6.A 3.0-kg mass is projected down a rough, circular track (radius = 2.0 m) as shown. The speed of the mass at point A is 4.0 m/s, and at point B, it is 7.0 m/s. What is the change in mechanical energy
done on the system between A and B by the force of friction?
(B) -9.3 J
(C) -8.1 J
(D) -7.2 J
(E) -6.6 J