A 50kg crate is being push on a horizontal floor at constant velocity. Given that the coefficient of kenitic friction between crate and floor is hk=0.1 . What is the push force?​

  F = 49 N

Explanation:

For this exercise we must use Newton's second law. Let's set a reference frame with the x axis parallel to the floor.

As they indicate that the box is going at constant speed, its acceleration is zero

Y axis y

       N-W = 0

       N = mg

X axis

        F-fr = 0

        F = fr

the friction force has the formula

      fr = μ N

      fr = μ mg

we substitute

            F = μ m g

let's calculate

            F = 0.1 50 9.8

            F = 49 N

radiation

explanation:

heat is transmitted by radiation

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.