A 4kg block is pushed against a vertical wall by a horizontal force of 7N.?

Question

The block is stationary but on the point of slipping. What is the coefficient of friction between the book and the wall?

POSSIBLE ANSWERS:
(A)0.3
(B)5.6
(C)1.75
(D)0.75
(E)9.6

Answer 1

here the reaction from the wall is 7N,

if F is the frictional force between the wall and the block.
R is the reaction and Mu is the coeff of friction

then F=Mu*R..

but we know the frictional force must be equal to the weight of the block in order to just keep it from slipping..

so Mu.R=W(weight of the block)

so Mu=W/R
=(4*9.8)/7 (becos W=4*g newton)
=5.6..

Answer 2

The 7 N force must provide 4x 10 = 40 N

40/7 = 5.7 . The answer is B.

Answer 3

7μ = 4*9.80665
μ = 4*9.80665/7 = 5.6