Certain neutron stars (extremely dense stars) are believed to be rotating at about 1 rev/s. if such a star has a radius of 20 km, what must be its minimum mass so that material on its surface remains in place during the revis's rotation?

Mass,

Explanation:

It is given that,

Angular velocity of the neutron stars,

Radius of the star, r = 20 km = 20000 m

Let M is the mass of the star. The magnitude of acceleration due to gravity is balanced by the centripetal acceleration of the stars.

So, the minimum mass so that material on its surface remains in place during the revis's rotation is . Hence, this is the required solution.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.