Measuring punching G-Force on a Pendulum

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Discussion Overview

The discussion revolves around the design and measurement of g-forces generated by a punching mechanism using a pendulum setup. Participants explore the theoretical and practical aspects of measuring acceleration and forces involved in martial arts training, including considerations of mass, moment of inertia, and sensor technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes using a pendulum with a padded punching surface and an optical rotary encoder to measure g-forces based on angular displacement.
  • Another participant suggests that the inertial moment of the pendulum and the distance from the punch to the rotation axis are crucial for accurate calculations.
  • A different participant expresses curiosity about the necessity of the pendulum mechanism, questioning if a hard-mounted target pad with an accelerometer could suffice.
  • Concerns are raised regarding the noise levels of accelerometers at high g-forces, with one participant indicating that a heavier punching bag might be required to reduce noise but could increase injury risk.
  • There is a discussion about calculating the moment of inertia by summing the contributions of different components of the pendulum system.
  • Clarifications are made regarding the relationship between torque, force, and distance in the context of the pendulum's motion.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and effectiveness of various measurement methods, including the use of accelerometers versus optical sensors. There is no consensus on the best approach or the implications of mass and moment of inertia in the calculations.

Contextual Notes

Participants mention the need to account for the mass distribution of the pendulum and the effects of gravity, but there are unresolved questions about the significance of these factors in the measurements.

Who May Find This Useful

This discussion may be useful for individuals interested in experimental physics, engineering design, or martial arts training technology, particularly those exploring methods for measuring forces and accelerations in dynamic systems.

lownlazy
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I am building a machine for my martial arts club so that I can have a bit of fun and measure the speed and reaction times of my club mates.

The design that I feel would yield the most reliable and accurate results would be to mount an upside down 'pendulum' on a frame. At the head of the pendulum would be a padded punching surface, the other end would be steel rod axis mounted on bearings connected to a optical rotary encoder. (rotation/angle measuring sensor)

The theory being that you hit the pad and a computer measures the nanoseconds between a given angle. Do a bit of high-school geometry and you can find the m/s acceleration and multiply it by the weight of the 'pendulum' to find the g-force. (F = MA) But...

Q1) What is the mass of punched bag?

I hope its as simple as this: if I removed the 'pendulum' from the machine and it weights 1kg. Then lay it down, with a fulcrum at the axis and scales under the head end, the head end weights 750grams. Would I be correct in guessing that the head end weight (750grams) would be use to calculate g-force? Or do I some how need to include the whole 1kg mass of the whole 'pendulum'?

2) Do you think I would need to bother dialing-out the effects of gravity? Or would the be somewhat insignificant give that the measures will be taken in the first 45 degrees (start from vertical position)

I hope that makes sence! Many thanks in advance :)
 
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If you want to do it right, you need the inertial moment of the pendulum and the distance between punch and rotation axis. If the punched bag weights much more than the rod itself, the assumption that the whole mass is at the location of the punch (and neglect the weight of the detector) could give some reasonable approximation.

To be more specific, it would help to see a sketch of the rotating object.
 
Yes I would like to calculate it properly, can you please expand on that? Or perhaps Can you point me at a website that has a formula?

 
Last edited by a moderator:
I googled "moment of inertia" and (correct me if I am wrong) it looks like I should to calculated each part (axis, rod, head/pag) of the swing arm construction and add them all up to arrive and mass figure?
 
I'm a bit curious as to whether or not all of that mechanism is necessary. Can't you just hard-mount a target pad with an accelerometer chip right under the surface?
 
I had consider an accelerometer but I decided against it because they get increasingly noisy the higher the G figure you want to read from them. Obviously the g reading is relative to the weight of the punching bag but by my approximate calculations you would need a 5-10g+ bag to bring the G-Forces down to a less noisy level but then you run the increased risk of injury or breaking bones in your hand.

With the swing arm rig using an optical sensor the force figures will be. hopefully accurate but, if nothing else, consistent and comparable within a sample group.
 
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lownlazy said:
I googled "moment of inertia" and (correct me if I am wrong) it looks like I should to calculated each part (axis, rod, head/pag) of the swing arm construction and add them all up to arrive and mass figure?
Right, but the result is not a mass, it is a mass multiplied by a distance squared.
Together with the measured acceleration, you can relate this to a torque. The torque is equal to the force multiplied by its distance to the center of rotation (assuming the direction of force is orthogonal to the rod).
 
lownlazy said:
I had consider an accelerometer but I decided against it because

Cool. You obviously already know far more about the subject than I do. (That, however, will not prevent me from popping in with opinions once in a while. :devil:)
 
Danger said:
Cool. You obviously already know far more about the subject than I do. (That, however, will not prevent me from popping in with opinions once in a while. :devil:)

thats cool :)
 
  • #10
mfb said:
it is a mass multiplied by a distance squared.

Ahh see that makes sence. I was see "kg m2" and I was thinking what does surface area have to do with it? :)

Thanks for you help mfb, that has me pointed in the right direction.
 

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