A box at rest is in a state of equilibrium half way up on a ramp. The ramp has an incline of 42° . What is the force of static friction acting on the box if box has a gravitational force of 112.1 N ? 70 N
80 N
75 N
85 N

the correct answer  is 75 N

Explanation:

This is a translational balancing exercise, let's set a reference system with the x-axis in the direction of the plane and the y-axis perpendicular to the plane.

In this reference system the force to decompose is the weight

         sin 42 = Wₓ / W

        cos 42 = W_y / W

        Wₓ = W sin 42

        W_y = W cos 42

        Wₓ = 112.1 sin 42 = 75.0 N

        W_y = 112.1 cos 42 = 83.3 N

The expression for equilibrium on the x-axis is

         Wₓ - fr = 0

         fr = Wₓ

         fr = 75.0 N

the correct answer  is 75 N

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.