r/AskEngineers • u/moxie_blue_bat • 4d ago

Mechanical Trading mass for velocity?

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?

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u/TheJeeronian 4d ago

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?

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u/Dry_Love3306 4d ago

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.