Solution:
Mass of the plane = m = 1.8 e5 kg
Initial speed = vi = 20 m/s
Final speed = vf = 0 (Plane Stops)
Length of the runway = 120 m
Acceleration of the plane = a = (vf2 -
vi2 ) / 2d
                                            Â
= (02 - 202) /(2*120)
                                            Â
= -1.67 m/s/s
Acceleration is negative since the plane stops finally.
Average force = ma = (1.8 e5)(-1.67)
                                Â
= -3.0 x 105 N
