Big Bang and the conservation of Mass

Harilal asked:
“During the big bang it is supposed that the small particles expanded in a very very small interval of time and there was nothing before that. If it is so, then how can it in be good agreement with law of conservation of mass?”
Ans:
After the Big Bang, the universe expanded rapidly. The big bang theory is not a theory which is completely resolved. There are many questions to be answered.
Further, mass alone is not conserved, it is mass+energy  which is conserved.
Energy can be converted into matter and vice versa. E=mc2
Dark Energy and Dark Matter are two widely discussed concepts in association with the Big Bang. These correspond to the mass (matter) and energy which we cannot account for by calculations and theory.
Further, BIGBANG is not the only possible cause of the Birth of Universe. There are many other possibilities.
Big Bang theory originated after the discovery that the universe is expanding. SO if we think in reverse direction, we can imagine that the entire universe was confined to an infinitesimally small (Compared to the present size of the universe) space. This idea started the theoritization of the Big Bang.
Please visit the following links for further reference:
http://www.aip.org/history/cosmology/ideas/bigbang.htm
http://www.aip.org/history/cosmology/ideas/journey.htm
http://en.wikipedia.org/wiki/Big_Bang
http://www.aip.org/history/cosmology/ideas/expanding.htm
 
(Comments and better answers are expected from visitors and experts visiting the site)














Gauss Theorem - Electrostatics

DIVYA asked:
“WHAT IS THE  MAGNITUDE OF ELECTRIC  FIELD IN THE  GAUSSIAN SURFACE OF  A CUBE,AT ITS FACE ,AT ITS CENTRE, AT ITS CORNERS OR AT ANY OTHER POINT INSIDE THE CUBE. ALSO TELL ME THE THE WAY TO KNOW IT.”
Ans:
According to Gauss Theorem, the total electric flux through a closed surface,\oint_S \mathbf{E} \cdot \mathrm{d}\mathbf{A} = \frac{Q}{\varepsilon_0},
The Gaussian surface is an imaginary surface. So, for calculating the electric field at a point using Gauss theorem, we have to imagine a Gaussian surface symmetric with the given charge distribution.
I have assumed that there is a point charge Q at the centre of the cube.
At its face (at a point on the face on the line radially outwards from the point charge at the centre of the cube and perpendicular to the face)
The distance is equal to half the length of side of the cube (Say L).
Therefore the electric field,  

 
On the corner,
Calculate the distance from the centre of the cube to its corner and replace (L/2) in the above equation with that distance.










Time Period of a Simple Pendulum in Space

Mary Beth asked:
“if you set up a pendulum aboard an orbiting space vehicle, would the period be less than, the same as, or greater than it would be in our lab? explain.”
Ans:
In a space vehicle orbiting around the earth, the apparent weight is zero as the entire gravitational pull is being utilized in providing the centripetal force required for revolving. This is called weightlessness in space.
i.e; in effect, the acceleration due to gravity is zero which make the time period of the satellite infinity. The pendulum will not oscillate at all.
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Tailpiece:
However, in manned space vehicles, there is an option to simulate gravity by making the system rotate at a particular rate so as to make the inhabitants feel as if there is weight and gravity.

Plus Two Physics