Determining the direction of friction

[OmniNewton]

Homework Statement

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.

Homework Equations

N/A

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.

 

[SteamKing]

Homework Statement

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.

Homework Equations

N/A

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.

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.

 

[OmniNewton]

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.

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 ------> ?

 

[SteamKing]

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 ------> ?

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.

 

[OmniNewton]

Alright thank you sir!