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For an earth sized planet to exist with no atmosphere would require it to be so hot, the lead bullet would melt.
Bob
I was just thinking?????????????? I had some time on my hands, and was wondering if a person stood on a planet the size of our earth (7912 miles in diameter and had the same gravitational force as our earth) , with no atmosphere (a vacuum), or Coriolis Effect (the planet is not rotating on its axis), and fired a projectile horizontal to the position they were standing; What muzzle velocity would be needed to circle this planet? And, how long it would take the projectile to orbit this planet and hit the shooter in the back side?
Answer: Muzzle Velocity = 45,952.91426 feet per second
= 31331.53245 miles per hour
= 0.004% the speed of light
Obit Time = 2855.992185 seconds
= 47 minutes 35.0165364167 seconds
These are the answer I came up with, but I might be wrong. Are there any Physicists out there that see a mistake.
Even though I walk through the valley of the Shadow of Death, I will fear no Evil, for YOU are with me; Remington 44 Mag:
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For an earth sized planet to exist with no atmosphere would require it to be so hot, the lead bullet would melt.
Gravity would pull the bullet into the ground before it made it very far.
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Orbital velocity should not change based on lack of atmosphere.
The orbital period for the earth is more like 90 minutes. The orbital velocity is closer to 17000mph
-Doc
Bob
The planets temperature would be dependant on its distance from a star (heat source), not the lack of an atmosphere. If heat was a problem, we can make the projectile from material similar to the tiles on the Space Shuttle.Originally Posted by thoricuncle
[QUOTE]Gravity would pull the bullet into the ground before it made it very far[QUOTE]
I would agree if the muzzle velocity was slower (under 45,000 feet per second). I was using g=9.81 meter/s(32.18 feet/s). With a muzzle velocity 45952.91426 feet per second I calculated the projectiles drop to parallel the planets circumference.
[QUOTE]Orbital velocity should not change based on lack of atmosphere.
The orbital period for the earth is more like 90 minutes. The orbital velocity is closer to 17000mph
[QUOTE]
I used a no atmosphere condition to remove air drag on the projectile to simpify the calculations. I agree orbital period is 90 minutes, and velocity 17,000 mph for a satellite 100-1240 miles above the planets surface. At that altitude the g-force is reduces, causing the drop of the projectile to also be reduced. At that altitude, any velocity faster than 17,000 mph(aprox) would break the gravitational hold of the planet, and the projectile would go off in space. My example was a person standing on the surface, firing horizontal to the surface (aprox 5' altitude) where the g-force is greater g=9.81m/s (32.18'/s), and more velocity would be needed to counter the increased g-force and projectile drop.
Even though I walk through the valley of the Shadow of Death, I will fear no Evil, for YOU are with me; Remington 44 Mag:
I'm still trying out the answers other important questions. What is the coefficient of friction on a hockey
puck at sea level? and Can you get down off an elephant?
NRA Life Member
Stirrer of the Pot
Bingo.
Two other questions -
1. How did you calculate that velocity? That's an awful lot of energy and that leads me to my second question....
2. Newton's 3rd law tells us the shooter will be subject to the same force/energy as the bullet. Could the shooter withstand that much force to properly send the bullet on the way or would the gun come flying back at him/her thereby bleeding off energy (to say nothing of the possibly disasterous consequences)??
(Insert random tough-guy quote here)
"See my gun?? Aren't you impressed?" - Anonymous sheepdog
Guns - the alternative to running for your life.
Stirrer of the Pot
Your bullet is travelling at approx 9 miles/sec. Assuming your shooter is of average height, gravity would pull the bullet down to the ground in about a second. I don't believe the slug is traveling anywhere near fast enough to bend with the planet's curvature. If I remember correctly from waaaaay back, the horizon (i.e. the edge of the curve, so to speak) for a 6' tall person is about 21 miles. Your bullet traveling 9 miles in the time it takes gravity to pull it down just won't cut it. And remember - your bullet's downward path is accelerating due to gravity (+3 meters/sec).
I believe it would be impossible to fire a bullet horizontally and have it circle the planet, not without some additional force like wings. You could increase the velocity but I believe at some point the bullet would just Break Free of the planet's gravity and simply speed out into space.
(Insert random tough-guy quote here)
"See my gun?? Aren't you impressed?" - Anonymous sheepdog
Guns - the alternative to running for your life.
That explains why I don't hit my target. My bullets are going into space.
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This is the part that bugged me. With out an atmosphere, how are you going to achieve the combustion of propellants to initiate the flight of your projectile. That being said, I have doubts that it would be possible to achieve that kind of velocity with simple high explosives anyway.
Its been a while since I have done any legit rocket science, but in that sort of environment you generally have to bring both a propellant and an oxidizer with you, and all of a sudden your projectile is looking pretty substantial. Soon you get to the point you need a multi-stage delivery vehicle, and if you are burning fuel carried on board while traveling, it becomes a differential equation rather than linear algebra and simple physics.
Kinda makes me want to get back into building model rockets.
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