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How much g's creates a 100 lbs punch to the head?

  1. Feb 14, 2015 #1
    Hi guys,

    I heard in the news today that a punch of a pro boxer can create about 50 g. I was just wondering how much g's a rather weak punch(100 lbs) can create?
  2. jcsd
  3. Feb 14, 2015 #2


    Staff: Mentor

    The human head weighs about 5 to 10 lbs. So a 100 lbs force applied to the head could cause an acceleration of 10 to 20 g, assuming no resistance from the neck.
  4. Feb 14, 2015 #3
    Thanks for the answer! One more question - how much Newtons creates the punch itself?
  5. Feb 14, 2015 #4


    Staff: Mentor

  6. Feb 14, 2015 #5
    So the result is 444N. Thanks for your help.
  7. Feb 14, 2015 #6


    Staff: Mentor

    You are welcome. And welcome to PF!
  8. Feb 15, 2015 #7
    I wanted to ask a similar question. I hope I am allowed continue this thread.

    So the punch to the head stays the same, but what would be the g force if the head only moves 1 or 2 mm and then suddenly stops?

    would be great if you could post the formulas for this calculation.

  9. Feb 15, 2015 #8


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    The acceleration (in units of g if you like) is the result of dividing force (100 pounds-force) by mass (5 to 10 pounds-mass). It does not matter whether that force is applied for a long time or short, a long distance or a short. The resulting acceleration is the same.

    If the head then suddenly stops, the g force for the deceleration would be infinite.

    One relevant formula is ##f=ma## -- force (f) is equal to the product of mass (m) and acceleration (a) in a coherent system of units. If force is measured in pounds-force, mass is measured in pounds-mass and acceleration is measured in g's then the units are "coherent" enough for this purpose.

    Another formula is ##p=ft## -- the change in momentum (p) from a constant force (f) exerted for a time (t) is equal to the product of force and time.
  10. Feb 15, 2015 #9
    Basically i undersand what you said,

    but what means 'the deceleration would be infinite'?

    Does it mean that the g forces can't be calculated in my example? My case refered to an actual situation, namely if a person stands very near to a wall(space 2 mm) and gets punched to the head.
  11. Feb 15, 2015 #10


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    Neither the head, nor the wall are perfectly rigid. You have to now how much they deform to compute the acceleration.
  12. Feb 15, 2015 #11


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    If the question really is about a boxer then why has it been mostly assumed that the 'force' of the punch will last for as long as you want? I think this assumption has been made without qualification and it is just possible that people may get the wrong idea about the reality of the situation. It's obviously right to quote F=ma but that is not really enough to deal with a punching situation. Imo, the proper answer to the OP is much more complicated and must include the momentum changes of the head mass, coupled to the body and the fist mass, coupled to the other body.
    I don't suggest that things should necessarily be taken further than they have been, here but that there is a massive caveat required, lest anyone jump to unjustified conclusions.
  13. Feb 21, 2015 #12
    I couldn't reply earlier, as I didn't have much time

    Ok I see now that this is indeed a complicated example. The only thing that really interested me here was, how much the stop of the head highers the g forces, compared to the case when there is no wall behind. To calculate this, we could simply take reasonable numbers. The question is, what parameters do we need?

    Some were mentioned in the previous post. Let's assume the wall moves 1 mm due to the impact, and the head caves in 1 mm as well. The fist weighs 1.5 lbs, the mass of the body is 165 lbs (both persons). The head weighs 10 lbs.

    As I said, the only thing that would interest me is the difference of the g forces, when the head is suddenly stopped.

    I hope it can be calculated now.
  14. Feb 21, 2015 #13


    Staff: Mentor

    If I understand what you want, you want to calculate the acceleration (g-force) of the head in the following two scenarios:

    A 10 lb head accelerated by a 100 lb punch with no neck or head restraint forces.

    A 10 lb head accelerated by a 100 lb punch for 2 mm and then decelerated by a wall for 2 mm (1 mm of give from the wall and 1 mm of give from the head). Do you want the force of the punch to continue during the phase where the wall is also decelerating the head (I would assume so).

    I agree with sophiecentaur that the real biomechanics of a punch are far more complicated than this model of a punch, but with that caveat what you ask can be calculated.
  15. Feb 23, 2015 #14
    DaleSpam, you understood completely right what my question was.

    It would be great if you could show me both ways.

    Thanks in advance.
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