A boy is standing on a flatcar that is moving with uniform acceleration on a level track. At the instant when flatcar is moving with velocity 10 m/s due east, the boy throws a ball with velocity 20 m/s in direction $45º$  above the east with respect to the flatcar in plane of motion of flatcar. What should be the acceleration of the flatcar so that the boy can catch the ball without moving anywhere on the flatcar? Assume acceleration due to gravity  $10m/s^2.$