Pushing force vs. Friction force

[RobinSky]

My teacher in mechanics says that when we have a force pushing an object (moving at constant velocity) forward we also have a friction force that equals to the force that is pushing the object. So if we call the friction force for Fr and the pushing force for F, F=Fr.

But in my mind, when we have 2 forces in opposite directions the object is standing still. So if my teacher is correct, how am I thinking wrong? Or can any give some tips to solve this fault of thinking.

Regards, Robin Andersson

 

[Doc Al]

But in my mind, when we have 2 forces in opposite directions the object is standing still.

Your teacher is correct. Your error is in thinking that a net force of zero implies zero velocity. What it does imply is zero acceleration. If the net force is zero, then the velocity will be constant, not necessarily zero.

 

[Naty1]

In other words, once the pushing force exceeds the force of friction, the object accelerates from rest until the friction force and pushing force are equal...from then on constant velocity ensues.