firing gun on the moon

Tomcat ha says:

".... Ill ask again why is it impossible to escape from earth when going up at a steady speed of 5 kmph or so."

The reason I have been "ignoring" you is that your question is a difficult one; despite my background, I am not sure why (at least theoretically) a craft with an unlimited amount of fuel could not rise at a steady 5 km/hr until it reached a point wherethe Earth's gravity was negligible.

I would invite anyone whose physics background is stronger than mine to educate both of us.


[edit on 11-3-2005 by Off_The_Street]

hey no problem, i would feel offended too if someone would badmouth one of my favorites. you were not the only one to disagree with me on this matter. i respect the passion you showed in defending the work of heinlein.

the result of your post is that i will give heinlein another try (this time in english), this is something i possibly will benefit from. you were absolutely right that my level of knowledge in physics is very low but as 2005 is the year of physics i plan to raise that level a bit.

[edit on 11-3-2005 by feyd rautha]

Originally posted by tomcat ha
As everybody is ignoring me. Ill ask again why is it impossible to escape from earth when going up at a steady speed of 5 kmph or so.

That's an interesting one. I would have thought it was possible. Look at, for example, driving up a hill. I can drive up one his at 5kph and I can also drive up a steeper hill at 5kph - it is just the amount of power and the gear I use. I would have thought the same logic would have applied. To leave the atmosphere is the same principle? Even if we don't have the actual ability yet?

NB: I am not a physics person, I am just speculating

If one fires a gun pointing straight away from hills/mountains on the moon the bulet will leave the moon and go into space, plus there's no air (Drag) so the bulet will be traveling 10x faster than on earth.

Originally posted by SpookyVince
In a word, it is the minimum speed you need to reach to be able to "detach" from the gravity of the body you're on. On earth, to be able to break the gravity and go into space, you need to reach 11km/s to do that.

An even better answer is here.

What that site is talking about is initial velocity. I.e. getting shot out of a cannon velocity (and completely ignoring friction with the atmosphere). Thus if you were on an airless planet the same size and density of Earth, and shot a cannon shell upward at 11 km/s, you would escape.

Originally posted by tomcat ha
But what if a object is steadily going upwards at say 5 miles an hour? wouldnt it get away from earth's gravitational field after a while?

I don’t see why not. It would take a pretty long “while”, though.

Originally posted by LandOnAir
If one fires a gun pointing straight away from hills/mountains on the moon the bulet will leave the moon and go into space, plus there's no air (Drag) so the bulet will be traveling 10x faster than on earth.

How did you found out both difference in gravity and lack of pressure? Please post it.

lololol really good stuff. I wish I had the math; I really do. I especially liked the nap thingy! I would like to see you guys take on the fundies who are poluting the second law of Thermodynamics in a pathetic attempt to refute evolution. As far as I can see the bad science just ignores the second half of the second law.
skep

Originally posted by whita

Originally posted by tomcat ha
As everybody is ignoring me. Ill ask again why is it impossible to escape from earth when going up at a steady speed of 5 kmph or so.

That's an interesting one. I would have thought it was possible. Look at, for example, driving up a hill. I can drive up one his at 5kph and I can also drive up a steeper hill at 5kph - it is just the amount of power and the gear I use. I would have thought the same logic would have applied. To leave the atmosphere is the same principle? Even if we don't have the actual ability yet?

NB: I am not a physics person, I am just speculating

an interesting analogy which made me think.
i can drive uo a steep hill at 5mph in 5th gear. eventually i find a steeper hill which causes me to drop down to 4th, then steeper so down to 3rd, etc. eventually i find a hill which i can`t climb at 5mph even in 1st. so i can have all the fuel in the world, but i don`t have the power to continue at 5mph.
at this point i run out of ideas.

having thought about it. if you have unlimited power then i suppose it is possible.
i wouldn`t want to be underneath you when you did it though.

".... Ill ask again why is it impossible to escape from earth when going up at a steady speed of 5 kmph or so."

Of course it is possible, the problem is that you will need more or less amount of power at diferent height levels...the power needed to keep a body at 5Km/h(vertical ascendency) at ground level is not the same as at 10 or 20 Km high.
But for sure it will take some time to scape the gravitational field at this speed.

[edit on 5/12/05 by Krpano]

[edit on 5/12/05 by Krpano]

Hello, this is my first post so be nice to me


I think that the escape velocity is irrelevant, it is the energy required to break into orbit that is important.

A space elevator can put objects into orbit, at a very low velocity.

I seem to remember some physics from school about moving a kilo one meter up and being able to calculate the energy required to do this. Couldnt this same calculation be used to work out the energy required to lift an object into space?