A 1.39 × 104 kg railroad car moving at 6.75
m/s to the north collides with and sticks to
another railroad car of the same mass that is
moving in the same direction at 1.41 m/s.
What is the velocity of the joined cars after
the collision?
Answer in units of m/s.

this is poiselle's equation from fluid mechanic in physics. mech eng

where is the photo and the answer choices?

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.

conservation of mass

mass before something happens = mass afterwards