If you push a box with $20$ N force, it pushes back on you with $20$ N force. How can you ever accelerate this box if it always pushes back with the same force you exert on it$?($a$)$ There is a very short time delay between the moment you exert the force and reaction of the box.$($b$)$ Normal force becomes smaller when you start to accelerate the box. $($c$)$ Two forces in action$-$reaction pair always act on different objects. $($d$)$ There is frictional force acting on the box, which causes it to move.