A block of mass 0.1 is held against a wall applying a horizontal force of 5 N on the block . If the coefficient of friction between the block and the wall is 0.5, the magnitude of the frictional force acting on the block is :

a) 2.5 N

b) 0.98 N

c) 4.9 N

d) 0.49 N

Normal force = F = 5 N

Ff = F*Cf = 5*0.5 = 2.5N The weight of the block is 0.1*9.8 = 0.98N The block isn`t moving which means the forces on it are balanced. The weight = 0.98N is balanced by the friction force so Ff = 0.98N But that block won`t move downard until the weight plus downward force exceeds 2.5N or until the upward force exceeds 2.5 + 0.98 N =3.48N