What will happen to an object at rest if no unbalanced forces act on it,
according to newton's first law?

Explanation:

Newton's first law says that an object at rest stays at rest, and an object in motion stays in motion, until acted upon by an unbalanced force.

If there's no unbalanced force, then an object at rest will stay at rest.

force tends to be low and long diustance masses are high

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.