A box of mass m is sliding along a horizontal surface.
 A. The box leaves position x=0 with speed v0. The box is slowed by a constant frictional force until it comes to rest at position x=x1. Find the magnitude of the average frictional force that acts on the box. (Since you don't know the coefficient of friction, don't include it in your answer.)Express the frictional force in terms of m, v0, x1

B. After the box comes to rest at position x1, a person starts pushing the box, giving it a speed v1. When the box reaches position x2(wherex2 > x1), how much work has the person done on the box? Assume that the box reaches x2 after the person has accelerated it from rest to speedv1. Express the work in terms of m, v0, x1, x2, v1.