A 100-kg mass is pulled along a frictionless surface by a force F such that its acceleration is 6 m/s2. A 20-kg mass slides along the top of the 100-kg mass and has an acceleration of 4 m/s2.(It thus slide back relative to the 100-kg mass). What is the frictional force in newtons exerted by the 100-kg mass on the 20-kg mass?