I How many Gs can a person pull during a punch

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I want to make a device that can actually measure the force in a person's punch or kick. So I am going to buy an Accelerometer but I need to know how many Gs should I get it for ?

would a9G one work or should I get a 50 G one?

or

 

sophiecentaur

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would a9G one work or should I get a 50 G one?
Estimate the distance that your fist travels after it has made contact to when it's stationary. (There are loads of youtube videos for that information). Estimate the speed of the fist before contact (distance it travels over the last few video frames: v = d/t and the frames will be 1/30 s for best quality). Or you could measure the same thing for your own best punches. No particular need to punch anyone.
Then a = 2v/s where a is the (negative)acceleration of the fist and head, v is the velocity you found previously and s is the distance travelled during contact until motion stops.
Divide the number you get for a by 10 (=1g) to find the G's

But that's the Physics of it (it assumes uniform acceleration, for a start) and the circumstances will make a big difference. There are many hits on Google when you search g force of a punch. This one, for example.
 

russ_watters

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I want to make a device that can actually measure the force in a person's punch or kick. So I am going to buy an Accelerometer but I need to know how many Gs should I get it for ?

would a9G one work or should I get a 50 G one?

or

What are you going to attach it to?
 

russ_watters

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A punching bag
9 may not be enough, but 50 should be plenty. You might want to try it with a cell phone first though.

You may also consider measuring energy transfer by videoing the bag from the side to see how far it swings.
 

sophiecentaur

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A punching bag
The bag will distort when punched so the different parts will be subject to different accelerations. The details of the structure of the bag will make a difference to the "G Force" that's relevant.
Is the purpose to monitor the progress of someone's training? If it is then just attach an accelerometer and take many readings over the training regime to see how much improvement is being made.
Or why not measure the deflection of the bag when it's suspended from a rope as a measure of the Energy delivered?
 
426
5
I want to make a device that can actually measure the force in a person's punch or kick. So I am going to buy an Accelerometer but I need to know how many Gs should I get it for ?

would a9G one work or should I get a 50 G one?

or

Can you go in to more detail on how to do this ?
how to find the G force from a video
 
426
5
The bag will distort when punched so the different parts will be subject to different accelerations. The details of the structure of the bag will make a difference to the "G Force" that's relevant.
Is the purpose to monitor the progress of someone's training? If it is then just attach an accelerometer and take many readings over the training regime to see how much improvement is being made.
Or why not measure the deflection of the bag when it's suspended from a rope as a measure of the Energy delivered?
Can you go in to more detail on how to do this ?
how to find the G force from a video
 

sophiecentaur

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how to find the G force from a video
(It's acceleration, not Force, that you are looking at with a camera but that's just me being fussy)
The successive frames of a video will show the distance travelled in the interval between frames. (A ruler scale in the shot will give you distance measurements). I suggest you do the measurements in good bright sunlight so the shutter will be fast to minimise blurring. You don't want the flicker of typical indoor lighting to add complications but integral lights on a camera would be fine. No punching is necessary to practice with the system - just film a dropping ball, for instance, and you will see the
You really need to know a bit of Physics if you want to be sure you are getting worthwhile, valid measurements.
The 'SUVAT' equations describe motion under constant acceleration. Real life situations seldom involve constant acceleration but it's a great method for estimating acceleration. You can work out the initial velocity and then the acceleration by using the time between video frames and the distances travelled.
Initial velocity is given by distance travelled for, say ten frames (time = 0.33s, or whatever the frame rate of the camera) before impact. Count how many frames to bring the fist to a halt and
acc times time = the velocity you already calculated
will give the average acceleration.
There's an alternative way to find the acceleration (see that link) by measuring the distance s that the fist travels during impact
v2 = 2as
 

sophiecentaur

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I want to make a device that can actually measure the force in a person's punch or kick. So I am going to buy an Accelerometer but I need to know how many Gs should I get it for ?
Returning to your OP, I think what you want is a Force Measuring device and not an accelerometer at all. There are a host of stress sensors available which will tell you the force, as it varies during the punch. There are also position sensors and the two instruments could tell you all you want to know. I think the "G force" thing is something that is often referred to but is not really all that relevant.

In short, you need to decide just what information it is that you really want. If you want to have an indication of the damage a punch could inflict then it could well be the amount of Energy transferred, rather than the acceleration.

The damage you could do to a flying ant by punching it would be very small and that would also apply to a 100kg block of metal (ouch!) That is because the energy transferred to the target would be so small. There would be an optimum size and structure of a target to do the most damage, depending on the mass of your fist / arm and the actual speed you can manage. Boxers are capable of very fast movement of fists with a very short length of jab.

You need to read around the subject a lot and try to identify and eliminate the nonsense that you are bound to come across. An excellent coach may well have little idea of Physics. Some answers that you could find useful are in there somewhere but won't be found in five minutes I think.
 

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