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Jumping on glass

  1. Oct 1, 2007 #1
    Hello,

    I've been to a couple of towers in the world (CN and Blackpool) with specially build glass floors that you can safely walk/jump on.

    In each case, claims have been made stating that the glass is capable of withstanding a particular weight or pressure.

    Intuitively, I feel I would be more likely to break the glass if I stood on a box and jumped on to the glass, and I'd be even more likely to break it if I was to run up a ladder and jump on to the glass.
    I can understand in each of these scenarios how I would have a greater energy - but how could the weight or the pressure I exert on the glass be affected by this?

    Has the wrong statistic been used? Should the text have given the energy required to break the glass instead? Or am I missing something fundamental? Any ideas what would be required to break the glass and how close I could get to doing so on my own?
     
  2. jcsd
  3. Oct 1, 2007 #2
    Jumping people exert the same force and pressure *on average* as do standing people.

    But jumping people periodically exert zero force or pressure for a large portion of the time, and hence must exert a greater force (and pressure) at other times.
     
  4. Oct 1, 2007 #3
    Normally, the force you exert on something is just your weight (more or less). Just standing there, you have no downward velocity (unless you fidgit or listen to some good beats on your iPod).

    Now, if you jump from some height, you will have a velocity down (the higher you jumped from, the higher the velocity). When you touch the glass, it has to stop you by changing that velocity to zero. That's called acceleration and, according to Newton's 2nd law, produces a force. That force can be several times your normal weight.

    You can try this by walking on a frozen puddle (no, not on a lake!) which has perhaps 1/2 inch of ice. You'll be able to stand on it, but if you jump up and down, you'll break through.

    But, please don't try this on the glass. Even if you don't get hurt (which you likely will), you'll wind up being charged with a crime. Chill.
     
  5. Oct 1, 2007 #4

    mgb_phys

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    It's also the reason you have knobbly-knees.
    When you run, the force on your legs when your foot hits the ground is upt 15x your body weight. You need to spead this load out - so the joints at the end of you bones have to have large surface areas, hence knees and ankles stick out.
     
  6. Oct 2, 2007 #5
    Any idea what the deceleration time for landing is? I always saw it as a rather instantaneous event. Though I suppose in practice that makes no sense - it must just be a very very short time.
     
  7. Oct 2, 2007 #6
    Depends on your shoes and the stiffness of the glass. You're right, it can be very short.
     
  8. Oct 2, 2007 #7
    the deceleration depends on what kind of shoes your wearing but also if you bend your knees and how much. Bending your knees increases the time of deceleration therefore decreasing the force extreted. (so best case scenario you bend all the way down to the ground giving the longest possible deceleration time.
    F=ma , where mass is some constant (in this case your mass) therefore we can say F is proportional to acceleration.
    Acceleration or deceleration is the (Change in velocity) / (Change in time). Therefore if time apporaches zero (instaneous) then acceleration becomes Infinite. This means force apporaches infinity. Hope that made sense...
     
  9. Oct 3, 2007 #8
    A "shock wave" does not exhibt the same characteristic of a longitudinal pressure wave. So, there is a real potential for catatrophic collapse with "sudden impact"
     
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