A 12.0-kg box is pushed along a horizontal surface by a 24-N force as illustrated in the diagram. The frictional force (kinetic) acting on the object is 6.0 N. (a) What is the acceleration of the object? (b) Calculate the value of the normal force acting on the object. (c) If the 12.0-kg object then runs into a 4.0-kg object that increases the overall friction by 3.0 N, what is the new acceleration? (d) What force does the 4.0-kg object exert on the 12.0-kg object when the two are moving together