A way to measure force...

[Cody Carey]

I am currently looking for a way to measure the amount of force exerted by an object... namely, my team's air cannon. I would like to be able to have a read-out of how hard a projectile from said air cannon hits a target. It would be nice if the solution were off the shelf, but I wouldn't be opposed to making one either.
I have looked, and found many ways to make my own, but if anyone has any prior experience with this, help would be appreciated

Thank you in advance,

-Cody C

[Tytus Gerrish]

the energy that a projectile has is kenetic energy and it is measured in Joules. try looking up how Firearms are tested.

[KenWittlief]

one way to measure the energy of a projectile is to fire it into something like a lump of clay, suspended on a cord, and measure how high the mass swings up after the impact.

[Cody Carey]

I was thinking more along the lines of what they use for measuring the force of a punch, so that we could make some force comparisons that might impress a usual High-Schooler, but I will definately look into firearms testing thanks

[Tytus Gerrish]

the same type of device would do the job. i saw on mythbusters on shark week they used a punching bag on a string and used a video camera to measure how far a punch would recoil the bag. you should do the same thing with any impact. Wow i wish i had thought of that before i had Marcus put a helmet on and take a potato to the head and tell me how hard the hit was.

[Nitroxextreme]

I know there are some devices out there that can have a digital readout of how much force is being exerted on them. I remember seeing one in a CSI episode a while back.

[Simon Strauss]

Dont bother looking up how firearms are tested, they do that by measuring how deep the projectile penetrates a slab of ballistics gel and that wont help you much.

[KenWittlief]

its not really force you are looking at - a bathroom scale measures force (the force that gravity is exherting on the mass you are weighing)

you need to measure the impulse that results from a collision - the energy contained in the projectile

another way would be to fire the projectile straight up, or to use a strobe flash on a camera and measure the velocity as the projectile passes.

or setup a light sensor light gate, with the projectile passing infront of two light sensors, spaced out a few feet apart

[Billfred]

Originally Posted by KenWittlief

or setup a light sensor light gate, with the projectile passing infront of two light sensors, spaced out a few feet apart

Wouldn't one photogate work, if the object to be measured has a known length? (That's how I think I recall us doing it in high school physics, combined with a 3-by-5 card.)

[Alan Anderson]

I think what you want to measure is either kinetic energy or momentum, not force. The force exerted on a target by a projectile depends on how long it takes to transfer the momentum -- the longer the time, the smaller the force.

Ken's suggestion of capturing the projectile with a pendulum and noting how high it swings sounds like a good one. The height will be directly related to the kinetic energy carried by the projectile.

[Kevin Sevcik]

To further borrow from the mythbusters, the often have fish scales that indicate the maximum deflection of the scale. If you had the reverse, a scale that measures compression and indicates the maximum compression, you can use it to measure the energy imparted to the scale by the ball/projectile. You could use a fish scale and a simple lever system to achieve this as well, actually. Note that how the ball richochets would affect things, so you'd want the deformable clay/perfectly sticky surface to shoot the ball into as well. On the plus side, you don't need a high speed video camera.

And now a little OT..

Originally Posted by Nitroxextreme

I know there are some devices out there that can have a digital readout of how much force is being exerted on them. I remember seeing one in a CSI episode a while back.

I'm terribly annoyed by this statement. If you do nothing else as a potential engineer, trust nothing you see on CSI or any similar show. It's fluff that has minimal basis in reality. I saw an episode where they purported to decode a conversation recorded on a clay vase that was spinning on a pottery wheel with straw dragging along its surface. While it might seem plausible with the similarity to how a record is made, clay is not wax, and a bundle of straw is not a metal needle attached to a diaphragm. Whatever science the have is secondary to their plot, and only vaguely based in reality.

[sanddrag]

Originally Posted by Kevin Sevcik

I'm terribly annoyed by this statement. If you do nothing else as a potential engineer, trust nothing you see on CSI or any similar show. It's fluff that has minimal basis in reality. I saw an episode where they purported to decode a conversation recorded on a clay vase that was spinning on a pottery wheel with straw dragging along its surface. While it might seem plausible with the similarity to how a record is made, clay is not wax, and a bundle of straw is not a metal needle attached to a diaphragm. Whatever science the have is secondary to their plot, and only vaguely based in reality.

I like how they can take a photo, zoom in on a person's eye, say "let me see if I can enhance this" and then seconds later they have a gigantic and perfectly clear picture of what the person was looking at (the reflection in their eye). Or howabout they take a finger print and say "let me run this" and then seconds later "boop boop boop bing!, it's a match!"

Anyhow, back to the thread.

[KenWittlief]

Originally Posted by sanddrag

I like how they can take a photo, zoom in on a person's eye, say "let me see if I can enhance this" and then seconds later they have a gigantic and perfectly clear picture of what the person was looking at (the reflection in their eye).

I think the first time this was done in a movie or TV show, it was a photograph. with a good quality 35mm camera this is possible

in fact, I have a photo of the steering wheel of a Coast Guard 44 footer. If you blow up the 2" diameter chrome center of the wheel you can see me sitting in the pilots chair, pointing the camera

but I know I have seen it 'used' in tv shoes with video from a survalince camera. Video is 640x480 at best, what you see is all that is there - you cant zoom in and see what was between the scan lines of the video camera (theres nothing there)

if you use a fish scale you are measuring the impulse - the response of the spring in the scale, so the max reading would mean something if you could capture it, but the reading on the scale itself would not be the maximum force applied, because the scale is not intended to be used on moving objects

you would have to experiment with the scale to be able to interprete what the readings mean

but as a relative measurement, you could say impact A was greater than impact B. How much greater would take some work to figure out. I dont think the readings would be linear.

[Kevin Sevcik]

It's still energy as long as you know physically how far the scale's deflected as well. You'd be measuring the maximum potential energy stored in the spring. PE=0.5*k*x^2. You don't know k, but F=k*x so PE=0.5*f*x. Only problem is if ball bounces off in random directions, imparting an inconsistent amount of energy to the system.

You could also come up with a comparison is you don't know the deflection, using an arbitrary constant/guess for k and saying PE=0.5*F^2/k.

[dlavery]

Originally Posted by KenWittlief

but I know I have seen it 'used' in tv shoes with video from a survalince camera. Video is 640x480 at best, what you see is all that is there - you cant zoom in and see what was between the scan lines of the video camera (theres nothing there)

Not to hijack the thread, but that is incorrect. There is more "there" there than is there.

In simple terms, the pixel represents a region of data in the source image, and not just an infinitely small single point at the center of the pixel region. The information assigned to a pixel is based on the aggregation of the image properties across the entire image region assigned to the pixel. Using techniques such as superresolution processing, cascading image enhancement, multiframe quiver analysis, and multiscale morphological smoothers (typically nonlinear smoothing filters for detail enhancement at large scales), information can be extracted from a digital image "from between the pixels."

For example, imagine that there is a region that is composed of a white square surrounding an inner red square. The outer white square completely fills the region that comprises the pixel. If the center of the region mapped directly to the pixel, then the CCD pixel would just "see" a point at the center of the inner red square, and record "red." No other information would be recorded, or available. However, in the real world, as the light from the region passes through the optical system and impacts the pixel on the CCD it is aggregated together and the pixel records "pink." When this happens, some of that information about the source region is available to be extracted. This also holds true for a series of adjacent pixels. Using this property, it is possible to determine precisely where dissimilar regions fall between pixel coordinates. From that information, image details and geometry that is not readily visible in the original image can be reconstructed.

Image processing labs have been using advanced versions of these techniques for years to extract information from digital images. The techniques are commonly used at places like The Multimission Image Processing Laboratory at the NASA Jet Propulsion Lab, the previous Intelligent Mechanisms Group at the NASA Ames Research Center, and many of the intelligence community facilities. Details on the fundamental algorithms associated with digital image enhancement are found in virtually every set of proceedings of the annual SPIE Conference or any of the several photonics society newsletters.

-dave