Frank Leany 
Member since Sep 1, 2015

Recent Comments

Re: “The true force of a boxer's punch

Thought experiment:

You have a mass hanging on a string. How do you change its weight without changing its mass?

You accelerate it. F=ma. The mass has a weight that is the product of its mass and the acceleration of gravity. So you could change that weight, measured in the tension of the string, by pulling on the string.

The key here is that when you get down to it a load sensor is actually measuring acceleration. Am I on the right track? Is that the secret to taking something dynamic like energy/impulse and converting it to more static units like force?

Okay, so you build a machine that uses a punch to pull the string. Measure the tension in the string (or rod), and you have a force measurement of a punch.

Oh, but the problem with all the machines I've come up with is that the free body diagram doesn't take into account the energy to accelerate the mechanism, even if it's balanced. D-oh!

Help me, Obi Wan Kephysics-major. You are my only hope.

6 likes, 0 dislikes
Posted by Frank Leany on 09/01/2015 at 6:18 PM

Re: “The true force of a boxer's punch

The units (lbs. force) that these are delivered in always messes with my head. Can someone with a physics background help me understand it? How do you measure 'force' (lbs.) when you're dealing with an impact?

To me that seems like it should come out in units of energy or impulse/momentum. You have a moving mass, it hits something . . . how do you measure an equivalent force?

I've searched for exactly how Sports Science and others measure the 'power' of a punch (power in a generic sense, not time rate of change of energy) and they don't get into that detail. I'd really like to find out, as force is a much easier quantity to deal with than energy or impulse.

In my limited understanding it seems like the only way you could equate that to a force is if you knew the precise mass of what was being moved by the impact and measured the acceleration. Easy enough in concept, but in the things that get hit on Sports Science, like the dummy, only a portion of the mass gets accelerated and in a way that's not very predictable (a head on the end of a spring). A ballistic pendulum seems the most obvious way to measure a punch, but that yields energy (ft-lbs).

I hope I've explained this well enough that someone can enlighten me. You hear it all the time--things like "By the time the baseball hits it has x lbs. of force." It would put my teeth less on edge if I understood how that's a valid measurement of energy equivalence . . .

5 likes, 2 dislikes
Posted by Frank Leany on 09/01/2015 at 6:04 PM

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