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Posted: 1/14/2003 2:41:02 PM EDT
 crossing fingers -Spaceman |
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ah ha -- now we'll find out who the physics majors are here... [:)] who can figure out the velocity of the bullet just from the photograph? everything you need to do it is illustrated right there, with two minor exceptions -- for the sake of argument let's assume the ambient temperature is 25'C (77'F) and that the ambient pressure is 1atm (14.7psia). so, who's got the answer? [;)] this isn't a trick question, you can in fact derive the bullet velocity just from the picture... so tell us all how, and take a shot at the velocity to prove you didn't sleep through physics or fluid dynamics. ps, an equally telling picture can be seen here: [img]http://www.fas.org/man/dod-101/sys/land/apfsds.jpg[/img] while this is clearly not an ar15 round (actually it's an m1a1 120mm sabot-equipped depleted-uranium fin stabilized penetrator [M829A1] -- aka "the kinetic energy killer"), the velocity can be "read" from this picture as well. ps: more info on this shell is here: [url]http://www.fas.org/man/dod-101/sys/land/120.htm[/url] |
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well it seems to me that since the shock wave is traveling from its point of origin at exactly the speed of sound, all you need to do is look at the angle of the primary shock wave off the line of axis of the projectile's motion. it forms a right triangle with the shockwave as the hypotenuse, the distance the wave has traveled as one leg, and the distance the projectile has travelled as the other leg. since these dimensions are all from the same instant in time, it is quie easy to give the velocity of the projectile as a function of the length of the two legs: Velocity = (length of projectile travel leg over shockwave expansion leg) times the velocity of sound under the conditions given. i estimate the projectile i posted at about mach 2.2 and the projectile you posted at about mach 3. -Spaceman |
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hurrumphhh -- i guess all the sharpies hang out here late at night (EST)... spaceman has hit the nail on the head. all supersonic bodies create a shock wave, in this case it can be termed a bow shock wave. at mach 1.0, the shock wave would be more or less perpendicular to the direction of projectile travel. but at mach numbers greater than 1.0, the shock wave is angled back -- creating a phenomena called the "mach cone". the faster the projectile is going, the more the cone is drawn back (also called the "oblique shock"). by measuring the angle of the cone, and knowing the local density of the air (which is dependent on temperature and pressure), you can calculate the Mach number and therefore the projectile's velocity. spaceman's estimates are close, close enough for government work. the M1A1 projectile is actually going faster than mach 3 but the most pronounced shock waves are caused by the sabot, not the sharp-tipped projectile itself. the sabot is slowing, very very rapidly, while the penetrator races on. last i read the penetrator is blitzing along at around a mile a second (~5280 fps), which at normal conditions is about mach 5. it's not hard to see why some soviet T72 tanks in the gulf war had two holes in them from this round -- one entry hole going in and one exit hole going out. ps, in case you wanted to know... these pictures employ a technique called "Schlieren photography", invented in the mid 1800's to detect flaws in glass ('schliere' being german for 'streak'). it is one method of visualising shock waves in wind tunnels. the technique takes advantage of a change in refractive index (ability to 'bend' light) with density. if you look directly above the filler spout when you are refueling your car, you can see some of the same effects: the gasoline vapor-laden air has a different density than the surrounding air and this creates local disturbances in the refractive index. and that's what causes the "wavy-ness" you see above the gas filler spout. |
| Huh????????????? all that time in college and my aircraft mechanic school. all thos eyears in high school.....algebra and trig. i always said we would never use that kind of math in the real world and yet here it is! and i let all that memory of sine and cosine slip away only to be filled with visions of rifles and internet porn. somehow my life seems so useless now. have i failed my parents? is there any reason to live? let me hold my sig and decide wha........................... |
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