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Physics in the Discus Throw

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  1. May 22, 2014 #1
    I am a discus thrower and a coach. It is clear that there are a number of physical attributes that make some throwers genetically superior. For example, height, weight, strength, and wingspan. What I would like to discuss, is what attribute is more important to the discus thrower, between height and wingspan. While any coach would contend that both height and wingspan give a thrower a greater potential to throw the discus farther, which one is a greater advantage, and why?

    Discus Throwers
     
  2. jcsd
  3. May 22, 2014 #2

    A.T.

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    What allows you to throw a ball further? A 1 meter long ball thrower (arm extension), or a 1 meter high table to stand on? What about a really heavy ball?

    The optimal combination depends on the mass of the projectile vs. your strength. I think, in the case of the discus thrower increasing wingspan would initially bring more benefit. But only within reasonable bounds. At some point you will need more height too.
     
  4. May 22, 2014 #3
    Thanks for the information. How would someone calculate the distance a discus thrower would throw on a particular throw?
     
  5. May 23, 2014 #4

    sophiecentaur

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    What data would you be able to supply? That's the problem.
    If you knew the speed and elevation on release, the problem would be simple (you could even factor in the wind speed, temperature and humidity) but modelling the athlete would be pretty difficult. I guess one strategy would be to use a lot of slow mo sequences of the athlete's action (plus distance results) and identify which parts of the throw correlated with distance. A slo mo sequence would show where in the throwing action the acceleration is greatest and least (high and low spots in performance). That could give you information to work on for improvement. Problem is that discuss involves pretty well all parts of the body - not just the arms - so I reckon the job can only be done heuristically.
    When the athlete happens to do a good throw, you need to know what he/she did right at the time and vice versa. A good coach will be doing that sort of thing all the time but a systematic approach (with a record of the event) could be even better.
     
  6. May 24, 2014 #5
    Aside from what on the surface appears to be simple physics, when we are discussing athletes it can be seen it is not that simple. Technique and timing come into play. Consider that at one time all high jumpers used the straddle form and now most use the "back over" form. In discus I am betting that many systems and fulcrum points are at play, right down to wrists and fingers. These actions are similar to multi-stage rockets where each successive stage adds it's power/speed to the previous. I have heard athletes say things like "It's all in the wrist" but this is untrue in an absolute sense. It really means don't leave out the wrists in the entire system that makes up the throw/punch whatever. They stack.

    As for the most basic answer for OP inquiry, I'm betting on wingspan having a slight edge, due to centrifugal force, assuming all else essentially equal.
     
  7. May 24, 2014 #6

    A.T.

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    What does centrifugal force have to do with?
     
  8. May 24, 2014 #7

    sophiecentaur

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    Nothing, directly but the higher the tangential velocity (that governs how far it will fly), the greater the centripetal force felt.
    But IMO, you need to be looking at the velocities of all body parts (knees, trunk, shoulders and arm sections) to maximise acceleration of each. I was thinking in terms of the technique used with Gollum etc. to see the detailed motion.
     
  9. May 24, 2014 #8
    I am thinking in terms of stored energy suddenly released on a tangent much like the radial devices that launch "clay pigeons" in skeet and also some baseball pitching machines. The longer the moment arm, the greater the potential for stored energy, right?
     
  10. May 25, 2014 #9

    sophiecentaur

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    The kinetic energy has no direct association with 'centrifugal force' though. You could use any means to accelerate the discus. The only "stored energy" is the KE if the discus itself.
    The KE is due to arm length AND angular velocity.
     
  11. May 25, 2014 #10
    That depends on the definition of "direct". Changing from an inertial system to a co-rotating system the kinetic energy turns into potential energy related to the centrifugal force.
     
  12. May 25, 2014 #11

    A.T.

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    Sure, the greater the radius, the greater the tangential velocity for the same spin rate, which is a limiting factor for a human standing on his feet. This is rather obvious, while you remark about "centrifugal force" isn't obvious at all to me.
     
  13. May 25, 2014 #12
    As a boy it seems to me the very first active description of centrifugal force I read was of a stone tied at the end of a string being spun around and then the string is released/cut. This seemed very like a discus throw to me. More recently, the revived popularity of the Trebuchet has demonstrated the great power of the combination of leverage and centrifugal force.

    However when I looked it up on a physics site it mentioned that the forces at play in a trebuchet are quite complex and include both centrifugal force and centripetal force. I am indebted to this thread for clearing this up for me... but now I am contemplating why it is that employing wheels on the base increases the throw distance... but that, I suppose, has little counterpart in discus and for another thread.
     
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