[faiza (OP)]

some one help me to answer it that why it happens

[chris]

Dear Faiza

the reason that you are 'weightless' in space is that, assuming you are in earth-orbit, relative to the earth you are in perpetual free-fall. In other words, the pull of gravity is keeping you in orbit around the planet, stopping you flying away at a tangent into space, so you follow a circular free-fall trajectory. The space craft carrying you is also in free-fall and hence relative to the space craft you float about in one place.

Chris

[Fossil]

Your weight changes according to the amount of gravity it takes to hold you where you are. Different planet, different weight.Your mass however remains constant.

[Quantumcat]

What he means be perpetual free fall is that gravity pulls you toward the planet but the planet keeps moving, and you are sucked around the planet forever ... (I think)

Don't forget that weight is defined by whatever gravity system you are in. Mesured in newtons. Mass is a completely different thing and stays constant no matter where you are, mesured in kg.

Am I dead? Am I alive? I'm both!

[neilep]

Hi Faiza....of course if you're just out on a happy floating session somewhere in space and not orbiting a body then the the gravity is virtually non existent....in fact you yourself would probably generate more gravity than is acting upon you.

'Men are the same as women...just inside out !'  

[Rokitansky]

Equation for Wieght is WEIGHT=m*g
m   mass
g   gravity acceleration

If you are in space far away from planets, force of gravity thet pulls you is mush smaller since it decline proportionaly with second degree of the distance. i.e. if you are two times as far, force will be four times weaker.

[tweener]

All these answers are correct, but I'll add a little more.

To elaborate on what Chris said, if you are in orbit you are falling, and because of this feel no weight.  Actually, anywhere you are, if you are falling freely, you will feel no weight.  So, if you were in an elevator and the cable broke, you would be completly weightless....until the elevator got the bottom of the shaft!

As Quantumcat (and Chris) said, in orbit, you are falling toward the earth, but because you are moving, you miss the earth.  It is just like swinging something on the end of a string.  The string is constantly pulling the object toward your hand, but because it it moving, it misses and goes around your hand.

And, as Rokitansky said, the acceleration due to gravity falls off inversly with the square of the distance from the center of the body (like the earth or sun).  So, the farther away you are the lowere the acceleration.  And, as Neil said, if you are way out in space far from everything, there would be very little gravitational acceleration.

I hope this helps - it's really just a summary of the other answers.

----
John - The Eternal Pessimist.

[gsmollin]

And to elaborate on what Tweener said, if you were in an elevator in space, far from earth, or sun, or any massive body, you could float freely, being weightless. Now if a rocket began pulling the cable of the elevator, so you were accelerating at 9.8 m/s2 (32.2 ft/s2), you would fall to the floor of the elevator, and feel like you were in a gravitational field on the surface of the earth. For all purposes, you would be in a gravitational field, and could not tell the difference between the acceleration due to the rocket, or due to the gravity on the earth. So the difference between having weight, or being weightless comes down to the difference between being accelerated or not. On the surface of the earth the acceleration is f=mg, and in the rocket it is f=ma.