How can a body moves if the net force is zero. And if the net force is not zero how can the body is said to be dynamic equilibrium.

FOR 1st PART
Consider a block of matter on a surface at rest. The surface is such that the friction between the surface and the block is zero. The block is in a surrounding of absolute vacuum.

1. I apply a force F on the block for say t seconds.
2. In t seconds the block will accelerate from zero to say a.
3. After t seconds say the block acquires a velocity v.

Now, after t seconds there is no force acting on the block (by point 1) but the block will continue to move with a velocity v as there is no friction i.e. no opposing force acting against its state of motion.

In a nutshell I can generalize that force is a prerequisite to set a body in motion but not to keep it in its state of motion.
FOR 2nd PART
 If a body is initially moving with some velocity and it continues its motion rectilinearly with the same velocity, or if the body is rotating with some initial angular velocity and the angular velocity remains constant, then the body is said to be in dynamic equilibrium.
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