A 1000-kg airplane moves in straight flight at constant speed. The force of air friction is 1800 N. The net force on the plane is

zero

it is said that the net force is zero. i don't know correctly.

The net force on the plane can be found by subtracting the force of air friction from the total force acting on the plane.

In this case, the force of air friction is given as 1800 N. Since the airplane is moving in straight flight at constant speed, we know that the net force on the plane must be zero. This is because there is no acceleration in the direction of motion.

Therefore, the net force on the plane is:

Net force = Total force - Force of air friction
Net force = 0 N - 1800 N
Net force = -1800 N

The negative sign indicates that the force of air friction is opposing the motion of the airplane. Hence, the net force on the plane is -1800 N.