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# Astronomy/solar flares & gravity

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Question
Please excuse any scientific misunderstandings beforehand:

When I watch footage of solar flares it appears that the matter from the flares eventually drops back to the surface of the Sun. But I thought there was no gravity in space. Therefore, why/how can material drop back to the Sun? Why doesn't the material just keep flying outward?

Answer
Hello,

In actual, technical fact gravimetric forces are found throughout space, just at different distances, e.g. from planets. (For example, from any given gravitating mass like Earth it falls off as the inverse square of the distance, or 1/ d^2) What may be happening is that you may be confusing this with the zero gravity experience - say by astronauts in a spacecraft orbiting the Earth.

In addition, every astronomical object of whatever mass exerts a gravitational pull which can be translated into an acceleration of gravity at its surface. In the case of the Earth this amounts to about 9.8 m/s^2 on average (it changes in small ways with each location depending on distance from the Earth's center)

The point here is that any mass (m) on Earth's (mass M) surface is attracted to the Earth's center and this is also a measure of the weight-  which must not be confused with mass. Thus w = mg and if we set this equal to the force of gravitational attraction, F, we obtain:

mg = G M m/ R^2

Or: g = G M / R^2

In the case of the Sun the same relationship applies except the Sun's mass is much larger than the Earth's by about 330,000 times.

Thus, in the case of the Sun the value for g = 273 m/s^2 approximately, so with a much greater g -value anything going up - say the mass of a solar flare- will also come back down. Unless it is energetic enough to achieve an escape velocity - as we see with some coronal mass ejections.

As with the Earth, the farther one gets from the mass or body the lower the g-value. For example, for a spacecraft at *twice the Earth's radius*  the g value at the Earth's surface reduces to g/4. For a craft at ten times the distance to the Earth the g-value goes to g/ 100. Thus, the g-value never actually goes to zero.

Again, this is distinct from an astronaut - say in a space station - feeling weightless or experiencing "zero g". In this case, because his craft is in the "free fall" condition orbiting Earth (centripetal force of craft (mv^2/ R)  balancing the force of gravitational attraction of planet) he senses no g-force.   This happens at the velocity v for which:

mv^2/ R  =  G M m/ R^2

At this v no "resultant force" would be left over to provide weight so one has "zero g" or weightlessness.

The situation is somewhat similar to being in an elevator for which the suspension cable is suddenly cut leaving the platform in "free fall". Before this happens there is the sensation of weight as a result of the reaction force due to the floor of the elevator platform - i.e. by Newton's 3rd law for every force acting F there is an equal but opposite reaction force F' or F = - F'

So long as the cable supports the elevator platform we have w = mg - ma = m (g - a)

Where in this case, a = 0 (i.e. no acceleration relative to the person standing on it) so ma = 0 or w = mg

But once the support cable is cut the reaction ceases to exist because now the elevator floor is ALSO accelerating at a = 9.8 m/s^2

In this case:  w = m(g - a) = m(g -g) = 0

Hence, the occupant experiences "weightlessness" - at least for as long as he lives!

Hopefully, this clarifies the issues for you !

Astronomy

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#### Philip Stahl

##### Expertise

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.

##### Experience

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.

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

Publications
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'.

Education/Credentials
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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