# Friction problem on video

1. Nov 18, 2014

### gracy

in this video question no 5 actually it is first question of the video but it's number is 5,i didn't understand this question.first i thought The force acting on block A is its weight in vertically downward direction, with no friction with the wall and Condition for equillibrium: Sigma Fx= 0, Sigma Fy= 0, & Sigma Fz= 0, Hence ,a vertical force can never be balanced by a horizontal force, whatever it (applied force) is, even infinity. Only friction force b/w wall and block A can balance weight of A. But here there is no friction b/w block A and the wall. Hence , option (d) is correct.but then i thought friction force is not necessarily be horizontal it just opposes motion in whichever direction ,so why it can't balance normal force and what about friction force between A &B ,it is especially mentioned in the question coefficient of friction between block b and a so won't it be used in the solution of problem?

2. Nov 18, 2014

### Dr.D

Looks to me like d is correct.

The friction force is always parallel to the surfaces in contact, so it cannot possible oppose the normal force on that contacting surface (which by definition of "normal" is perpendicular to the surface.)