In Case 1, is the magnitude of the force exerted on block A by the platform greater than, less than, or equal to the magnitude of the force exerted on the platform by block A.
a. mag. of force on A by platform > mag. of force on platform by A
b. mag. of force on A by platform < mag. of force on platform by A
c. mag. of force on A by platform = mag. of force on platform by A

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.

365.25 earth days are in the year of a light year