To ensure that the block is in equilibrium, the block must move down the inclined plane at a constant velocity.
Explanation:
When a body starts moving, at first, the kinetic friction exceeds the applied force, and more force is applied to counter this effect. The result is that the body accelerates until the point where the applied force equals the kinetic friction. At the point where the externally applied force becomes equal to the kinetic friction, the body moves with a constant velocity.
From the last sentence, in order to be able to determine coefficient of kinetic friction, one must try to achieve equilibrium by creating a condition, in which the body moves down the inclined plane at constant velocity. At this constant velocity, the kinetic friction is known, since it is now equal to the applied force, which in this case is the normal force, which is the product of gravity and the mass of the body down the inclined plane. The coefficient of kinetic friction can then be easily calculated from this values.
