When a body moves with constant velocity, the magnitude of the net force applied to keep it moving is equal to the frictional force acting on the body. This is because, according to Newton's first law of motion, an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force. In the case of a body moving with constant velocity, the net force acting on the body is zero, which means that the frictional force is equal to the force applied to keep the body moving. If the net force were greater than the frictional force, the body would accelerate, and if the net force were less than the frictional force, the body would decelerate. Therefore, in order to keep a body moving with constant velocity, the magnitude of the net force applied must be equal to the frictional force.