HuntingNut » News » An intro to physics for shooters.

Home • Community Forums • Photo Albums • Register

Modules

• Home - Top 30
• Treasury
• Tell a Friend

Articles: Submit New
• Articles: Topics
• Contact Us
• Community Forums

Members List
• Photo Albums
• PointBlank Ballistics
• PointBlank Online
• Privacy Policy
• Private Messages

Reloading Database
• State F&G Agencies
• Statistics
• Your Account

User Info

Welcome Anonymous

(Register)

Membership:

Latest: IPutMoInYoA

New Today: 0

New Yesterday: 0

Overall: 13131

People Online:

Members: 0

Visitors: 730

BOT: 2

Total: 732

Who Is Where:

Visitors:
01: Forums
02: Forums
03: Forums
04: Forums
05: Forums
06: Photo Albums
07: Forums
08: Photo Albums
09: Forums
10: Home
11: Home
12: Forums
13: Forums
14: Forums
15: Forums
16: Your Account
17: Your Account
18: Forums
19: Your Account
20: Forums
21: Forums
22: Forums
23: Forums
24: Forums
25: Forums
26: Photo Albums
27: Statistics
28: Photo Albums
29: Forums
30: Forums
31: Forums
32: Forums
33: Forums
34: Forums
35: Forums
36: Home
37: Forums
38: Forums
39: Photo Albums
40: Forums
41: Forums
42: Forums
43: Forums
44: News
45: Your Account
46: Your Account
47: Home
48: Forums
49: Your Account
50: Forums
51: Home
52: Home
53: Forums
54: Forums
55: Forums
56: Home
57: Forums
58: Forums
59: Forums
60: Forums
61: Your Account
62: Forums
63: Forums
64: Forums
65: Home
66: Forums
67: Your Account
68: Forums
69: Forums
70: Forums
71: Forums
72: Your Account
73: Forums
74: Your Account
75: Forums
76: Your Account
77: Forums
78: Forums
79: Forums
80: Forums
81: Home
82: Forums
83: Forums
84: Forums
85: Your Account
86: Forums
87: Forums
88: Forums
89: Your Account
90: Forums
91: Forums
92: Forums
93: Forums
94: Forums
95: Forums
96: Photo Albums
97: Forums
98: Forums
99: Forums
100: Forums
101: Home
102: Forums
103: Forums
104: Forums
105: Forums
106: Your Account
107: Forums
108: Forums
109: Forums
110: Forums
111: Forums
112: Forums
113: Forums
114: Photo Albums
115: Forums
116: Forums
117: Forums
118: Your Account
119: Forums
120: Forums
121: Forums
122: Forums
123: Forums
124: Forums
125: Forums
126: Your Account
127: Forums
128: Forums
129: Forums
130: Forums
131: Forums
132: Photo Albums
133: Forums
134: Forums
135: Forums
136: Forums
137: Forums
138: Forums
139: Forums
140: Statistics
141: Your Account
142: Forums
143: Your Account
144: Forums
145: Home
146: Forums
147: Forums
148: Forums
149: Forums
150: Photo Albums
151: Forums
152: Forums
153: Forums
154: Your Account
155: Your Account
156: Your Account
157: Forums
158: Forums
159: Forums
160: Forums
161: Forums
162: Photo Albums
163: Forums
164: Your Account
165: Home
166: Forums
167: Your Account
168: Forums
169: Forums
170: Forums
171: Forums
172: Forums
173: Home
174: Your Account
175: Home
176: Forums
177: Forums
178: Forums
179: Forums
180: Photo Albums
181: Forums
182: Forums
183: Forums
184: Forums
185: Your Account
186: Forums
187: Forums
188: Forums
189: Home
190: Forums
191: Forums
192: Photo Albums
193: Photo Albums
194: Forums
195: Your Account
196: Photo Albums
197: Home
198: Forums
199: Forums
200: Forums
201: Home
202: Forums
203: Home
204: Forums
205: Forums
206: Forums
207: Forums
208: Forums
209: Your Account
210: Forums
211: Forums
212: Photo Albums
213: Forums
214: Home
215: Forums
216: Forums
217: Forums
218: Forums
219: Your Account
220: Forums
221: Forums
222: Forums
223: Home
224: Your Account
225: Forums
226: Your Account
227: Forums
228: Your Account
229: Your Account
230: Your Account
231: Forums
232: Your Account
233: Photo Albums
234: Forums
235: Forums
236: Forums
237: Forums
238: Forums
239: Forums
240: Forums
241: Forums
242: Forums
243: Forums
244: Forums
245: Home
246: Photo Albums
247: Forums
248: Forums
249: Forums
250: Photo Albums
251: Forums
252: Forums
253: Forums
254: Forums
255: Your Account
256: Photo Albums
257: Forums
258: Forums
259: Photo Albums
260: Forums
261: Forums
262: Forums
263: Forums
264: Forums
265: Your Account
266: Forums
267: Home
268: Your Account
269: Forums
270: Forums
271: Forums
272: Your Account
273: Forums
274: Photo Albums
275: Home
276: Your Account
277: Photo Albums
278: Your Account
279: Your Account
280: Forums
281: Forums
282: News
283: Forums
284: Forums
285: Forums
286: Home
287: Forums
288: Forums
289: Forums
290: Your Account
291: Forums
292: Forums
293: Forums
294: Forums
295: Your Account
296: Forums
297: Forums
298: Forums
299: Home
300: Forums
301: Photo Albums
302: Forums
303: Your Account
304: Your Account
305: Forums
306: Forums
307: Forums
308: Forums
309: Forums
310: Forums
311: Home
312: Forums
313: Forums
314: Forums
315: Forums
316: Forums
317: Photo Albums
318: Forums
319: Forums
320: Forums
321: Home
322: Forums
323: Forums
324: Forums
325: Your Account
326: Your Account
327: Forums
328: Forums
329: Forums
330: Forums
331: Forums
332: Forums
333: Forums
334: Forums
335: Forums
336: Forums
337: Forums
338: Home
339: Forums
340: Forums
341: Forums
342: Your Account
343: Your Account
344: Forums
345: Photo Albums
346: Forums
347: Forums
348: Forums
349: Forums
350: Your Account
351: Forums
352: Your Account
353: Forums
354: Home
355: Home
356: Forums
357: Forums
358: Forums
359: Forums
360: Photo Albums
361: Home
362: Forums
363: Your Account
364: Forums
365: Forums
366: Forums
367: Forums
368: Home
369: Forums
370: Your Account
371: Home
372: Forums
373: Forums
374: Forums
375: Forums
376: Home
377: Forums
378: Forums
379: Forums
380: Forums
381: Home
382: Photo Albums
383: Photo Albums
384: Photo Albums
385: Forums
386: Home
387: Your Account
388: Your Account
389: Forums
390: Your Account
391: Home
392: Home
393: Forums
394: Home
395: Forums
396: Forums
397: Forums
398: Forums
399: Forums
400: Your Account
401: Forums
402: Your Account
403: Forums
404: Photo Albums
405: Your Account
406: Home
407: Your Account
408: Photo Albums
409: Your Account
410: Forums
411: Forums
412: Forums
413: Forums
414: Forums
415: Your Account
416: Forums
417: Forums
418: Forums
419: Forums
420: Forums
421: Photo Albums
422: Photo Albums
423: Photo Albums
424: Forums
425: Home
426: Forums
427: Forums
428: Your Account
429: Forums
430: Your Account
431: Forums
432: Your Account
433: Statistics
434: Forums
435: Home
436: Forums
437: Forums
438: Forums
439: Photo Albums
440: Forums
441: Forums
442: Forums
443: Home
444: Your Account
445: Forums
446: Forums
447: Forums
448: Forums
449: Photo Albums
450: Forums
451: Forums
452: Forums
453: Home
454: Forums
455: Forums
456: Your Account
457: Forums
458: Your Account
459: Forums
460: Forums
461: Forums
462: Forums
463: Forums
464: Your Account
465: Forums
466: Forums
467: Forums
468: Forums
469: Your Account
470: Your Account
471: Home
472: Your Account
473: Forums
474: Your Account
475: Forums
476: Forums
477: Forums
478: Home
479: Photo Albums
480: Forums
481: Forums
482: Forums
483: Home
484: Your Account
485: Forums
486: Forums
487: Photo Albums
488: Your Account
489: Forums
490: Your Account
491: Forums
492: Forums
493: Forums
494: Forums
495: Forums
496: Photo Albums
497: Your Account
498: Forums
499: Forums
500: Forums
501: Forums
502: Your Account
503: Forums
504: Photo Albums
505: Forums
506: Your Account
507: Forums
508: Forums
509: Forums
510: Forums
511: Forums
512: Forums
513: Forums
514: Forums
515: Forums
516: Forums
517: Forums
518: Forums
519: Forums
520: Photo Albums
521: Forums
522: Forums
523: Forums
524: Your Account
525: Forums
526: Your Account
527: Your Account
528: Home
529: Forums
530: Home
531: Home
532: Forums
533: Forums
534: Forums
535: Forums
536: Home
537: Forums
538: Photo Albums
539: Forums
540: Your Account
541: Photo Albums
542: Home
543: Forums
544: Photo Albums
545: Home
546: Forums
547: Forums
548: Forums
549: Photo Albums
550: Home
551: Forums
552: Forums
553: Home
554: Forums
555: Photo Albums
556: Your Account
557: Forums
558: Your Account
559: Home
560: Photo Albums
561: Forums
562: Forums
563: Forums
564: Forums
565: Home
566: Home
567: Photo Albums
568: Your Account
569: Forums
570: Your Account
571: Your Account
572: Home
573: Forums
574: Forums
575: Home
576: Forums
577: Forums
578: Forums
579: Forums
580: Forums
581: Forums
582: Forums
583: Forums
584: Forums
585: Your Account
586: Forums
587: Forums
588: Photo Albums
589: Home
590: Forums
591: Forums
592: Forums
593: Forums
594: Home
595: Forums
596: Forums
597: Forums
598: Forums
599: Photo Albums
600: Forums
601: Forums
602: Forums
603: Your Account
604: Forums
605: Forums
606: Forums
607: Forums
608: Photo Albums
609: Home
610: Forums
611: Forums
612: Forums
613: Forums
614: Forums
615: Photo Albums
616: Home
617: Photo Albums
618: Forums
619: Forums
620: Forums
621: Your Account
622: Forums
623: Your Account
624: Forums
625: Forums
626: Forums
627: Forums
628: Forums
629: Forums
630: Forums
631: Forums
632: Forums
633: Forums
634: Your Account
635: Your Account
636: Forums
637: Your Account
638: Forums
639: Forums
640: Forums
641: Photo Albums
642: Forums
643: Forums
644: Forums
645: Your Account
646: Statistics
647: Your Account
648: Forums
649: Forums
650: Forums
651: Home
652: Forums
653: Forums
654: Forums
655: Photo Albums
656: Photo Albums
657: Your Account
658: Forums
659: Photo Albums
660: Your Account
661: Forums
662: Your Account
663: Forums
664: Forums
665: Forums
666: Forums
667: Photo Albums
668: Home
669: Your Account
670: Your Account
671: Forums
672: Photo Albums
673: Forums
674: Photo Albums
675: Forums
676: Home
677: Your Account
678: Forums
679: Photo Albums
680: Your Account
681: Forums
682: Photo Albums
683: Forums
684: Forums
685: Forums
686: Forums
687: Home
688: Forums
689: Forums
690: Forums
691: Forums
692: Forums
693: Photo Albums
694: Forums
695: Forums
696: Forums
697: Forums
698: Photo Albums
699: Forums
700: Forums
701: Your Account
702: Home
703: Forums
704: Forums
705: Forums
706: Forums
707: Forums
708: Forums
709: Your Account
710: Forums
711: Forums
712: Forums
713: Forums
714: Home
715: Your Account
716: Your Account
717: Your Account
718: Your Account
719: Forums
720: Forums
721: Photo Albums
722: Your Account
723: Home
724: Forums
725: Forums
726: Forums
727: Your Account
728: Forums
729: Your Account
730: Forums

BOT:
01: Forums
02: Forums

Staff Online:

No staff members are online!

Coppermine Stats

Photo Albums

• Albums: 308
• Pictures: 2452
  · Views: 824587
  · Votes: 1316
  · Comments: 86

An intro to physics for shooters.

There is always a great deal of chatter among hunters, as well as shooters primarily interested in defense, about "energy" and "momentum" and how they relate to "knock down power", etc. I find, however, that the terms used are often misunderstood. Having been involved in so many of these discussions I thought I'd offer some definitions and examples of what these terms mean with regard to terminal ballistics, that is, what happens when a bullet hits a critter or other target.

Before I even get started I want to recommend an excellent book every serious shooter ought to have. "Understanding Firearm Ballistics" by Robert A. Rinker should be required reading for anyone who wants to really know how internal, external and terminal ballistics works. The book offers relatively simple explanations of the nuts and bolts of ballistics and also includes the formulas to go deeper into it if you want to. I don't know of any better reference for those of us who want to really understand ballistics.

The first thing we need to do is define a few of the terms that get used so often. I'll have a list of my references at the end of this article. Forgive me for using "English" units instead of metric units but I'm just used to them!

Mass - Mass is the amount of matter that makes up an object. The unit used for this is the "slug". It isn't the same thing as weight. If an object is in space it has almost no weight but still has the same mass as on earth. An object's mass can be calculated by dividing its weight in pounds by the acceleration due to gravity, which is 32.12 ft/sec/sec. Since we used "grains" to measure bullet weight we also have to convert grains to pounds before dividing by gravity. There are 7000 grains in a pound. For example: a 150 grain bullet would weigh 150/7000 = 0.02143 pounds. Its mass would be 0.2143/32.12 = 0.000667 slugs.

Velocity - Velocity is simply how fast our object is going. In shooting we usually measure velocity in feet per second (ft/sec).

Momentum - This is the "push" you get from a bullet leaving the gun and also hitting its target. Momentum is calculated by multiplying an object's mass by its velocity. When people are talking about an "equal and opposite reaction" in shooting this is what they mean. The bullet and propellant gases go one way and the rifle goes the other way. The unit used is the slug-foot/sec. A 150 grain bullet going 2900 ft/sec has a momentum of 1.935 slug-ft/sec.

Energy - Energy is the ability to do work. In shooting we are talking about "kinetic energy", that is, the energy of a moving object. The unit we use for energy is the "foot-pound". To calculate an object's kinetic energy you have to know its mass and its velocity. The formula for kinetic energy: KE = 1/2 x mass x velocity x velocity. A commonly used method to calculate energy of a bullet is; bullet weight x velocity x velocity / 450380. The constant of 450380 takes care of the conversion from grains to slugs. Our 150 grain bullet going 2900 ft/sec has an energy of 2801 foot-pounds.

So, what's all this stuff mean to us shooters? Let's look at what momentum and energy actually do for us, and to the targets. Keep in mind that there are a lot of variables that will muddy the water here.

As mentioned above, momentum is the "push" the moving bullet gives to both the rifle and the target. When the bullet leaves the muzzle the "equal and opposite reaction" is pushing the gun back against us. That's recoil. When we calculate recoil we also have to take into account the weight and velocity of the powder gases. The velocity of the gas is usually estimated as 1.5 times the velocity of the bullet. If we take that 150 grain bullet at 2900 ft/sec, along with a 50 grain powder charge exiting the muzzle at around 4350 ft/sec, and fire it in an 8 pound rifle we'll have the rifle coming back at us at about 12 ft/sec. But, if we are holding the rifle correctly, you also have to include our mass in the calculation. A 180 pound man firing that rifle would be given a backward velocity of about 0.5 ft/sec. Now for some of those variables. Our muscles aren't rock hard (as much as we'd like to believe they are!) so they soak up part of that recoil. So do any recoils pads. The stock material and design can make a difference too. The point to take away here is that the recoil isn't going to knock us into the next county, even if it does smart some.

Now, how does the momentum affect the target? In every single case the bullet will hit the target with less momentum than the gun hits the shooter. Remember that part of the gun's recoil comes from the propellant gases and those don't make it to the target. If you're shooting a steel plate like a metallic silhouette ram, which weighs around 50 pounds, you can give the plate a velocity of 1.24 ft/sec. That's enough momentum to tip it over, if the base isn't too wide. Since the bullet doesn't go through the plate it transfers all of its momentum to the plate. You might even get a little extra push if the bullet spatters back toward the shooter.

If you are shooting a 150 pound deer (or bad guy), and the bullet doesn't go all the way through, you'll again transfer all of the momentum to the target. In this case the velocity you'll give the deer is a little over 0.4 ft/sec. If the bullet passes through the target the momentum would be even less. We've all heard stories of deer being knocked down, flipped over, etc. when struck by a bullet but those incidents are the result of the reaction the animal made to being struck, not the momentum itself. It's like a person reacting to a bee sting. The momentum of the sting doesn't cause all those contortions and gymnastics, the person's reaction to the sting does. The big variable here is whether the bullet stays in the critter or passes all the way through. The take away, however, is that there really isn't any such thing as "knock down power" except maybe on very small critters.

How about this energy thing? As mentioned above, energy is the ability to do work. In the case of a moving bullet the energy gets used up on several ways. Initially some of the available energy is used up by pushing through the air on the way to the target. More is used creating heat and noise. On impact energy is used by making a hole in the target and deforming the bullet. The big variables to remember here are the bullet's construction and the target's construction. When you start comparing loads using their energy levels you need to make sure that you're comparing apples to apples. Let me offer a couple of examples to illustrate my point.

If I take identical 150 grain .30 caliber spire point bullets and load one in a .30-30 and the other in a .30-06 I'm going to get somewhat different results when the bullet hits the target. The .30-30 will start the bullet off at around 2200 ft/sec and at 100 yards will have an energy of 1300 foot-pounds. The .30-06 will start the same bullet at 2900 ft/sec and at 100 yards will have an energy of 2317 foot-pounds. With that extra energy the .30-06 will do more tissue damage and also deform the bullet more.

What if we took two bullets of the same weight but very different construction and loaded them so that they would strike the target with exactly the same energy? Let's say that I load a very frangible bullet, like a Barnes "Varmint Grenade" in one case and a monolithic bullet, like a Barnes TSX, in another. In this case the frangible bullet would come apart on impact, creating a shallow wound. The monolithic bullet would stay together and keep penetrating the target. If we're talking about a small critter this doesn't make much difference but if we are talking about a deer-sized critter it could easily mean the difference between a lost animal and venison steaks on the grill.

The point to all this is that neither momentum nor energy are magic numbers. Momentum won't knock big game over and energy alone won't cause humane kills. When you start making comparisons of cartridges you also must keep in mind the bullet construction. Also keep in mind that there are a number of other variables that will affect the terminal performance of the loads. The biggest variable of all while hunting (and defensive shooting) is shot placement, and that takes practice with the gun, not the calculator.

References:
"Understanding Firearm Ballistics" by Robert A Rinker
"Applied Physics" by Paul E. Tippens
"Principles of Physics" by Frederick Bueche

Posted by Pumpkinslinger on Friday, September 16, 2011 (21:08:43) (2571 reads) [ Administration ]