Question

A 1000 Kg aeroplane moves in straight flight with a constant velocity. The force of air friction is 1800 N. The net force on the plane is

Answer 1

is 1800 N.
because acceleration is zero. 
f=ma
net force =ma - 1800
                =0-1800
               = -1800N

Answer 2

2800answer is by me for this question