Hmm. I'm quite confused with this: Does no resultant force means no external force? i.e. if there is an external force, there is a resultant force? Like say, sand falls onto a moving conveyor belt at the rate of 5.00 kg/s. The conveyor belt is supported by frictionless rollers and moves at a constant speed of 0.750m/s under the action of a constant horizontal external force Fext supplied by the motor that drives the belt. so the friction force is the force required for the sand to move at same speed as the belt right? then can i say there is no resultant force acting on the sand? anyway i duno if this is considered as h/w qns.. sorry.