Does a body move when static friction is equal to applied force?

Question

Just a quick question,

When a force on an object is equal to its static coefficient * normal force , would the object move or would it stay stationary? Or does the force always need to be greater than this value for the object to move?

My guess is that it need to be greater but im not completely certain.

Answer 1

The answer to this question depends on whether you're working "in real life" or on a physics problem.

On a physics problem, the object doesn't start to move. The net force is exactly 0 at this time, so there is no acceleration.

In reality, however, the object would possibly move. There's a variety of reasons for this:

• The normal force, static coefficient, or applied force may be slightly different than estimated. Measurements are never exact (they are at minimum restricted to the precision of the device being used), and there's always some range of error present in any measurement, so if the true normal force was lower than thought, the object would begin to move.
• If the coefficient of kinetic friction is less than that of static friction, a very, very small push would suffice to encourage the object to begin to move, after which the provided force would suffice to continue motion. (Note: I'm not sure the Coulomb friction model - the one you've likely learned in your physics class - suffices for such slow motion. Atomic effects may be significant here.)