Punch Force Gauge

[PVC Arsenal 17]

A few of my friends asked me to help out with their high school physics project. The project has to be related to kinematics but other than that there are no restrictions. Spud guns have of course been considered but there's no easy way to relate their function to physics without a ballistic pendulum or chrono and that's too involved for this project.

We were thinking about a simple gauge to measure the force of a punch. Basically it would consist of a pipe with a spring inside, and another pipe inside that one to depress the spring when punched. (Basically a spring piston or shock without the dampening elements) On the outside of the inner pipe will be a tight fitting collar that slides along the inner pipe during a punch and then stays in place afterwards.

Knowing the specs of the spring and by measuring the distance x that the ring has slid, you can apply Hooke's Law to find the force of the punch, correct? Or are we overlooking something? Input is much appreciated.

[saefroch]

there's no easy way to relate their function to physics without a ballistic pendulum or chrono and that's too involved for this project.

I beg to differ. I won't patronize by explaining why, because you seem to already know, but I do disagree. Block of wood, string, cannon, projectile, ruler, balance, video camera. Shoot, measure, chug.

Your device should work. Hooke's law isn't perfect, but it's a close enough measurement *cough*source of error*cough* for this. As drawn it will work, but I think you might have some problems with the little red object sliding around after the punch, because in all likelihood the device will either fly into two pieces (which might actually be the better scenario) and then hit something, or its inertia will carry it on farther towards what I assume to be a grey pad when it hits the stop.

[PVC Arsenal 17]

I would have liked to help them with a ballistic pendulum (in fact it was my first suggestion) but their teacher wants to take their projects and use them as basic lab experiments for next year's class. I had the guy's class; he wouldn't bother with trying to get a bunch of kids to work on something like that and I'm not sure if they would be able to focus on it. Sticking two pipes and a spring together is little bit more simple.

And yeah I was concerned about the ring sliding under it's own inertia. It makes the whole thing difficult as a punch force gauge but at the very least it could make for a crude scale since that issue wouldn't be a factor.

I can replace the ring with a dry erase marker&holder so that it draws a line on the inner pipe as it moves...

[saefroch]

Might have some measurement difficulties with that, but they'd be minimal, and mostly related to the length of the marker. Other than that, the device looks like it'd work quite well. To me, at least.

[jackssmirkingrevenge]

Why not cut out the spring variable and bounce factor and just shoot at a weighted trolley on rails?

Or how about this other option for a chrony, I had set it up in the physics lab in high school

[PVC Arsenal 17]

Here it is:

[jackssmirkingrevenge]

So, does it work? And how are you marking travel?

[PVC Arsenal 17]

Physically it does what it should. Whether it's any accurate is up to my friend to determine. First he's going to test if the constant listed for the spring on McMaster is accurate.

I'm marking travel with a short section of one of these: . It seems to work just fine.

[metalmeltr]

What about an air cylinder with a pressure gauge and a check valve?

EDIT
updated photo

Ball valve for releasing prsessure after tests added to picture.

[Lockednloaded]

metalmltr, that seems like a great way to test spudgun energy, I might try it out soon Wouldn't you have to have a pre calculated sized cylinder first?

[metalmeltr]

If by calculated you mean knowing the diamiter of the cylinder bore the answer is yes.

This like many cylinders states the bore diamiter in the specifications.
Edit
The bore is 5/8"

pi x r squared, the area = 0.30679615757713 square inches

gauge reading of 90psi/by 0.30679615757713= 293.354391 lb applied to the cylinder

Edit the cylinder linked to initailly may not work as the cylinder stroke is only 1/2"

This one may because it has a stroke of 2", bore 9/16"

[Lockednloaded]

you would need a very long stroke and a large bore for this to work with spudguns, and those are the most expensive type of cylinders

[metalmeltr]

I was thinking primarily in terms of small pneumatics.

Edit

What about this?

2" bore 6" stroke max psi = 200

bore area=3.14"

at maximum psi up 63.6 pounds could be measured, it is likely that the cylinder could take slightly more than 200psi

[Lockednloaded]

Or you could charge the cylinder with say 100psi of air, so it would be harder to compress. I don't know how you would calculate the energy then...

[metalmeltr]

Measure for final pressure and divide that by bore area? include the 100psi in the measurement, this should work the same as if the cylinder built up this pressure fom compression. Am i looking at this wrong?