Impending Friction vs nonimpending?

[CProgWiz]

Hi all,

How do you know if friction is impending in a system?

I'm working on some problems, and in textbooks it tells you when you're dealing with equilibrium equations, sometimes you can consider Frictional force = (Us) * Force normal...

Next it says you can only consider that if there is impending motion. Then it says with no apparent impending motion, you can't use this... But it doesn't specify how you would know whether or not it is impending? Many of the problems seem very similar..

Thanks

 

[grzz]

One (obvious) case is:

A block at rest on a horizontal surface has no tendency to move.
But it WILL have a tendency to slide if the same block is on an inclined plane.

 

[middling]

I think "impending" would mean that something is about to slip.
As soon as it starts to slip, the coefficient of static friction
needs to be replaced by the (smaller) coefficient of moving friction.

As you tilt the block ,as described by grzz, the resisting frictional
force increases to exactly counter the resolved component of
gravitational force along the block. And, of course, the normal (at
right angle) reaction of the slope against the block also lessens -
with an effect on the realisable frictional force.