A way to measure force...
 

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

Re: A way to measure force...
 

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

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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 :D

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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

Re: A way to measure force...
 

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.)

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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..
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.

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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.

Re: A way to measure force...
 

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

Re: A way to measure force...
 

Check this: http://www.gigapxl.org/

Go to the image gallery, pick any picture, then scroll down and look at succedingly zoomed views and look at the detail. In this case, I don't think they are enhanced in any way. It's equivalent to a 4 Gigapixel camera! Now think of what detail image enhancement could show.

Back on topic, just shoot the balls into a pendulum of known dimensions & mass and calculate from there. Now think about what the mass of the pendulum should be relative to the projectile to achieve maximum transfer of momentum and avoid losses.

Re: A way to measure force...
 

I guess I should have been more specific describing the techniques 'used' on these type of TV programs. A surveillance camera will capture someones face in one or two frames, maybe filling in 25% of the screen (whole face is ~200 by 200 pixels)

they freeze on that one frame of video and zoom in on the center of the persons eyeball (which might be 5 by 5 pixels)

and then zoom in till that center black area fills the screen, hit the "image enhancement" button, and there you see someones face you can clearly recognize.

Not going to happen. The techniques Dave is talking about are common with big screen TVs, and projection video systems. When video is moving there is more data present, and you can interpolate pixels from frame to frame, and in 3x3 or 4x4 pixel blocks to determine what is between the pixels (as the image moves and pans) to scale the blocks up to 5x5 or 7x7 pixels. This is zooming in roughly 2x or 3x.

in fact, if a camera is panning sideways you can grab successive frames and get stereoscopic 3D images from a single camera (similar to the way an owl bobs its head from side to side to increase its depth perception)

OK, so yes, you can take multiple frames of video and pull more info out, and you can interpolate and project what is probably between the adjacent pixels

but you cant zoom in on video 100x and clearly see a persons face reflected in a 5x5 pixel eyeball

but with film, it is possible. A 35mm frame has the equivalent resolution of 10M pixels, compared to 0.3M pixels for a high quality NTSC video single frame.