work done is zero if displacement of point of application of force is zero but why can't we assume forces on COM and its displacement suppose i jump from ground then work done by normal is zero as point of application  displacement is zero(work is doneby internal forces) but during the pushing part  COM has upwards displacement then shouldn't work be done if apply force on COM  ??
 A: When you jump, your legs don't leave the ground instantaneously.

When you bend, you do work on your body (your muscles are applying force and your center of mass gets displaced). Your muscles do work against the gravity to slow your downward motion down.
After you bend completely, you try to jump. In the process, your muscles begin to move your body upwards. Your body is being displaced and you jump (you are gaining kinetic energy). At one point, your body has sufficient velocity and momentum that you can left your legs.
A: Normal force doesn't do work . You are thinking right that your Center of Mass has got a displacement and it's the displacement of the center of mass which counts in the equation. 
What your are not seeing is that  The Normal Force doesn't last during the interval of displacement . Infact Displacement begins after Normal Force has gone down to 0.
For eg: You hit a pencil . During the time of your hit the pencil displaces only a very small amount. And that displacement causes the work done by you. After that ( without friction ) the pencil moves with a constant velocity. 
Here the Normal Force doesn't even cause the displacement . Its your muscles. 
Consider a block on an inclined plane . You apply force horizontally to the plane . The normal force being perpendicular to the displacement doesn't do any work. But its the vertical component of normal force the cause the block to rise. Effectively the Normal Force has caused your applied force to raise the block ( your applied force doesn't raise it directly ).
Similarly the normal force here causes you muscles ti do the work .
