artg 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 Ff, 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, and x1. View Available Hint(s) Ff = nothing Part B After the box comes to rest at position x1, a person starts pushing the box, giving it a speed v1.