A 50kg obect falling through the air experiences a drag force of 90N. The acceleration of gravity is #9.8m/s^2#. What is the magnitude of the net force on the mass?
The weight of the object acting downwards
The drag which acts in the opposite direction
So the net force
At the point where the weight equals the drag, there is no net force acting on the object. Since Force = mass x acceleration the acceleration is zero and the object moves at constant velocity.
This is known as "terminal velocity".