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Author Topic: Measuring the speed of a projectile  (Read 7369 times)
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Quote
It is a 2 meters long vacuum cannon.
P = Atmospherics pressure = 101325 Pascal
r = 0.02 meters
A = (pi)(r)^2

F = PA
F = (101325)(3.14 X 0.04) = 12732.8750
W = Fd = (12732.8750)(2) =  25465.75
W = (1/2)(m)(V)^2

V = square root((2 X 25465.75)/0.04) = 1128
V = 1128 m/s

Redo your calcs, this time square r,  F=(101325)(3.14x0.02x0.02)=127.328
W=127.328x2=254.657
V=sqrt((2x254.657)/0.04)=112.83 m/s I think

Tom

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I don't think you connected the grounds, Dave.
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Well spotted!
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I hope I got it right, it was 10 to 1am here when I did the calc.
Tom
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I don't think it is possible to fire a ping pong ball at 300 metres per second.
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Quote
F = PA
F = (101325)(3.14 X 0.04) = 12732.8750

So  the 3.14 x 0.04  which you have there,  is supposed to be the cross-sectional area of the bore of the cannon,
calculated by   pi *  r^2  ?

and r is 0.02 m = 2 cm ?

and r squared is ???
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I don't think you connected the grounds, Dave.
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Psst, it's there in reply #15
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Have a look at Infrared LED's/Photodiode...

e.g.
http://arduino-info.wikispaces.com/InfraredBeamPair

http://www.me.umn.edu/courses/me2011/arduino/technotes/irbeam/irbeam.html

Basically, you need TWO beams a known distance apart, not time (uS) when the first beam was broken, then the second.
Given time interval and distance you can calculate average velocity.
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www.phys.csuchico.edu/~lbuchholtz/Papers/VC_AJP.pdf

The above paper says it could be max 287m/s and 4700g (measured with ping-pong balls and photo gates 125-250m/s typically for 0.5 -2m cannon lengths).

What a fun paper! Thanks for posting it. To clarify the above comment, with a 2 meter long vacuum cannon they measured exit velocities of ~200 +/- 30 m/s. That is indeed fast.
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Though I'm a great fan of the arduino I've finished a similar Project without the arduino but with two laser-photocells, two NE555's (one bistable, the other astable) and many CD4026. The distance is 10 cm and I can measure times down to 0.0001 sek. Therefore the velocity-range has a Maximum of 1000 m/s = 3600 km/h  smiley-wink


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I can not recommend this tool enough
http://www.thehalls-in-bfe.com/GGDT/

It is a tool for finding all sorts of info about your cannon design
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