If the net external torque acting on the particle is zero, the it satisfies - 1st condition of equilibrium - 2nd condition of equilibrium - Both 1st & 2nd condition of equilibrium - 2nd law of motion

a) the distances are zero, Both 1st & 2nd condition

c)  the torques are equal but of the opposite sign, 2nd condition of equilibrium

Explanation:

The equilibrium conditions are

1 translational

        ∑ F = 0

2 rotational

        ∑ τ = Σ (F_i x r_i) = 0

They tell us that external torque is zero.

Therefore we have two various possibilities

a) the distances are zero, in this case we have a pure translation movement

    for this situation the two equilibrium relations are fulfilled

b) the forces are zero, there is no movement

    It does not make sense to use the equilibrium relations since there are no forces

c) the torques are equal but of the opposite sign, the forces are on the opposite side of the body.

    In this case the 2 equilibrium relation is fulfilled, but not the first one that the force has the same direction

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.