Question

why no net force acts on a body moving with constant velocity??

Answer 1

We know that

F = ma

the above equation is central equation of Newton's first law and states that force acting on a body produces an acceleration in it. In other words, a net force causes change in velocity.

so,

F = m(dv/dt)

Thus, if a body is moving with constant velocity

dv/dt = 0

so,

the net force acting on it will be zero