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Definition of Weightlessness:
Weightlessness is the sensation that is experienced when the only force acting upon an object is the force of gravity.
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Many students enter Physics class with the incorrect conception that orbiting astronauts feel weightless because there is no gravity in space. But don't be fooled! If there were no gravitational force acting upon the astronaut, then the astronaut could never orbit the Earth. Gravity is not absent on the astronaut. Rather, it is the ONLY force that is present on the astronaut. Feeling weightless has to do with being in a situation where all other forces besides one's weight have been removed. See the Think About It section for more details.
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If a person hits you in the nose, you feel it. An applied force is a force that results from physical contact. The force of gravity is quite different; it is a non-contact force which results from Earth's gravitational attraction for your body. It results whether you're physically touching the Earth or not touching the Earth. If you were on a free fall ride at an amusement park, the force of gravity would act on your body even during the 3 seconds of falling from 150 feet up. In fact, the force of gravity would be the only force on your body during those 3 seconds of free fall.
Being a non-contact force, there is no way to directly feel the force of gravity (a.k.a., your weight). What you can feel are the various contact forces exerted on your body to counteract or support your weight. The most common supporting force is the normal force. As you sit in a chair, lay in a bed or stand on the floor, you experience a normal force - the support force from the chair, bed, and floor. A 600-Newton person has a good sense of what 600-Newtons of weight feels like because he/she experiences a 600-Newton normal force all day long. But if the same person were orbiting Earth in a space shuttle then the free fall environment of orbit would result in the absence of any support forces. The 600-N person would no longer feel his/her weight and might describe him/herself as feeling weightless. The weightless sensation does not result because they have lost weight but rather because there is no longer any other force that is capable of supporting their weight.
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