Can an object be in equilibrium if it is in motion?
Yes! Such an equilibrium situation is called dynamic equilibrium .
A body is said to be in equilibrium if the net force acting on the body is zero. By Newton's second law, the acceleration of such objects will be zero. Zero acceleration implies a constant velocity. Constant velocity also includes zero velocity as a special case. So an object in equilibrium can either have zero velocity or a constant non-zero velocity.
Static Equilibrium : When the net force acting on an object is zero and if the object is not moving, then it is called static equilibrium.
An example of static equilibrium is a book lying on a horizontal surface and remain stationary. The two forces acting on the book, gravitational force & normal force, are equal and opposite giving a zero net force. Here the velocity is zero and remains unchanged.
Dynamic Equilibrium : When the net force acting on an object is zero and if the object is moving at a constant non-zero velocity, then it is called dynamic equilibrium.
An example of dynamic equilibrium is a book sliding straight down along an inclined plane with a constant speed. The three forces acting on the book, gravitational force, kinetic frictional force and normal force, all add to give a zero net force and the object is sliding down the incline at a constant speed.