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Why do stones bounce off the water?

  1. Nov 6, 2012 #1
    Why does a stone bounce off the surface of the water?

    Why does a bullet bounce off of the surface when it strikes a material at an angle?

    When I was learning about why glass water bend light, the use of a toy car rolling over different surfaces was used. The slowing down of one side of the car made it steer in that direction. Likewise, light acts similarly when striking glass at an angle.

    So why don't other objects do this? When you try and bounce a stone off the water, the stone hitting the surface of the water should slow down causing it to steer into the water.

    Same thing when a bullet hits the surface of anything it can go through, it should steer into the slower medium.

    So why do they bounce off instead?

    And for that matter, why do things bounce? I never got ho Newton's cradle worked.

    Personally, I think it's witchcraft.
     
  2. jcsd
  3. Nov 7, 2012 #2

    A.T.

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    Gold Member

    Some do. It depends on the shape. A non spinning cube thrown at the water surface at an oblique angle will flip over forward, because the bottom is slowed more. Just like you say. Initial rotation around the vertical axis can stabilize an object and prevent the steering into the water, but if the object is symmetrical enough, this doesn't matter. A sphere can bounce off well.

    A flat or even a roundish stone deforms the water surface such, that it forms a sloped ramp, on which the stone glides forwards & upwards:
    http://en.wikipedia.org/wiki/Stone_skipping#Scientific_explanation
    http://arxiv.org/pdf/physics/0210015v1

    In WW2 they even bounced bombs. Conventional ones:
    http://en.wikipedia.org/wiki/Skip_bombing
    And special ones, that were initially spinning backwards:
    http://en.wikipedia.org/wiki/Bouncing_bomb

    ETA: Ultimately the main difference to the light & toy cart models you mentioned is how the object interacts with the media. Unlike in your examples the stone must actually displace the medium to move through it, and the boundary between the two media is not static.
     
    Last edited: Nov 7, 2012
  4. Nov 7, 2012 #3
    Thank you A.T. for your conclusive and complete answer. My curiosity is satiated for today :approve:
     
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