How can a skateboarder move without pushing anything?

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Ayomide

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Ayomide asked the Naked Scientists:
   
A skateboarder on a horizontal surface can accelerate from rest just by moving the body without touching external support.  

How does this work?

What do you think?
« Last Edit: 14/05/2010 00:30:06 by _system »

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Offline Geezer

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How can a skateboarder move without pushing anything?
« Reply #1 on: 14/05/2010 06:31:04 »
That's a very good question. I have some ideas, but the last time we tried to figure out how a bicycle stays balanced, we got into a very lengthy and somewhat heated debate.

As this one is potentially far more complex than the bike question, I'm going to pass.

Any takers? There must be at least one skateboarder out there.
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline Vern

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How can a skateboarder move without pushing anything?
« Reply #2 on: 14/05/2010 06:39:45 »
The skateboarder does actually push against the wheels friction against the surface. This can give an acceleration in the direction of the push. It is similar the push you get by pumping a swing. It is the same push you get when skating with roller skates or skis. You push from side to side and convert your side to side energy to forward motion.

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Offline JP

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How can a skateboarder move without pushing anything?
« Reply #3 on: 14/05/2010 06:56:29 »
Maybe the skateboarder ate a lot of beans and is ejecting hot gasses just like a rocket...

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Online syhprum

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How can a skateboarder move without pushing anything?
« Reply #4 on: 14/05/2010 09:04:43 »
Skateboarders seldom if ever just stand on a stationary board and set it in motion they normally stand with one foot on the board and push off with the other.
To start off with both feet on the board they could use a technique similar to tacking on a boat by pushing in a sideways direction where the friction to the ground is greater than the rolling friction.
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Offline LeeE

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How can a skateboarder move without pushing anything?
« Reply #5 on: 14/05/2010 20:56:00 »
As Vern says...

Quote
It is similar the push you get by pumping a swing

Vern also mentions roller skates and skis, but although skateboards, roller skates and skis all steer by using the same fundamental mechanism it is only generally with skateboards and roller skates that this mechanism is also used to accelerate or maintain speed; with skis you're generally going downhill (although cross country skiers will use skis that have a directional tread or pattern on the bottom face of the ski, allowing the skis to slide forwards but not backwards, so that the skis can be 'walked' forward, aided by pushing with the ski sticks).

However, it is the steering mechanism that allows speed to be maintained or increased with skateboards and roller skates.  Skateboards, roller skates and skis are all steered by rolling them over, to one side or the other.

Skis are designed so that they are relatively wide at the front and rear ends, but are narrower where the feet are positioned (typically about two thirds of the way along the length of the ski) and this means that when the ski is rolled to one side or the other the weight of the skier bends the ski into a curve as the narrower part of the ski has to flex further before it touches the surface than the wider ends of the skis.

The turning effect with both skateboards and roller skates is achieved by using pivoted 'trucks' to carry the wheels, the pivot axis being oblique between the longitudinal and vertical axis, with the result that when the skateboard or roller skate is rolled to one side or the other along the longitudinal axis it also rotates the wheels around the vertical axis, turning them left or right.

In practice, it just as hard (if not slightly harder) to start a skateboard moving from rest, just by moving your weight around or varying the pressure through your legs, as it is to start swinging on a swing from stationary, but once you are moving it is relatively easy to maintain or even increase your speed.  On a swing, you increase your speed by shifting your Center of Gravity (CoG) forwards and backwards when at the top of each arc of the swing to increase your speed at the bottom of the arc.

With a skateboard, this mechanism is effectively turned through 90░ but instead of the skateboarder shifting their CoG back and forth to increase the energy at the end of each turn (or arc) against a constant force of gravity they use their legs to push sideways against the skateboard, which more like varying the force of gravity through the arc of the a swing.

In the end, maintaining or increasing your speed on swings, roller skates and skateboards are all examples of Parametric Oscillation.
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