# Physics of a punch: compression, stress/strain, etc.

Hello, I am working on a project for my physics class and I'm not sure how to proceed. I decided to do mine on the physics of a punch, and I have a few ideas of how I can collect data, but I'm not sure if I'm looking at it the right way.

I was going to make a soft clay block and find it's mass, volume, density, etc. And then I was going to punch it and measure how much the force of my punch compressed it. There would obviously be some sort of fist imprint (given that I have quite a strong punch), but I'm not sure how I would use this to find the force my fist exerted on the block. I was thinking it could be measured by finding the change in density or volume of the block, but I'm just not sure.

My problem is, different materials have different resistance to being compressed. It would take more force to dent a metal bar than soft clay, would it not? If I measured the volume/density change from dropping objects of known masses from different heights onto it, would that give me some sort of constant for the material? Because I could calculate the change in energy and momentum for the block, I could easily find the force it exerted, correct?

So please, if anybody can help me with this or clear me up on the physics of compression (or stress and strain, as my book calls it) that would be extremely helpful.
Or if you could link me to a website that explains it very well, that would also be good.

256bits
Gold Member
The volume and density of the clay block will not change - only the shape changes.

Why not just set up a heavy punching bag and record how far it will swing from your strong punch? You can calculate the impulse of the punch from that data.

But the volume will change, wont it? My fist compacts it and forces it downward. as long as it doesn't just go out to the sides, the volume will have decreased.

And the reason why I don't just hit my punching bag is because the bags are not equally compacted on the inside. It's more compacted toward the bottom and less (therefore softer) at the top. So I don't know where the center of mass is. Also, it's attached to a chain and the motion is limited. I was going to do it this way originally, but I also wanted to limit the amount of friction involved. With a mass that great, this would be hard to do. There would be too many variables as well.

BruceW
Homework Helper
I've seen in documentaries they use some kind of plasticine or clay to measure how much force something exerts on it. I guess they would already know the details on how that material compresses before they do the experiment.
Your idea of dropping something on the clay to see how the clay compresses with force seems good. It would be easy to calculate the exact momentum of the object (due to height it was dropped from, or its density), I believe this is the right equation:
$$p = m \sqrt{2 g h}$$
where h is height dropped from, and g is acceleration due to gravity.
The momentum will be related to the compression of the clay, so you could test this by dropping the object from several heights (or using different densities) and then measuring the compression. This would give you a graph of compression against momentum. So then when you punch the clay, you measure the compression, which should give you the momentum of the punch (using your graph).

hillzagold
I think there are arcade games that do this. I also remember the high tech Russian gym in Rocky IV had one. If you just want total force, can't you punch a scale? Clay sounds fascinating for the force distribution of a punch, though.

Okay so I guess I'm going through with this. Thanks for the help guys.

I don't want to use a scale because it does all the work for you. I know they have things that do this in gyms, but that takes the fun out of it. Plus, i can describe the physics this way and get a higher grade on my project.

Keep in mind that "Rocky IV" was greatly exaggerated. Nobody has a 2150 psi punch. It's just a movie, but the technology is real.