Why do the scientists that work on the International Space Station experience weightlessness?
Weightlessness is experienced by anyone or anything that breaks free of the gravity of Earth (or any other celestial body) and is exposed to the vacuum of space. The International Space Station is large, but nowhere near large enough to produce its own gravitational force. I'm afraid that artificial gravity is something only seen in the movies. The only way that we currently have to produce a feeling of gravity in a spacecraft artificially is to have a large, circular section of a spacecraft that rotates at a constant speed on its axis. This does not produce an actual gravitational force, but the centrifugal force produced by the rotation simulates gravity if the rotation is at the correct speed. Keep in mind that we have never actually built such a spacecraft, but it is theoretically possible with current technology. To see an excellent example of this, see the movie 2001: A Space Odyssey.