Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How and why are shock waves produced in breaking objects?

  1. Jan 5, 2013 #1
    The example I have in mind is when a pole vaulter's pole breaks. I remember at last year's Olympics one of the commentators made a commenf about thw shock waves thag would've been travelling through the pole and the guy's hands when it broke.

    I did a search and it was mentioned in some comments on YouTube that it would feel kind of like (but worse) hitting a something with a baseball bat but neither object breaking. Would whatever hurts your hands in this case be the same as the above?

    And one more example...when I cut the inner plastic tube of a pen it started to hurt my fingers more and more as it got shorter (kept cuting it in half).

    Assuming these are all shock waves, how are they caused? What would be going on at a molecular level at the breaking point? also, would they be able to do damage if you are close to but not touching the surface of the object?

    Thanks
     
  2. jcsd
  3. Jan 5, 2013 #2

    jedishrfu

    Staff: Mentor

    when you bend a pole vaulters pole you need to apply a force to bend it a certain amount and if it snaps that energy is released and the pole tries to go back to being straight but it does this by oscillating back and forth. The oscillation is felt by the pole vaulter as he tries to hold onto his pole.

    A simple analogy is to look at a diving board after someone has dived off. It oscillates back and forth until the energy is dissipated.
     
  4. Jan 5, 2013 #3

    Bobbywhy

    User Avatar
    Gold Member

    autodidude, It’s important to always use the correct scientific terms when describing physical actions. I am not sure the breaking of a pole vaulter’s pole creates a shock wave. Although solids do support the propagation of a shock wave, I could find no example given on this Wikipedia page:

    “A shock wave (also called shock front or simply "shock") is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma)”
    http://en.wikipedia.org/wiki/Shock_wave

    Hopefully some real physicist can enlighten us all: Does the breaking of a pole vaulter’s pole cause a shock wave to propagate in it?
     
  5. Jan 5, 2013 #4

    jedishrfu

    Staff: Mentor

    Not sure if this is a good resource but this blog post forum discussion I found talks about one break initiating a shock wave that in turn causes other breaks in the pole.

    from blog:

    http://www.polevaultpower.com/forum/viewtopic.php?f=8&t=19848#p142082
     
  6. Jan 7, 2013 #5

    jedishrfu

    Staff: Mentor

  7. Jan 15, 2013 #6
    So after it breaks, it's oscillating and it's this oscillating that causes the pain?

    Yeah, I'm not knowledgeable to know the difference yet though! :p

    Thanks, I came across that when I did the search but I'm not it sure it really explains what I'm after
     
  8. Jan 16, 2013 #7
    How were you cutting the pen? A shorter segment is more rigid and the natural frequency of oscillation would be higher. But the increased level of pain may be an accumulative effect.
     
  9. Jan 16, 2013 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You really need to think of the definition of a shock wave. That, afaik, is what happens when a part of a substance is forced to travel faster than the propagation of sound (wave propagation) in the medium. (As with the shock wave produced in supersonic flight). When an object is broken, the speed of part of the object would need to be faster than the wave speed in the substance. I guess this is possible if the shape of the break is suitable but I can't actually think of an example.
    Shock waves do not last long when travelling in a medium - they slow down and become normal sound waves within a wavelength or so.
     
  10. Jan 16, 2013 #9

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    What that article fails to make clear is that the secondary break is in the reverse direction. This shows that it is not the simultaneous failure hinted at by Decamouse, but rather a result of recoil.
     
  11. Jan 24, 2013 #10
    I used a pair of scissors. Ah, that makes sense.

    So then technically, it doesn't really make sense to speak of shock waves in breaking objects? It's actually these oscillations that have been mentioned that are dangerous?
     
  12. Jan 24, 2013 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Perhaps you could say it's sometimes the initial shock wave that starts the fracture, then the local stress breaks the obhect further and further. The shock wave explanation probably only applies to certain (brittle) materials like ceramics and not to most metals.
     
  13. Feb 15, 2013 #12
    Sorry to bring this up again but I came across the usage of the term 'shock wave' again and not sure if it's the correct term. The context is the effects on impact on the human body during car crashes.

    (~15:15)

    '..the impact produces a shockwave that moves through the body similar to a sound wave moving through air...'

    And he hits a gel with a mallet and there's a visible wave that moves through the gel - is that technically a shock wave? He also says the wave changes speed as it moves through the human tissue of differing densities producing 'complex wave interactions' that cause stress and strain in the tissues and organs.

    What are these 'complex wave interactions' and how do they cause stress and strain?
     
    Last edited by a moderator: Sep 25, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How and why are shock waves produced in breaking objects?
  1. Shock waves (Replies: 1)

Loading...