Calculating force on a punching bag?

[hxtasy]

i have a punching bag, say it weighs 70 lbs, hung by a chain. i duct tape an acceleromter (sparkfun) on the back of it, sampling at ~300 hz (is that enough?). punch the bag, i get a force in G force. how can i calculate this to say, foot lbs? i really am not sure where to start with this.

any guidance in the right direction is appreciated. thanks!

 

[nasu]

You don't get a force but an acceleration. You could multiply this by the mass of the bag to get some force.
foot lbs would be the unit for torque or maybe energy (or work).
It is not clear what you really need, you are mixing units a little.

 

[sophiecentaur]

The peak of acceleration will be at the same time as the peak value of the force. If you are measuring the acceleration in units of g then the acceleration, in ft/second will be your g number times 32. Force is mass times acceleration so you can get your answer at the peak time.
I always get confused with imperial measurements because of the use of the pound as a force and a mass and the 'slug'. SI is so much easier to deal with because mass and weight have distinct names. Using a mass of 30kg (near enough 70lbs) and a g value of 10 (near enough) then the Force, in Newtons will be 30 times the acceleration in g's times 10.
I could point out that the Force is not the only thing that counts. A punch needs to carry Energy with it and also the Momentum is relevant when knocking someone over or out. So, for the whole picture, you would need Force times Distance and also Force times the time it's applied for. I wouldn't be surprised if your equipment would show you that the same peak F (peak acceleration) can be obtained for a 'long' and a 'short' punch, and the results on a 'body' could be very different. I guess that there will be many informed opinions among the boxing fraternity that could be more relevant than just the straightforward Physics.

 

[hxtasy]

ok I am trying to make a cheap version of this



and yea g force is a very confusing unit of measure

 

[hxtasy]

The peak of acceleration will be at the same time as the peak value of the force. If you are measuring the acceleration in units of g then the acceleration, in ft/second will be your g number times 32. Force is mass times acceleration so you can get your answer at the peak time.
I always get confused with imperial measurements because of the use of the pound as a force and a mass and the 'slug'. SI is so much easier to deal with because mass and weight have distinct names. Using a mass of 30kg (near enough 70lbs) and a g value of 10 (near enough) then the Force, in Newtons will be 30 times the acceleration in g's times 10.
I could point out that the Force is not the only thing that counts. A punch needs to carry Energy with it and also the Momentum is relevant when knocking someone over or out. So, for the whole picture, you would need Force times Distance and also Force times the time it's applied for. I wouldn't be surprised if your equipment would show you that the same peak F (peak acceleration) can be obtained for a 'long' and a 'short' punch, and the results on a 'body' could be very different. I guess that there will be many informed opinions among the boxing fraternity that could be more relevant than just the straightforward Physics.

very good info, a lot more to think about here than i original thought. this needs to be more precise than accurate, even if its an arbitrary number, its a good way for people in our martial arts gym to understand their performance.

 

[sophiecentaur]

very good info, a lot more to think about here than i original thought. this needs to be more precise than accurate, even if its an arbitrary number, its a good way for people in our martial arts gym to understand their performance.

But a simple measure of maximum Force could lead you to a false conclusion as to who is the 'better' puncher. I would say that measuring the angle by which the bag is moved would be a better way of assessing performance. That would be easy to do and would be a good measure of the 'effect' their punches have.
If you wanted to justify the (possibly) simpler Force measurement, you could take a group of fighters and see how the maximum chain angle correlates with the maximum measured force. The way to do that would be to use a simple scatter graph with Force on one axis and angle on the other. Plotting the (X,Y) points for each fighter would give a set of points on or near a line or curve if you are correct or a random looking pattern if you are wrong. That would be a 'good' scientific method and you could place the fighters in a heirarchy of performance. You would not actually need to do any 'physics calculations' - just to look at the data points would be enough.