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The physics behind a Punch

  1. Sep 18, 2014 #1
    I was thinking about the physics of a punch (not that I´m interesting in getting into a fight).
    I was wondering what would enter into consideration.
    Obviously the formula F= m.a is the key, but it´s clear that there are some factors that are involved, like the grip you have on the floor (if you were standing over a skateboard the punch would suck), and the pressure (I guess if you can concentrate all the force into a tiny spot, it would be better).
    So, I was thinking that if you were to throw a side uppercap and your ellbow acted as the pivot point and is really steady to resist the reaction of the inpact you´d still be conditioned by how much mass you happen to have in your fist, and the rest would be how much you could accelerate that mass.
    What would be the key then? how much momentum you can get in there before it hits the target?
    Also, are there any keys to messure the grip?
  2. jcsd
  3. Sep 18, 2014 #2

    Simon Bridge

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    The punch is very complicated.
    The arm acts as a lever, the rest of the body contributes.
    How you treat it depends on what questions you want to answer.

    Per your example, the forearm moment of inertia is also important - but the simple swing from the elbow would basically work like a hammer.

    Grip between feet and floor is handled by friction models - if you don't slide, that's static friction.
    The way your weight is distributed will be important.
    Also bear in mind that your legs and most of your body will swing with the punch - adding to it or at least countering the conservation of angular momentum effect and acting as shock absorbers.
  4. Sep 19, 2014 #3


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    That is a good question, for which I don't have an answer. There are four related physical quantities that you could think of as being important for a punch, one would be the force, another would be the pressure (force/area), another would be the energy transfer (force*distance), and the third would be the momentum transfer (force*time).

    I think that just force probably is not the critical factor, since a lot of things exert larger forces than a punch with less damage, so pressure is probably more important than force. Momentum transfer is probably counter-productive since then your punch turns into a push. So I suspect that energy transfer is the other important factor besides pressure.

    This is all just conjecture, I don't know of any studies on the topic. But energy and pressure as the key factors seem to be consistent with what I know of punching and kicking. You always want to kick or punch "through the target" which implies increasing the distance through which you are applying the force and thereby increasing force*distance.
  5. Sep 19, 2014 #4


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    When hitting the head, transferring momentum is actually quite dangerous. High pressure causes external damage to your face. But high accelerations of the entire head can knock you unconscious, or even tear the brain's blood vessels and lead to internal bleeding, which is usually deadly.
  6. Sep 19, 2014 #5


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    I agree, but in my mind that is more closely related to force than momentum transfer. My car can transfer momentum to my head so that it goes from 0 to 60 mph over a few seconds (OK, over many seconds) with a harmless low force. A fist may do the same change in momentum, but it is over a very short time because the force is high. The damage is associated with a high force despite equal momentum transfers.

    They are all closely related, so it is not unambiguous.
  7. Sep 20, 2014 #6


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  8. Sep 20, 2014 #7

    Doc Al

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  9. Sep 20, 2014 #8
    The stretch shortening cycle (SSC) plays a dominant part is most athletics. In analyzing the role played in tennis, I used the example of the once famous 'punch from nowhere' - the most dramatic example of the stretch shortening cycle that I know of.

    See the thread and video for what happened.

    To better understand the microscopic force behind the SSC research Titin.
    Last edited: Sep 20, 2014
  10. Sep 20, 2014 #9
  11. Oct 3, 2014 #10
    Thanks a lot for all the answers, all this really helped me clarifying a few doubts I had.
  12. Oct 3, 2014 #11


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    There are so many different punches, but many of them get the power from the same place, your hips. The straight punch, the upper cut, the hook punch, and the overhand punch all derive power from a strong rotation of your hips which then proceed through your shoulders. Of course a good solid base on the ground is important for these punches, because the hips can't rotate if your feet slip, you'll go off balance and fall.

    Some punches are spinning, like the spinning back-fist, and those punches do not require so much of a strong base as a quick spin. Hammer fists derive power more from your torso and triceps because they come down in a hammer-like fashion. Also, if you are punching on the ground, e.g. from mount position or from bottom position, the power will come from mostly your torso since you no longer have the luxury of rotating your hips in these instances.

    But of course, a punch is not measured purely by its power. The most powerful punch you can deliver is almost never the best punch that you can deliver. A very powerful punch will likely require you to over-commit and make counter-punching easy for your opponent in the event that it's blocked or evaded. An upper cut or hook punch for which you are only considering power might need you to move your hands away from your chin in order to create the most leverage; but this gives your opponent a much easier target (your open chin) to which he can throw a jab (which will get to your chin much faster than your hay-maker hook or uppercut can get to his chin) and put you away. There are many many things to consider when considering punches! Accuracy, speed, range, potential for counter-attacks, all account for a lot as well. Power is only one small aspect of a punch.
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