[omid (OP)]

Since its half term (omid's favourite time) and omid is off from BORING college so omid is more into scientifc research these days.

recently omid find out that ball point pens cannot be used in spaces.
and Astronauts use a special pen to use on the space known as Zero Gravity Pen.

so why cant we use ball pen in space?

[norcalclimber]

Without gravity the ink can't "fall" towards the ball.  Try to write with a ballpoint pen upside down and you will see the same effect.

[flr]

Without gravity the ink can't "fall" towards the ball.  Try to write with a ballpoint pen upside down and you will see the same effect.

 Actually the gravity acceleration g in close orbit around Earth (at about 200-300km from Earth surface)is about 2/3 from gravity acceleration g₀ on Earth surface(one can do easily the exact calculation). The "weightless" sensation is close orbit is due to compensation between the centrifuge force and gravity force. The gravity in close orbit is not small, but actually quite comparable with that at Earth surface.

 See the link:
 http://education.jlab.org/qa/gravity_01.html

[neilep]

ewe can still use it to scratch your back !

[norcalclimber]

Without gravity the ink can't "fall" towards the ball.  Try to write with a ballpoint pen upside down and you will see the same effect.

 Actually the gravity acceleration g in close orbit around Earth (at about 200-300km from Earth surface)is about 2/3 from gravity acceleration g₀ on Earth surface(one can do easily the exact calculation). The "weightless" sensation is close orbit is due to compensation between the centrifuge force and gravity force. The gravity in close orbit is not small, but actually quite comparable with that at Earth surface.

 See the link:
 http://education.jlab.org/qa/gravity_01.html

I guess I could have been more clear...the observed gravity in orbit is not enough to allow the ink to fall onto the ball.

[flr]

ewe can still use it to scratch your back !

Not sure about that either. One may have in space a different sense of Newton third law than on Earth.

[flr]

Without gravity the ink can't "fall" towards the ball.  Try to write with a ballpoint pen upside down and you will see the same effect.

 Actually the gravity acceleration g in close orbit around Earth (at about 200-300km from Earth surface)is about 2/3 from gravity acceleration g₀ on Earth surface(one can do easily the exact calculation). The "weightless" sensation is close orbit is due to compensation between the centrifuge force and gravity force. The gravity in close orbit is not small, but actually quite comparable with that at Earth surface.

 See the link:
 http://education.jlab.org/qa/gravity_01.html

I guess I could have been more clear...the observed gravity in orbit is not enough to allow the ink to fall onto the ball.

I don't believe 11-20% less gravity alone will not allow the ink to fall onto the ball. It may have to do with the fact that objects are completely "weightless". I mean: centrifuge force if pushing the ink away from the ball with the same force gravity is pushing it into the ball, therefore there will be no net force pushing the ink into the ball.   

[norcalclimber]

Exactly, that's why I said "observable" gravity, it might be there, but objects think they are weightless regardless, therefore gravity is essentially not working on them.

[Geezer]

Without gravity the ink can't "fall" towards the ball.  Try to write with a ballpoint pen upside down and you will see the same effect.

 Actually the gravity acceleration g in close orbit around Earth (at about 200-300km from Earth surface)is about 2/3 from gravity acceleration g₀ on Earth surface(one can do easily the exact calculation). The "weightless" sensation is close orbit is due to compensation between the centrifuge force and gravity force. The gravity in close orbit is not small, but actually quite comparable with that at Earth surface.

 See the link:
 http://education.jlab.org/qa/gravity_01.html

Actually, for a body rotating with the Earth, the only force is gravity, or, if you prefer, the "centrifuge force" is equal to the force of gravity. If the "centrifuge force" was ever less than the force of gravity, we would lose contact with the Earth.

[chris]

Doesn't the ink, being water-based, have a cohesive property and hence pull itself out of the pen as you write?

[Madidus_Scientia]

That's what I would have thought, maybe gaps form in the tube though?

[omid (OP)]

OK guys

Omid guess, omid knows the answer to this one but still wana confirm with all you intelligent scientists.

"why don't Astronauts use pencil instead?" []

[flr]

....... if you prefer, the "centrifuge force" is equal to the force of gravity.

I definitively prefer it that way. Note that one does not necessarily need to go to space to feel "weigthless". See the link: http://www.incredible-adventures.com/zerog.html

If the "centrifuge force" was ever less than the force of gravity, we would lose contact with the Earth.

If the centrifuge force (due to rotation in orbit around Earth) will be less than gravity force, the object will go to lower orbit or fall toward Earth.
The centrifuge force (F_c=m*ω²*R)due to Earth rotation is less than gravity at the Earth surface. See the link: http://www.newton.dep.anl.gov/askasci/phy05/phy05087.htm

[RD]

"why don't Astronauts use pencil instead?" []

Pencil "lead" is actually graphite, which conducts electricity,
so bits of pencil "lead" floating about could short-circuit electrics in the spacecraft,
(and get in astronauts eyes).

Another rumor has it that the Apollo 11 astronauts accidentally snapped off a switch which was necessary to permit them to fire the engine to return to Earth, and that a Fisher Space Pen was used to press this button. While the incident did occur, Buzz Aldrin has stated that, in fact, he used a felt-tip pen for this.

http://en.wikipedia.org/wiki/Fisher_Space_Pen

[fontwell]

According the BBC program QI which never makes mistakes a ball point pen does work in zero g (I think it was due to surface tension type stuff) . The problem we have on earth is that they don't work at -1g i.e. upside down. And the "Russians used a pencil" thing, is an urban myth. As has been stated already, dealing with the debris of sharpening a pencil in 0g would not be much fun.

[Mazurka]

Ok if pencil's are no good how about a chinagraph (grease pencil)?