What is angular momentum?

What is Bohr's postulated of angular momentum?

An electron is revolving in the third orbit of a hydrogen atom. What is its angular momentum? [h = 6.6 xx 10^-34 JS]

In a merry-go-round a boy of mass 20 kg is revolving along a circular path of radius 10 m with an angular velocity of 6 rpm. What is the angular momentum of the boy?

Angular momentum of a particle revolving with uniform speed is L. If the frequency of the particle is doubled and its kinetic energy is halved then its angular momentum becomes

Statement I : The magnetic moment of an electron rotating in an atom is proportional to its angular momentum . Statement II : The electrons in an atom can rotate only if those orbits for which the angular momentum of the electron is an integral multiple of (h)/(2pi) (h - Planck 's constant ).

Show that the angular momentum of a particle under the action of the force vec(F) = k vec(r ) is conserved , where K is a constant quantity.