Does static friction always act on objects at rest?

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Static friction does not act on objects at rest unless an external force is applied. For instance, a person standing still does not experience friction unless they attempt to move, at which point static friction prevents slipping. If the surface is slippery, static friction may not be sufficient to keep the object in place, leading to sliding friction once movement occurs. Similarly, a book on a table experiences zero static friction when at rest, but friction increases when a force is applied to push it. Overall, static friction only comes into play when there is an attempt to move an object at rest.
Nstraw
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Will the friction act on a body at rest. Do not take in the case of inclined plane.
 
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It depends. What situation do you have in mind? Is the body being pushed?
 
Well just consider a man standing.
 
In that case, no there is no friction acting on the man. There is, however, a distinction between "static friction" and "sliding friction". If the man attempts to move forward, he moves one foot forward which causes his other foot to exert a force backward. That foot does not move backward because of the static friction which acts on it even though that foot is not moving. If the static friction is not enough, if he is on a slippery floor, the force backward will be greater than the static friction and that foot will slide backward (now subject to sliding friction which is less than the static friction) making it difficult to walk forward.
 
If the man was standing on a slippery floor than he will be just moved by throwing some object in direction opposite To him but he can't do it on less slippery surface than isn't friction is preventing him To move
 
Nstraw said:
Well just consider a man standing.
As Halls' says, there's no friction acting in this case.

Another example would be a book on a horizontal table. If it's just sitting there, the static friction is zero. But start pushing it and the static friction will increase to prevent slipping--up to the point of maximum static friction. (Once you get it moving, kinetic friction takes over.)
 

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For simple comparison, I think the same thought process can be followed as a block slides down a hill, - for block down hill, simple starting PE of mgh to final max KE 0.5mv^2 - comparing PE1 to max KE2 would result in finding the work friction did through the process. efficiency is just 100*KE2/PE1. If a mousetrap car travels along a flat surface, a starting PE of 0.5 k th^2 can be measured and maximum velocity of the car can also be measured. If energy efficiency is defined by...
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