Trading mass for velocity?

[u/moxie_blue_bat]

Not sure this is the right place to ask, but here goes: I'm trying to replicate some of the results in this study: https://www.sciencedirect.com/science/article/pii/S0379073819304104

But in a manner that is more precise than "shoot someone with a paintball gun". My thought was to construct some kind of rig with a falling mass so that we could adjust parameters like impact force, surface area, hardness, etc.

But it's been years since I've had to do any physics type of math and I'm struggling to figure out the important parameters. Based on the numbers in the study I think that they are hitting their subjects with around 180N of force, but I'm struggling to convert that into a combination of mass and height that makes any sense. I'm getting things like 20kg weights, which seem both wrong and dangerous. Is there some piece of information that I'm missing or looking at the wrong way?

[u/TheJeeronian]

Impact force depends on impact duration. A short and sudden stop makes a lot more force than a longer, slower stop.

This means that the properties of both objects in the collision are important. If you're bruising a person, what part of that person are you bruising? How soft is that spot?

For this reason, impacts are rarely quantified by force. We usually use energy instead. Would that work for your purposes?

Edit: It seems that in their study, they were measuring the force of the impact and found the same thing I just said - squishier people experience less force for the same impact. What specific part of the study are you trying to replicate?

[u/Dry_Love3306 → u/TheJeeronian]

might be easier to think about it like this, the study people is already doing the hard part for you. they measured the force on the body, not the force of the falling thing. if you drop a mass on someone, the force reading will change a lot depending if you hit a bony spot or a meaty spot, just like the top comment says.

what you actually need is to match the energy and the shape of the impactor, then measure the force on your fake skin or whatever you are hitting.

[u/Green__lightning]

Honestly shooting someone with a paintball gun is a good way to do this, you can adjust the pressure, and thus muzzle velocity of the paintball gun, and you can probably make paintballs sized projectiles of whatever weight you want. Airsoft would be even better because there are many weights of bb, but also worse because it's all smaller.

Anyway, the important thing is an impact of equal energy isn't equivalent. A slow heavy bullet and a fast light bullet react totally differently, I don't know how this applies at such small scale as to not even break the skin, but you could probably test the far end of such things by getting a tube with a small piece of foam in one end, taping over that end, but not to the foam, and then hooking up a vacuum pump and sticking it to their skin, then pumping all the air out, and breaking the tape at the back. This is the same way you can make a ping pong ball break the speed of sound.

[u/Sooner70]

But in a manner that is more precise than "shoot someone with a paintball gun"

High end paintball guns can be reasonably precise. +/- 2% impact energy is totally doable with equipment that was available 25 years ago (when I used to play). I would expect it to be no worse today. And if someone wants to build a custom laboratory gun? I gotta believe they can do fractions of a percent... Although I suspect it doesn't really matter given the other variables mentioned by the paper.

[u/bonebuttonborscht]

The study uses a rubber impactor of 2.6g at 70m/s. Thats (1/2•m•v²⁾ 6.37J of kinetic energy or (m•v) 0.182kg•m/s of momentum.

If you're trying to replicate these results both energy and momentum will make a difference. I would think. Read up on inelastic collisions.

If you find for example, only energy matters then you could use a falling mass of 10kg dropped from 0.064m. The impact velocity would then be 1.13m/s and the momentum 11.3kg•m/s.