Why is the reaction force inclined on a smooth surface but perpendicular at corners?

[Like Tony Stark]

Homework Statement

Find the reaction force acting on the rod

Relevant Equations

$F=ma$
$M=I\alpha$

Hi, I have a doubt about reaction forces... I've attached a picture that shows two similar situations. The first one shows a rod left with an angle $\beta$ while it is on a smooth surface. The second one shows a rod leaning on a smooth surface and wall.

My question is: why is the reaction force "inclined" in the first case but perpendicular to the wall and surface in the second one?

 

[TSny]

Do you have the complete problem statement for the rod at angle $\beta$?

 

[Like Tony Stark]

Do you have the complete problem statement for the rod at angle $\beta$?

It says "A rod is released from rest at an angle $\beta$ on a rough surface so that there's no slip. Determine angular acceleration, reaction force, friction force and the minimun coefficient of friction so that there's no slip"

 

[haruspex]

It says "A rod is released from rest at an angle $\beta$ on a rough surface so that there's no slip. Determine angular acceleration, reaction force, friction force and the minimun coefficient of friction so that there's no slip"

Originally you wrote it was a smooth surface, no?
In both cases there is, potentially, a normal force and a frictional force. You can choose to represent these components separately or as a single reaction force.

In the left hand picture, it shows the net force as being along the rod. I can't see why it should be. Maybe the task is to find at what angle that would be true.

In the right hand picture, there is no friction so you only have the normal forces.