Question

if a body is not at rest, can the net-external force acting on it be zero?

Answer 1

According to the law of intertia Newton says that if a body is at rest then it will remain at rest and if a body is in motion then it will remain in motion until and unless any extetnal force is applied, So if body is in motion then its external force cannot be zero.

Answer 2

Yes,it can be zero if the external forces balance each other and thus allow the body to move in uniform motion