A 100-kg mass is pulled along a frictionless surface by a horizontal force such that its acceleration is 6 m/s² (figure). A 20-kg mass slides along the top of the 100-kg mass and has an acceleration of 4 m/s². (It thus slides backward relative to the 100-kg mass.) 

(a) What is the frictional force exerted by the 100-kg mass on the 20-kg mass? 

(b) What is the net force acting on the 100-kg mass? What is the force F? 

(c) After the 20-kg mass falls off the 100-kg mass, what is the acceleration of the 100-kg mass? (Assume that the force F does not change.)

20kg az = 4 m/s2 Frictionless surface 100kg a =6 m/s? the