# Homework Help: Determining the direction of friction

1. Nov 30, 2015

### OmniNewton

1. The problem statement, all variables and given/known data
Given the following diagram:

When examining the FBD for member AB how does one determine the direction of friction at B? If surfaces B and C exhibit friction.

2. Relevant equations
N/A

3. The attempt at a solution
I know that impending motion for post BC is in the direction of P so friction acts opposite but I am unsure as to how one determines the impending motion for Member AB.
The answer provided is that the friction for member AB at surface B acts horizontally in the same direction as P.

2. Nov 30, 2015

### SteamKing

Staff Emeritus
The motion of member BC produced by the force P will create a friction force at B which opposes the motion of BC. That's the force which is shown on the FBD for member AB at point B.

3. Nov 30, 2015

### OmniNewton

Would the force at of friction on member BC not be in this direction <----- so why is Member AB's force of friction in this direction ------> ?

4. Nov 30, 2015

### SteamKing

Staff Emeritus
As long as member AB is not moving, the net force between AB and BC must equal zero. When you draw a FBD of AB, a force opposite the friction force on BC must be included to bring the net force on this member to zero.

5. Nov 30, 2015

### OmniNewton

Alright thank you sir!