Physics, Magnitude

Question

A 4.1 kg block is held at rest against a vertical wall by a horizontal force of 139 N.

(a) What is the magnitude of the frictional force exerted by the wall on the block?

? N

(b) What is the magnitude of the minimum horizontal force needed to prevent the block from falling if the coefficient of friction between the wall and the block is µ_s = 0.40?

? N

Answer 1

a) If the block is at rest that means the vertical forces cancel. These forces are the frictional force F_friction, pointing up, and the gravitational force mg, pointing down. Thus F_friction = mg.

b) The frictional force is determined by the normal force F_friction = µN. Since the block is at rest N must equal the pushing force F_push and F_firction must equal mg ⇒ µF_push = mg ⇒ F_push = mg/μ.