In most cases the frictional force is proportional to the force pushing the surfaces together (N). This proportionality is usually written as F_friction = μN. The Greek letter u (mu) is called the coefficient of friction. It is important to remember that the two forces, F_friction and N are not in the same direction.

You probably noticed that it took more force to start the couch moving than it was to keep it moving. Physicists say that the coefficient of static friction is larger than the coefficient of kinetic (moving) friction.