Page 2 of 4
Re: Minor chuckle
Posted: Mon Jul 28, 2008 9:56 am
by lunchbox
what about loss of frictioon
Re: Minor chuckle
Posted: Mon Jul 28, 2008 9:57 am
by mr.72
WildBill wrote:
I am sure the recoil would feel different because you are "lighter" in terms of weight but the recoil would not likely knock you over unless you were shoulder-firing a 10 gauge shotgun or something like that.
As KBCraig pointed out it is the mass, not the weight that is the determining factor of recoil. The shooter's mass and the mass of the gun and bullet are the same on the moon as on earth.
Well of course if you fired the gun from a height equal to your center of gravity then this assumption would be correct.
But if you fired the gun as most of us do, about 2 feet above your center of gravity, then the recoil force has a mechanical advantage to tip you over. It is indeed gravity (which is why I said you are "lighter" in terms of weight, and didn't mention mass) that counteracts the force of recoil that tends to rotate your body around your center of gravity when you fire the gun.
In other words if you were space walking in outer space and fired a gun, then the recoil would cause you to spin around your center of gravity. If you are coupled to an object's surface due to gravity, then other forces including gravity and friction between your feet and the ground will prevent you from simply rotating. So the lever arm is your entire body, the fulcrum is your feet on the ground, and the force vectors are gravity pulling downwards on your center of gravity and the force of recoil perpendicular to the gravity vector. If the recoil vector is high enough to tip you back so that your center of gravity falls behind your heels then you will fall over.
I'm not going to do the math but I think it's nearly impossible for this to happen. But it would definitely feel different to shoot under those circumstances since the gravitational force is 1/6 or so what it is on earth, so comparatively speaking, the force of recoil would have 6 times as much influence over your balance as it does on earth.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 9:59 am
by lunchbox
could you even get the glove in the trigger guard
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:00 am
by Wildscar
That’s all well and good but have you ever seen Astronaut gloves. You would never get a finger in the trigger guard to get a shot off to begin with.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:03 am
by WildBill
lunchbox wrote:what about loss of frictioon
I don't understand your question.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:05 am
by WildBill
mr.72 wrote:I'm not going to do the math but I think it's nearly impossible for this to happen. But it would definitely feel different to shoot under those circumstances since the gravitational force is 1/6 or so what it is on earth, so comparatively speaking, the force of recoil would have 6 times as much influence over your balance as it does on earth.
I understand your point. I am not going to do the math either.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:05 am
by lunchbox
WildBill wrote:lunchbox wrote:what about loss of frictioon
I don't understand your question.
i dont even remember where i was going with that now
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:19 am
by Kerbouchard
The reason I said the thing about recoil knocking you back is I recall sometime many moons ago, taking some online quiz that had a list of 20 or so items and you were supposed to rank them in usefulness if you were stranded on the moon. I put the pistol very near the bottom of the list, and when they compared my list to the astronauts list, a pistol was pretty high up for the astronaut because they said it could be used for propulsion, if necessary.
Perhaps somebody remembers the quiz or knows where it is; I'm having trouble finding it again.
ETA: I found it, and the pistol isn't that high up on the list.
Here's the quiz, and below are NASA's answers:
The year is 2040. You are a member of a space crew that was to rendezvous with the mother ship on the lighted surface of the moon. You experienced mechanical difficulties and your ship was forced to land about 200 miles from the point you were to be. During re-entry and landing, much of the equipment on your ship was damaged. Your survival depends on you reaching the mother ship. You will need to survey what is left that is useable and determine the most critical undamaged items that you will take for the 200 mile trip.
Your task is to look over the list below which contains the useable, undamaged items left on your ship, and rank them in order of their importance for your crew. Remember you need to rank each item in terms of its value in allowing you to reach the mother ship. Copy the list below or print out a copy. Place the number 1 by the most important item and keep going to number 15 which will be the least important. Be ready to explain why you have given each item the rank it received. Use your knowledge of the Moon and its environment to help you make your decisions. When you are done you can check how you did against the rankings given this same list by NASA. If you are doing this activity in your classroom, compare your rankings with other groups or individuals and hear their reasons for their rankings before checking the NASA list.
___ Box of matches
___ Food concentrate
___ 50 feet of nylon rope
___ Parachute silk
___ Portable heating unit
___ Two .45 calibre pistols
___ One case dehydrated milk
___ Two 100-pound tanks of oxygen
___ Stellar map (of moon's surface)
___ Life raft
___ Magnetic compass
___ 5 gallons of water
___ Signal flares
___ First aid kit containing injection needle
___ Solar-powered FM receiver-transmitter
And here's NASA's answers
Here is the ranking that NASA scientists gave the 15 items. See if you selected the same items as the top 5 - or most important - and identified the 5 least important. Even if your ranking numbers are not the exact same ones, your knowledge of the Moon and conditions there should have helped you decide which items fell to the very top and bottom of the list.
Items below are listed in order from Number 1 (most important) to Number 15 (least important).
1- Two 100 pound tanks of oxygen. Oxygen to breathe would be the most pressing need for survival.
2 - 5 gallons of water. Water is essential to all life.
3 - Stellar map (of moon's surface) This would be your primary means of navigation.
4 - Food concentrate. A good source of food and efficient way to carry food is in concentrated form.
5 - Solar-powered FM receiver-transmitter. FM needs short range and line of sight. Needed to talk to mother ship.
6 - 50 feet of nylon rope. This will be useful in scaling the cliffs you may cross and if their are injured it would be helpful for tying them to others or objects for transport.
7 - First aid kit containing injection needle. Used for injecting vitamins through special aperture in space suit.
8 - Parachute silk. Provides protection from sun's rays.
9 - Life raft. CO2 bottle in raft may be used for propulsion.
10 - Signal flares. Distress signal can be used when mother ship is sighted.
11 - Two .45 calibre pistols. Possible means of propulsion.
12 - One case dehydrated milk. duplication of food concentrate but much bulkier to carry.
13 - Portable heating unit. Useless on dark side, not needed on lighted side.
14 - Magnetic compass. Magnetic field on moon is not polarized, compass worthless.
15 - Box of matches. No oxygen on Moon to sustain the flame, matches won't work.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:34 am
by mr.72
well as far as using a pistol for propulsion ... even if you didn't count the weight of a space suit and all of that, in a completely weightless environment, I think the recoil force of a typical 9mm pistol would propel a 200 lb man (by my rough calculations) to about 0.1 fps. Each additional round you fire might accelerate you one more tenth of a fps. So unload a whole hi-cap 9mm and you still will be barely drifting. I guess that would be helpful if you had absolutely no other way to propel yourself, like if you were just floating in space. This is not accounting for the amount of energy absorbed in the recoil spring or frictional losses in the gun etc.
On the surface of the moon, of course, walking or jumping are far and away more effective means of propulsion.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 10:43 am
by WildBill
I can't imagine a .45 Pistol or a CO2 cylinder from a life raft being used as a propulsion device. If the astronaut jumped real high and fired the .45 Pistol it might turn him end-over-end and land on his head 10 feet away.
I remember being given a similar list for a class in graduate school. It was quite an interesting exercise.
SeamusTx - A lot of interesting conversation from a cartoon strip!
Re: Minor chuckle
Posted: Mon Jul 28, 2008 11:22 am
by Keith B
Russell wrote:Holy bologna guys, 2 pages of comments about the physics of firing a gun on the moon?!
Are you serious?!
It's a slow news day.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 12:56 pm
by Venus Pax
Seamus, thanks for posting that.
We played a newly-wed type game at a church function recently, and Mars and I were drafted.
One of the questions was, "How many shoes does she have, and how many guns does
he have?"
I informed the associate pastor that the later part of the question really wasn't fair to us, as most of the guns are mine.
Re: Minor chuckle
Posted: Mon Jul 28, 2008 1:11 pm
by anygunanywhere
Venus Pax wrote:Seamus, thanks for posting that.
We played a newly-wed type game at a church function recently, and Mars and I were drafted.
One of the questions was, "How many shoes does she have, and how many guns does
he have?"
I informed the associate pastor that the later part of the question really wasn't fair to us, as most of the guns are mine.
Venus, you are a beacon in the darkness!
Anygunanywhere
Re: Minor chuckle
Posted: Mon Jul 28, 2008 1:30 pm
by LarryH
We were given that moon equipment exercise at AFROTC summer camp in 1967.