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Measure your speed (Read 16645 times)
wanderer
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Measure your speed
Nov 5th, 2006 at 3:28am
 
I guess a lot of us would love to know this.

I don't fancy my chances following Holly's suggestion and sweet talking the local cop/State Trooper into using their speed gun. Instead, here's an idea that I think could work well, but I can't myself try at the moment  Embarrassed since I don't have the equipment.

The idea is just an an application of the school book stuff:
  speed = distance/time.

ie, measure how long it gets for the slingstone to get from one place to another. So:

Set up a target of two sheets of something like newspaper, thin plastic etc. some fairly short distance apart (maybe 5ft to 10ft). As you might guess, the idea is to arrange them so that you can sling your bullet through both these targets, so if you sling overhead like me you might hang them from the roof of a garage or something similar.

You measure the time it takes to get from one sheet to the other by making a (sound) recording of your slinging, and turning that recording into a sound file on your computer. Then use whatever software you've already got/you can get off the net/write yourself  Wink to look at the sound file and measure the number of samples between the sounds made by the bullet passing through the two sheets. From that the time, hence your speed.

I think it should be at least as accurate as a radar gun, although you've got to work a little more to get the answer Smiley

A few comments which are probably obvious, but....

You want to shoot through something thin because you don't want to slow the bullet down significantly.

For the same reason this is going to be a lot easier if you are slinging something heavy and small than say a golfball or a tennis ball.

Also sling something small because you get to have more shots before you rebuild your targets Undecided.

Also something small will give a cleaner sound.

As a bonus of having the holes in the target, you can measure exactly how far the bullet travels between the sheets even if you don't hit it square. For me at least this would be a good thing Wink.

You get to try lots of things to make it work better - if that's the kind of think you like Cheesy.

Could at least make a great school science project, I think.
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Stringman
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Re: Measure your speed
Reply #1 - Nov 5th, 2006 at 3:38am
 
Excellent idea!  As the slingstone experiances constant deceleration the more sheets of newspaper the better but old newspaper is cheap.
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Re: Measure your speed
Reply #2 - Nov 5th, 2006 at 11:06am
 
hey i was looking through a gun store catalogue yesterday, and it is two loops of wire sitting a few inches away from eachother attached to a box.  the bullet would fly through both loops and the box would measure how long it takes to pass from one loop to the other.  this is the same principle as your suggestion, and if you are good enough for accuracy then you could maybe borrow one of these from a friend that shoots guns.
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ArizonaSlinger
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Re: Measure your speed
Reply #3 - Nov 5th, 2006 at 12:02pm
 
Freshmen Physics textbooks typically have ballistics problems.   The problems are based on the following truths:

Energy is always conserved.
Kinetic-Energy is not always conserved.
Momentum IS ALWAYS conserved.

The speed of a projectile can be very precisely measured using a pendulum a ruler and a weight scale.

The setup:
The pendulum should have a 'bob' on the end that the projectile will sink into.  It must not bounce off.  The porjectile must imbed inside the bob for the following calculations to work. The 'bob' of the pendulum is the target.

Use the ruler to measure how high the pendulum swings after being hit. Use the scale to measure the mass of the 'bob' and the projectile.

Then consult your physics book to see how they work out the problem. Basically it works like this:

The momentum of the projectile before it strikes the 'bob' is equal to the momentum of the 'bob' with the projectile in it right after impact.  That momentum causes the kinetic energy to be transferred to the 'bob', wich in turn gets converted to potential energy as the bob starts to swing up in an arc.  I'll post a final solution later. This method can be VERY accurate.  The accuracy can be as good as your scale for measuring the bob's mass and the accuracey for measuring how high the pedulum swings.

A back of the envelope calculations would yield the following:
Givens:
projectile = 100 grams
bob swings 33 cm upward
bob =3900 grams

Answer
Velocity of projectile = 100 m/s.
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Re: Measure your speed
Reply #4 - Nov 5th, 2006 at 2:46pm
 

Kewl!  Let us know how it turns out!


I can't wait to see the size of the 'bob' and how many attempts it takes....Youtube that video if you get any!


TS


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Re: Measure your speed
Reply #5 - Nov 5th, 2006 at 3:50pm
 
ArizonaSlinger, I was thinking the exact same thing just a few days ago.  I was thinking shooting a blob of clay at the target/pendulum, which would just be a weighted box.  However I decided it would be too hard to do, especially getting the swing height measured accuratly (and because it would take me many tries to hit the box).  Maybe a witness or video camera would help. 

Wanderer - That's a good idea!  As long as you used very thin targets like you said, I think it would work well!  Some large sheets of tissue paper would probably be best, but news paper would also work.  Microsoft sound recorder, which all window users have, lets you time sounds down to the 1/100th of a second, so with that you could easily do it.  You could go through it pausing at the bursts of sound that appear on the line, and note the times.  Also i'm guessing that ten as opposed to five would maybe be better, because it would be easier to tell the two sounds apart on the recording.
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wanderer
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Re: Measure your speed
Reply #6 - Nov 5th, 2006 at 7:33pm
 
Umm.. well thanks for the replies everyone!

I thought I would post this idea because it seemed a relatively simple way to get good results, using as a timing method something which almost every computer now comes with as standard, audio recording Smiley. I wasn't proposing it as a 'gold standard' to supercede all others. I know Matthias has experimented extensively with ballistic pendulums, or is it pendula - darned if I know.

It's a lot easier to hit arbitrarily large sheets of paper or whatever than the rather small space you've got to sling through for either the wire timing loops or the ballistic pendulum. Besides, what's wrong with having a different way of measuring the speed of a slug, particularly if you can probably do it in a short afternoon? There's obviously arguments that you're slowing the slug down as it passes through the sheets, but you can surely correct for that if you really want. Even if you don't, just make the stuff as fragile as possible provided you get some kind of sound out of it!

My school science text books may be a little dusty but I do know of the ballistic pendulum Wink. I've found them in practice to be a bit of a PITA to set up unless you exercise a lot of care. And I think you are always stuck with hitting in a limited area to get the simple equations to work. Great for muzzled weapons but not conducive to full power slinging.

Cliff. Thanks for the info re. Windows machines. I don't have one, so I didn't know what was available. 0.01s resolution is OK, but the sound is sampled at a great deal higher rate, so I think one could quite easily get 0.001s resolution or higher. I suppose the slug will travel about a foot or more in a hundredth of a second so yes, you'd need distances on the larger size to get any accuracy at all.

Swing height measurement is easy, 'cos you don't actually measure that Smiley, you measure the maximum angle of deflexion of the bob, or something closely related to that, and given that the  pendulum swings on the end of a known length you get the height, but then in practice you often don't need to compute the height either Wink.
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Re: Measure your speed
Reply #7 - Nov 5th, 2006 at 8:26pm
 
I think most measureing systems would have some built in inaccuracies, with the pendulum some energy would be lost to heat,  think of fire pistons, with paper energy spent on breaking the barriers but probably a tiny percentage.  I like the ease of set up for paper.  Maybe sound could be used another way too.  What if one made two recordings, one through a mike behind the slinger and one through a mike behind the target.  Use a sling that cracks and sling a buzzer.  Load both recordings into a computer, use the crack  and sound of impact to line them up on the screen and compare doppler effects.  It may be posable to calculate the velocity and deceleration  based on the differance in  pitch between the two recordings..
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Re: Measure your speed
Reply #8 - Nov 5th, 2006 at 10:56pm
 
Stringman,

I like your method for slinging a buzzer.  As you said, wi such a setup you could calculate the velocity as a function of time or of position!
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Re: Measure your speed
Reply #9 - Nov 6th, 2006 at 1:39am
 
I've been mulling this one over and thought of a frill.  If one synchronized two watches and put  one by each mike beeping occasionally at the same times ( or one watch in the middle if one was sure that one wouldn't hit it on a unlucky  bad shot)   it would help to line things up back at the pc and serve as  check on the playback and recording speeds being the same.  The check would matter more for tape based machines, which may be more accurate as they are analogue but tend to slow as the battery drains.  it will end up chopped up into digital on the pc anyway.  This form of measurement would also waste energy of course, buzzers only buzz because they are irregular rocks and so a bit slower.  The sound as they rotate through the air is probably irregular too, maybe "comparing individual waves" would have been better than saying pitch.  Still it would be easy, just set it up and apart from scaring passers by shouting things like "Shot 32, apache!!" one could just be slinging normally.
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wanderer
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Re: Measure your speed
Reply #10 - Nov 6th, 2006 at 6:11am
 
I like the idea of slinging a buzzer also. Smiley.

Trouble is, for some reason, very few of the rocks I throw buzz, I guess I'm just blessed with a source of good rocks Smiley. Making the sling projectile some kind of active sound source opens up a lot of possibilities, but maybe getting beyond the level I pitched my original post at - now we're up to say freshman engineering project?!

To do the analysis off the audio, it would probably be nice if the projectile whistled rather than buzzed. Ideally it would be some kind of active source, piezo or similar, which would give you access to what goes on before you launch it.

I think I read a hint of something similar on this site somewhere, but don't know if anyone ever built one.

Anyone have any ideas to make it simply and robustly enough that you don't end up having to treat it like stringman's gold glande? Grin

With the two mike setup, wouldn't it be easier to lay both signals down on the same recorder, although I suppose the cabling is a pain?

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Re: Measure your speed
Reply #11 - Nov 6th, 2006 at 5:42pm
 
I found two old threads About making humming or whistling projectiles.  I've been meaning to try slinging iron nuts myself to see how well they hum.  Both of the threads have some decent info, but neither is very long.

http://www.slinging.org/forum2/yabb/YaBB.cgi?board=1;action=display;num=11299135...

http://www.slinging.org/forum2/yabb/YaBB.cgi?board=1;action=display;num=11452740...

However, as much fun as freshman engineering projects can be, if I ever clock my speed, I'll stick with the two sheets of paper and a microphone  Wink.
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Re: Measure your speed
Reply #12 - Nov 6th, 2006 at 7:21pm
 
I am not very good with math or converting things on computers so most of what you guys said went right over my head. However you seem to have a good grasp on a sound idea. Make sure to post results. good luck.
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Re: Measure your speed
Reply #13 - Nov 7th, 2006 at 8:42am
 
If I did have a gold glans I'd give it to some mathematician to sort this out, I've been trying to work out the method for converting the  data into information and it's driving me mad, any physicists please help.  The experiment would be easy enough to set up, though I'd have to cadge a second recorder from somewhere ( if I mention that  mine would be the one in the "target bunker" that should be ok).  As I live inland and not near any handy mature rivers almost all the rocks I sling travel loudly ( you're a fortunate man Wanderer ) but my guess is that the sound will be a bit inconsistent, no idea if that is a good or a bad thing, so something more regular may be helpful.  I've just read those threads ( cheers Cliff )  and the one about the singing arrow gives me an idea.  I don't have a masonry drill but maybe molding glans  around sections of drinking straws would give the same effect, try to work out where they go with a bit of plasticine first before wasting high density concrete,  worth a try anyway.

The experiment is not too much of a problem,  but probably won't happen any time soon  as I only have the one afternoon a week and it's cordage at the moment, plus it would be nice to actually sling a bit plus weather, my bottom of the range mini disc can't handle damp.  My lack of knowledge is a problem.  This is where I could use some help.  So .....factors which will be known and available tools..
1/  The distance between target and slinger
2/ The speed of sound, which varies far more in air than I would have guessed,  is no problem as I found this http://www.sengpielaudio.com/calculator-speedsound.htm ; enter temperature and it does it for you, a link of the same page has another for humidity and pressure.  I can measure temperature and just hope that the local met report is close enough for the rest.
3/ I found a sound editor http://audacity.sourceforge.net/ ;  One can display sound as a waveform with a  time line underneath it.  One can zoom in on the wave/time line  to pretty high resolution.  Easily enough I think.

Lets imagine it's done and on my machine.  I'm going to pretend the distance from the mics to the target/slinger don't exist as I have enough problems without trig', it'll only be half a meter or so anyway.  On the assumption that the speed of sound is 340 meters per second on the day and that I sling 34 meters ( I'm lazy that way ).   I would expect the sound of the crack to be live at the slingers end and  delayed by a tenth of a second at the targets, and visa versa for the sound of impact.  I would also expect the Doppler effect, the waves would arrive closer together for the target microphone and further apart for the slinger microphone. does this mean the the slingers end recording will be slightly longer?   I have read that the Doppler effect does not change the speed of sound.  So if I zoom in on a distinctive pointy bit ( forgive the technical term from now on I'll call it a “dpb” ) and note how long it is after the crack and before the impact, then do the same with it's twin dpb.  How do I turn this into location?

Oh drat, just realized I may have spent hours on the wrong thought experiment.  'o' level geology!  Put the microphones either side of slinger,  quite a long way apart.  Put a timepiece in the middle beeping at regular intervals, or one with each.  The time the sound of the dpb's take to reach the microphones will give the distance to the glans, then it is just drawing two circles.  When one detects earth quakes one needs a third in order to tell which crossing point is the epicenter but hopefully I won't miss by 180 degrees. If the glans is really loud one could even forget about targets all together and just sling as hard as one can in the right general direction, the microphones would track the glans for as long as they could hear them. Unless the Doppler effect messes it up, which depends on how fast the glans retreats .....which is what we're trying to find out.... because ....we ...don't... know!   Unless there's some way round that...we have the beeps.  Or would it just be timeing when dpb's happen without knowing where?  Third mic after all?  Oh I don't know.

Arggghhh it's another one of those itchy ideas.  If anyone cares to throw some knowledge,ideas,or  just do it properly I'd be grateful, this one has kept me from sleeping most of the night. I have that  sort of mind, once it was “I wonder what the surface area of  the earth is in light years?” this is worse.   The wiki writing people want the velocity info too. 

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wanderer
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Re: Measure your speed
Reply #14 - Nov 7th, 2006 at 12:59pm
 
Buxton eh, stringman. Yep, somewhere down south!

I'd love one of your gold projectiles, but it gave me a headache trying to figure it out..

Maybe the way to get a grip on this is to get as simple as possible? (I can hear the laughs from here Grin).

If we wanted just to time as precisely as we could the interval between the sling crack and the thud of hitting the target, then just use one mike, and put it an equal distance between the target and you. You can of course put it well out to the side:). Then it doesn't matter what the speed of sound is. Whatever it is, the mike will get both the snap of the sling and the thud into the target delayed by the same amount. If you actually want to know the time delay, then it's distance/speed_of_sound.

Rest of my tortured reasoning in a little while, gotta go Sad.

Before I do, if ArizonaSlinger is still reading this, I'm afraid what I wrote before came across rather heavy. I can only blame too much coffee, a long time without sleep and a boss who plays mind games. You got a bit of the frustration from my 'real' life!

Oh and 2.8x10-24 square light years, I think Smiley
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