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Natural Science Forum / Physics / Acoustics / October 2004MULTIPLE CHOICE
Hi, First off, I have absolutely no knowledge when it comes to firearms. So I was hoping you guys might be able to help me with the following. I don't know if the following two questions are both so elementary that they are "slam dunks" or if they actually could be harder to answer than they seem to appear. QUESTION ONE: A 22-caliber pistol fires a bullet indoors. The bullet travels through the room in which it was fired and continues on through an open doorway and into a corridor. In that corridor, the bullet then hits a doorway about 52 feet away from the gun and imbeds itself into this doorway. The impact of the bullet going into this doorway produces a "thump" sound. If we are standing closer to this doorway where the bullet impacts than we are from the gun, we will: a) hear the gunshot before we hear the "thump" of the bullet's impact. b) hear the "thump" of the bullet's impact before we hear the gunshot. c) hear the gunshot and the "thump" of the bullet's impact simultaneously. QUESTION TWO: A 22-caliber handgun fires a bullet indoors. The bullet travels through the room in which it was fired and hits an open doorway. The bullet then ricochets. As a result of the ricochet, the bullet travels into a corridor. In that corridor, the bullet then hits a second doorway about 52 feet away from the gun and imbeds itself into this second doorway. The impact of the bullet going into this second doorway produces a "thump" sound. Will the ricochet: a) effect bullet velocity, thereby slowing it down before impact. b) not effect bullet velocity, thereby not slowing it down before impact. If you wouldn't mind helping me with this, I'd greatly appreciate it. I apologize if there's not enough information being presented. If that's the case, it would greatly help me to know what additional information would be required. Thanks very much! Brad bkj333@aol.com Question One - speed of bullet could be helpful Question two - The gun has "effected" the bullet velocity. I would expect the first impact to at least "affect" the velocity by changing direction, and probably speed, but this is not an acoustics question. > Hi, > [quoted text clipped - 45 lines] > > bkj333@aol.com Try a google search on "bullet velocity". You'll need to know the speed of the bullet, which can be dependent on power charge, mass of the bullet, and gun barrel length. The search may give some information on change of velocity with a deflected change of direction. Once you have this information and the temperature, pressure and humidity, it should be a simple matter of calculating times for the bullet travel between points and the sound travel. You should then be able to do a comprehensive paper, with diagrams, to get a good grade on your sience project. Dave > Hi, > QUESTION ONE: > [quoted text clipped - 37 lines] > > bkj333@aol.com Note that the questions use the word "hear." It is not just a matter of which sound arrives first at the ear if the question involves actual perception and the arrivals are close together. You may not hear them as separate events. > Try a google search on "bullet velocity". You'll need to know the > speed of the bullet, which can be dependent on power charge, mass of [quoted text clipped - 53 lines] >> >> bkj333@aol.com > Note that the questions use the word "hear." It is not just a matter of > which sound arrives first at the ear if the question involves actual > perception and the arrivals are close together. You may not hear them as > separate events. Exactly. This whole question depends mainly on the muzzle velocity of the bullet. If we assume a really slow speed, then the bullet will be travelling around 470 fps. However, if a military .22 firearm is used, then the muzzle velocity could exceed 2500 fps. We have already been told that (in question 1) the gun and the target are 52' away from each other. If the observer is equidistant from gun and target (I.E. 26'), then the sound of the gun firing will reach the observer at t=0.023s (assuming speed of sound to be 1125 fps). The bullet will pass observer at t=0.055s (assuming a muzzle velocity of 470 fps) and hit the target at t=0.119s. This event will create a sound that travels back to the observer, arriving at t=0.142s (from 0.119 + 0.023). The sound of the gun firing and the sound of the target impact are separated in time by the flight time of the bullet - which is dependent upon the muzzle velocity, and this value has not been specified. A time interval of 0.119s between acoustic events is easily discernible by the human brain. If however, the muzzle velocity is raised to 2500 fps, then the bullet will hit the target before the sound of the gun firing has reached the observer. The sound of the gun firing and the sound of the bullet hitting the target will now arrive at the observer within 21ms of each other. The human brain has an integration time of approximately 20-25ms, so if this test is performed with a high velocity round, then the observer will probably perceive a single acoustic event. Since neither the muzzle velocity of the bullet, and the position of the observer have been specified to any degree of accuracy, the question cannot really be answered. Chris W  SignatureThe voice of ignorance speaks loud and long, but the words of the wise are quiet and few. -- correction ; in my prior post, power charge should read powder charge. |
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