Why astronauts feel weightlessness in space? A free falling body will have accelaration of g downward but I read that accelerometer will read zero in free falling objects, is that true?

How ancient people were aware of planets even before telescope were discovered. Did the people came to know that saturn have rings only after telescope discovery.



Weightlessness, the sensation of it, is a result of the absence of a reaction force to one's weight. If one is standing someplace on the Earth's surface, he will experience a weight W = mg where g is the acceleration of gravity (about 9.8 meters per second per second).

The existence of this weight, however, depends upon the equal and opposite normal force exerted in the opposite direction (from ground up). This is a consequence of Newton's 3rd law of motion, generally stated as "for every action there is an equal but opposite reaction".

If you take the example of an elevator, if you are standing in it normally - say before it descends- you will again experience weight W = mg. If say the downward acceleration of the elevator is a =  5 m/s/s then the *net acceleration* you experience is: (g - a)= (9.8 - 5) m/s/s/=  4.8 meters per second per second. (Note: If you are accelerating *upward* in an elevator, the g forces increase. So the effective weight experienced is W' = (g + a) where a is the upward acceleration of the elevator).

But, if the cable supporting the elevator snaps and you are in genuine "free fall" then we have a = g so your effective weight becomes:

W' =  m(g - a) = m (g - g) = 0

so you will be weightless in that situation,

Again, if the elevator were *stationary only*  - no accelerating motion up or down you'd have:

W = m(g - 0) =  mg

The weight W experienced is fully the result of the normal reaction force upwards on you.

The analogous situation to weightlessness applies in space, where that reaction force becomes absent so the normal force (upwards) is 0. One then would have to compensate by creating gravity artificially, say by allowing the space craft - say space station - to rotate so that the spinning force (centripetal force) provides the reaction force.


Ancient peoples, i.e. Sumerians, became aware of the planets by observing the objects in the sky consistently and noting that some moved more than others and over time their motions repeated themselves. These objects were called "wanderers" or planets. In addition, they didn't display twinkling (as a rule).

Re: the rings of Saturn, these couldn't be observed until Galileo constructed his first telescope which we estimate magnified sky objects about 30 times.  


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Philip Stahl


I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.


Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

Past/Present Clients
Caribbean Examinations Council, Barbados Astronomical Society, Trinidad & Tobago Astronomical Society.

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