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Thread: Question about skimming stones

  1. #1 Question about skimming stones 
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    do you know when you go to a pond or lake and you find a nice flat stone and decide to throw it at the pond/lake how does the stone bounce off the water? If you need me to explain this further just say.


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  3. #2  
    Forum Freshman Jinn's Avatar
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    I'm not sure what exactly happens but basically at a certain speed even water can be as hard as cement, so as long as you throw it hard enough it will skip until it looses that momentum.


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  4. #3  
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    http://en.wikipedia.org/wiki/Surface_tension

    Imagine a body of water; a stone (symmetric, smooth, relatively flat, and with slightly sharp progressive edges) is thrown from a height of 2 meters at a speed of 20m/s with a rate of 1 rotation per contact unit of time between the water and the rock. Ignoring the effects of the atmosphere (for the distance), it would arrive at the surface of the water 12.76 meters from its projection point. Upon contact, the stone would still have a neutral angle due to the stabilizing effects of its spin. Now you need to take into consideration very many hydrodynamical effects and processes in order to calculate what the stone does upon impact with the water. In general, most of the needed properties come from the study of liquid surface tension. The first property that decides foremost whether or not the stone bounces, is the static force of the water. Water likes to attract to itself, and due to its need to reduce its surface area, does not like to be in any other shape than a sphere (under ideal conditions). Because of its resistance to change, the rock upon impact has its y-direction force acted against by the waters need to maintain its shape, and the natural density that it has. The surface area reduction comes from the fact that if its surface area were higher, a higher energy level would result; in a similar way to which photons are expelled from an atoms orbital after impact, the intruding object is negated. That effect alone though is not enough to keep the stone moving forward. Other forces are acting on the stone while touching the water; the need for the water to fill in the small imperfections of the rock and therefore pull it to the surface, and the building up of x-axis speed depleting water. The imperfections would normally prevent the rock from continuing, but the smooth surface and the spin minimize this. The smooth surface reduces the number and strength of the imperfections for the water to hold the rock back, and the spin constantly eliminates a contingent stream of water from gripping hold of the rock. If it were not spinning, the water, while even so moving over areas of the rock, would create an arc behind it because of the waters intermolecular attractional force. This spin throws any impending arc in either left or right direction, preventing a detrimental hold from taking place. The second effect that the spin has, is to prevent an over arc from forming and trapping the rock beneath the water. The spin like with the arc, shreds the impeding impediment, in this case the lip that starts to form because of the still existent attractional forces between the rock and the water. If it were not for the spin, the lip would pull itself over the top of the rock, hence displacing water behind its origin, and therefore causing the negative energy to pull the rock into it in order to fulfill the void-leading to the rock sending itself into the water rather than the air.
    With everything considered, the rock uses the water as a compressive spring and flies into the air, with only energy in the y-direction lost to thermal conversion.
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  5. #4  
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    thank you
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