Weightlessness is the state of not experiencing the effects of gravity. Most people assume there is only weightlessness in a no gravity situation such as on the space shuttle. But what most people don't realize, if you are in a state of freefall you experience weightlessness. You are in a freefall skydiving, on a rollercoaster at an amusement park, or when NASA trains astronauts.
When riding a roller coaster, there are times when you experience weightlessness or think you are experiencing weightlessness. At the top of a loop it feels like you are out of your seat experiencing weightlessness. In reality, you are experiencing freefall. When you are on your descent down from the loop everything else is moving with you.
When people go skydiving, they are also experiencing a freefall. When the skydiver jumps out of the plane he is slowed by air resistance. When the upward force of the air resistance equals the jumper's weight then he experiences zero acceleration or a freefall.
The heavier the jumper the higher the magnitude of the freefall, this is because air resistance is proportional to the falling body's velocity squared.
In training astronauts for a real weightlessness environment NASA incorporates weightlessness training programs. One such program is nicknamed the "vomit comet." This is an airplane, the KC-135, which allows the astronauts to experience weightlessness for 30 second intervals. The plane flies in a series of parabolic patterns. This was first done in July of 1978 by Guion S. Bluford. Another way they experience weightlessness is in a simulator. Although they don't physically experience weightlessness, they learn to maneuver the space craft in a weightless environment.