What force is needed to keeps 20kg box moving at a constant velocity when the mk is 0.3021 and the force is a push at 20 degree to the ground

63.0 N

Explanation:

According to Newton's second law, the resultant of the forces acting along the horizontal direction is equal to the product of mass and acceleration:

(1)

The box is moving at constant velocity, so the acceleration is zero: a=0, and so the net force is also zero.

The net force consists of two forces:

- The horizontal component of the push, given by:

where

- The frictional force, acting in the opposite direction of the push:

where is the coefficient of friction, m=20 kg is the mass of the box and g=9.8 m/s^2.

Therefore, eq.(1) becomes

From which we find the push, F:

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.