Question

if a body  is moving  with uniform speed in a particular direction on a perfectly smooth surface no net force acts on it .why?

Answer 1

see for change in velocity there must be net force
According to newton's 2nd law
rate of change of momentum is directly proportional to net force
so as there no change in speed or velocity there is no change in momentum so if the change in momentum is zero the force is zero

Answer 2

rate of change of momentum is zero
so net force is zero