[ARCHIVED THREAD] - Non-Homework Phyics Question (Page 1 of 2)
Posted: 2/5/2010 11:51:56 AM EDT
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Situation:
Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? |
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Quoted: Is it a LaRue mount?Situation: Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? |
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Gut reaction is the bullet flies out of the gun like you would normally expect. Bullet and gun are neutral. Explosion imparts additional energy on the bullet. Recoil acts to slow the gun down.
Scratch that. I missed the part about firing in the opposite direction of travel. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction?
Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? |
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Quoted: I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. The bullet travels at x speed relative to the ground while the barrel travels at x speed in the opposite direction. Which is what you said, I guess. |
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Quoted: The bullet travels at x speed relative to the ground while the barrel travels at x speed in the opposite direction. Which is what you said, I guess. I was addressing his concern that the bullet wouldn't leave the barrel if you fired it when the gun was facing the front of the vehicle. In that case, the observer would see the bullet travel at 2x. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? |
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Quoted: When did they make FIREarms with muzzle velocities of 2*10^7 m/s?Quoted: Quoted: I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c 2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? |
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Quoted:
I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? Thats what im thinking, lets say the car/round is going 800fps. So the round is already going 800fps, when fired it would go 1600fps? Or do I fail at physics? |
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Quoted: Quoted: I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? Thats what im thinking, lets say the car/round is going 800fps. So the round is already going 800fps, when fired it would go 1600fps? Or do I fail at physics? Correct. |
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The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. |
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Quoted: I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction?no Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel?yes <–– + ––> = . ––> + ––> = ––––> For speeds much much lower than the speed of light. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? No, it will be moving at perhaps 90% of C. Don't they cover relativity in highschool? 
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This is a conservation of momentum problem. Sum of mv initial = Sum of mv final "The plane will take off." Plus, there's that whole "action/reaction" thing to consider. Discharging the firearm in the same direction of the vehicle's vector of travel will decrease it's velocity, while discharging it in the opposite direction will increase it's velocity. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? No, it will be moving at perhaps 90% of C. Don't they cover relativity in highschool? Yes, they did. That was kinda' my point. Didn't they cover reading comprehension at your high school? |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction?no Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel?yes <–– + ––> = . ––> + ––> = ––––> For speeds much much lower than the speed of light. In th words of my drill instructors: Explained Barney Style. |
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Situation: Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? 
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There is no absolute speed only relative speed. You can pick any object and assume that it’s the one standing still.
The bullet is in the rifle. So, as far as it’s concerned the rifle (and the car it’s in) is stationary. It’s the ground and the air that’s moving. So, when you pull the trigger the rifle fires backwards and the bullet leaves the barrel at 3,000fps (or whatever). By an amazing coincidence the ground and the atmosphere is also moving backwards at 3,000fps. From the standpoint of a person on the ground it’s the car that’s moving. The gunshot actually deaccelerated the bullet by 3,000fps. Since that was the speed it was originally moving, the bullet stops and falls to the ground. |
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I'm not a physics guy but I would think since with a motionless gun the bullet travels down the barrel at "x" speed; if the barrel is moving in the opposite direction at that same speed then the bullet will travel down the barrel twice as fast as normal. However when it leaves the barrel would it not still fly in a "normal" direction? Now if the gun was firing in the SAME direction as the vehicle is traveling... would the bullet ever leave the barrel? The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? Actually no, once the bullet goes over 100% of c it begins to travel back in time and would actually reverse course and head back towards the vehicle and back down the barrel that it came out of. |
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Quoted: Quoted: Quoted: Quoted: The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c 2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. I rounded and dropped an OoM. That doesn't change the fact that for the question as stated Newtonian mechanics is a perfectly valid approximation. |
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This has already been covered.
Watch the video http://www.youtube.com/watch?v=-EpQIc153H0 Start at 3:25 if you just want the end. |
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The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. I rounded and dropped an OoM. That doesn't change the fact that for the question as stated Newtonian mechanics is a perfectly valid approximation. Actually, the question as stated is missing so much information we don't really know whether or not Newtonian mechanics are sufficient. We might make some assumptions about the parameters that would make your position valid, but we might also make assumptions that make it invalid. And if we're going to make the assumptions that make Newtonian physics sufficient, we also need to take into account some important things that are being ignored here. For instance, accelaration of the vehicle due to the firearm being disharged. Also, exactly what is being asked by the question, "What happens?" |
| neglecting every force but gravity, it would be identical to you shooting it standing on a ground with no velocity from the same position. In this case your velocity in the x direction doesnt matter. Its wierd you asked because we talked about this in my physics class yesterday. |
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The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c
2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. I rounded and dropped an OoM. That doesn't change the fact that for the question as stated Newtonian mechanics is a perfectly valid approximation. Actually, the question as stated is missing so much information we don't really know whether or not Newtonian mechanics are sufficient. We might make some assumptions about the parameters that would make your position valid, but we might also make assumptions that make it invalid. And if we're going to make the assumptions that make Newtonian physics sufficient, we also need to take into account some important things that are being ignored here. For instance, accelaration of the vehicle due to the firearm being disharged. Also, exactly what is being asked by the question, "What happens?" Already been covered Quoted:
Situation: Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? |
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Quoted: Quoted: Quoted: Quoted: Quoted: Quoted: The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c 2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. I rounded and dropped an OoM. That doesn't change the fact that for the question as stated Newtonian mechanics is a perfectly valid approximation. Actually, the question as stated is missing so much information we don't really know whether or not Newtonian mechanics are sufficient. We might make some assumptions about the parameters that would make your position valid, but we might also make assumptions that make it invalid. And if we're going to make the assumptions that make Newtonian physics sufficient, we also need to take into account some important things that are being ignored here. For instance, accelaration of the vehicle due to the firearm being disharged. Also, exactly what is being asked by the question, "What happens?" Fine. The OP clearly wanted to know what happened when a midget on a skateboard traveling near the speed of light and carrying the Paris Gun fired a neutronium projectile with 30 solar masses. |
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Quoted: Quoted: Quoted: Quoted: Quoted: Quoted: Quoted: The bullet leaves the barrel at the same velocity independent of the orientation the gun is facing with respect to the vehicle. I.e. the bullet will travel at speed "x." If the vehicle is traveling at speed "x," and the gun was facing forward, a stationary observer on the ground would see the bullet leave the barrel at 2x. Said stationary observer on the ground sees the vehicle traveling at 60% of c (velocity of light). x = .6c 2x = 1.2c Does that same observer now see a bullet exiting the barrel at 120% of c? When did they make FIREarms with muzzle velocities of 2*10^7 m/s? At about the same time they started making vehicles that travel along gravitationally flat surfaces at bullet speeds....in a vacuum. Oh, and.....check your math. 20,000,000 m/s doesn't match any of the values cited. I rounded and dropped an OoM. That doesn't change the fact that for the question as stated Newtonian mechanics is a perfectly valid approximation. Actually, the question as stated is missing so much information we don't really know whether or not Newtonian mechanics are sufficient. We might make some assumptions about the parameters that would make your position valid, but we might also make assumptions that make it invalid. And if we're going to make the assumptions that make Newtonian physics sufficient, we also need to take into account some important things that are being ignored here. For instance, accelaration of the vehicle due to the firearm being disharged. Also, exactly what is being asked by the question, "What happens?" Already been covered Quoted: Situation: Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? The projectile delivers an impulse to the vehicle equal and opposite to that which is imparted to the projectile. |
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Actually, the question as stated is missing so much information we don't really know whether or not Newtonian mechanics are sufficient. We might make some assumptions about the parameters that would make your position valid, but we might also make assumptions that make it invalid. And if we're going to make the assumptions that make Newtonian physics sufficient, we also need to take into account some important things that are being ignored here. For instance, accelaration of the vehicle due to the firearm being disharged. Also, exactly what is being asked by the question, "What happens?" Already been covered Quoted:
Situation: Vehicle traveling on a Gravitationally Flat Surface. Mounted on the Vehicle is a Firearm. It is Positioned so that it fires EXACTLY in reverse of the direction of travel of the vehicle. The Vehicle travels at the EXACT same rate of speed as the Bullet will travel. The Firearms is fired (Neglecting Wind Resistance) What Happens? No, not covered. The bolded statement from the original question simply says that the vehicle is traveling at the same speed the bullet "will" travel. This gives us the velocity of the vehicle BEFORE the firearm is discharged. Not the velocity of the vehicle at the moment that the bullet exits the barrel. |
