS OF THE TRADE
Key stage 3
It's possible to demonstrate how a skater rotates his body in mid-air if you have access to a trampoline. If a pupil bounces in the air and rotates the upper half of his body with his arms extended, you'll see his feet rotate in the opposite direction (angular momentum is conserved). When a falling cat turns in the air in order to land on its feet, it performs the same trick as a skater of rotating in mid-air while keeping its angular momentum zero. Discuss how the cat is able to do this. What do ice-skaters do when they spin?
Key stage 4
Skateboard and surfboard tricks can form a useful basis for a discussion about physics. Use their actions to discuss gravity, moment of inertia, centripetal force, and Newton's three laws of motion