When you fire a shot gun, you feel that you are pushed back by the gun. can you explain why this happens? is the recoil velocity of the gun equal to the velocity of the bullet? if not is there any other quantity that is equal for the bullet and the gun?

The recoils is caused by the gun trying to balance out the momentum of the bullet and the gas being discharged. The gas helps balance out the momentum of the bullet a little, but the recoil is still needed for the complete balance. This need for balance is proved by Newtons third law. 

The formula for finding the velocity of recoil is as follows:
(Weight of Bullet * Velocity of Bullet + Weight of Gases * Velocity of Gases) / (7000 * Weight of Gun)

Hope this helped!

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -

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