Why object will not move if force on object is equal to frictional force?

Answer 1

Even though friction force is in some ways a fictitious force, the question can

still be answered in terms of Newton's plain old laws of motion:

-- If the object is not moving, then in order to move, there would have to be

some acceleration ... from zero speed to somespeed.

-- If the force you exert on the object in this --> direction is equal to the frictional

force in this <-- direction, then the two forces on the object add up to zero.

-- Since the net force on the object is zero, there's no acceleration.

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As correctly stated by the previous answer in general terms the body velocity is constant, that is the body is not still, but it moves with a constant velocity.

Two observations are relevant regarding this problem:

1 - The velocity of a body depends on the movement of the observer (in physics we say on the reference system where the motion is observed).

Let us consider for simplicity reference systems moving with constant velocity once with respect to the other (this has an important meaning in classical physics, but it is quite out of the present discussion).

Consider for example an highway parallel to a railway (a quite frequent situation).

A car moving on the highway at constant velocity seems to be still if observed from a train moving with the same velocity in the same direction, while it is moving with double velocity if observed from a train having the same velocity in the opposite direction.

Thus, if a body is moving with constant velocity in a given reference system, (for example the car observed by a person still on the highway border) it always exists another reference system where the body is still (in the example a train moving with the same velocity and direction of the car).

2 - In nature friction forces are either attrition or viscous resistance. Attrition plays a role when a body moves in contact with a still, rough surface. If the surface is horizontal, the attrition is proportional to the body weight, in any case it depends on the active force component that is perpendicular to the contact surface, while it is independent from the active force component parallel to the contact surface.

Due to this rule, and since the motion is essentially determined from the parallel component of the active force, the attrition is almost independent from velocity.

Viscous resistance, that is the force exerted by fluids like air or water to oppose the motion of bodies moving through them, is proportional to a certain power of the velocity (from one to more than two depending on the type of fluid and on the motion regime).

The two above laws renders very difficult to have a situation where a friction force is constantly equal to the active force pushing a body, this explains why the described phenomenon is rarely observed in everyday life.

Answer 2

If the force on an object is equal to the frictional force, the object will not move because the two forces are in equilibrium. The frictional force opposes the motion of the object, and when it equals the applied force, the object remains stationary. Any further increase in applied force will overcome the frictional force and set the object in motion.