Can a body have displacement with net force acting on it is zero? Suppose a block is residing on a rough horizontal plane. A force F is acting on the block towards rights and friction f acts towards left such that net force is zero. But the block is moved with displacement s .Thus Force F does work F.s .is it correct?   In my book they solved a question by this method. Is their approach is right?
 A: 
A force F is acting on the block towards rights and friction f acts
  towards left such that net force is zero

You need to distinguish between static friction and kinetic (sliding) friction. Static friction prevents sliding until the force acting to the right exceeds the maximum static friction force, given by $F_{s max}=u_{s}N$ where $u_s$ is the coefficient of static friction and $N$ is the force normal to the surface, generally the weight of the block $mg$. So in this case up to the point where impending motion occurs, the net force is zero and there is no motion of the box on the surface.
When the force to the right exceeds the maximum static friction force, the box begins to move. The friction force that now opposes the motion is kinetic friction given by the same equation except the coefficient is that of kinetic friction which is generally less than the coefficient of static friction. Now, two things can happend.


*

*If the force to the right remains the same as the max static friction force it will be greater than the kinetic friction force to the left, for a net force to the right which will cause the block to accelerate.

*If the force to the right is reduced to exactly equal the kinetic friction force to the left, the net force will be zero, but the block will continue to move at constant velocity equal to that when it started moving. 
Bottom line: It is  not necessary to have a net force to have something move at constant velocity as long as some force got it started moving. 
Hope this helps.
A: 
A force F is acting on the block towards rights and friction $f$ acts
  towards left such that net force is zero. But the block is moved with
  displacement $s$.

The only way the block will start to move is when a net force acts on it, as per Newton's 2nd. Due to the acceleration displacement then occurs. In the kind of problem you describe it usually means a specific component of gravity is larger than the friction force. But there might be other forces at play too.

Thus Force F does work F.s .is it correct?

It's correct but a little incomplete.
$$W=F_{net}\times s$$
works only if $F_{net}$ is constant. But if $F$ is a function of $s$, i.e. $F_{net}(s)$, then we need find $W$ by integration:
$$\int_0^W\text{d}W=\int_0^sF_{net}(s)\text{d}s$$
A: Body can have coordinate displacement only if it moves:


*

*with acceleration $a \neq 0$. In such case $F_{net} \neq 0$

*with constant speed $v = const, F_{net} = 0$ in inertial frame reference (all bodies keeps current speed until forces change them)

