A box is pressed against a vertical wall by a force F which is directed horizontally. If the magnitude of F is the minimum required to hold the box at rest, what is its value

 F = mg /μ

Explanation:

For this exercise we define a coordinate system with the horizontal x axis and the vertical y axis, let's write the equilibrium equations for each axis

X axis

         F -N = 0

          F = N

Y axisy

         fr - W = 0

         fr = W                (2)

the force and touch have the expression

           fr = μ N

we substitute

          fr = μ F

we substitute in 2

         μ F = m g

          F = mg /μ

answer: the exterior temperature of commercial jet planes as they leave the stratosphere and return to the ground increases steadily.

explanation:

the lower most layer of atmosphere is troposphere which extends from 0 to 12 km above the surface of the earth. the temperature initially decreases as distance is increased from the earth's surface. commercial jets fly in lower part of the stratosphere. the stratosphere extends upto 50 km. the temperature in stratosphere starts rising again due to presence of ozone which traps the uv rays.

thus, when the planes leave the stratosphere and return to the ground, the exterior temperature increases steadily.