Question

how can the moment of force be zero when the force itself is not zero give one example

Answer 1

moment of force = force*perpendicula distance from the force

so if the distance is zero moment of force is zero

Answer 2

The turning effect caused by force is called Torque or Moment of Force. For this to happened, the system must have a fixed point along which it rotate and some distance between the fixed point and the part of the system where the force is being applied. And more importantly, the angle at which the force is being applied must be 90° for maximum effect.

Mathematically, it is expressed as
Torque = Force * Displacement from fixed point * sin(x)

Here x is the angle at which force is being applied.
If x becomes 0, the Torque will be 0 too. That means, you are applying force but there is no Torque or Moment of Force.

For example, if you hold a door knob and apply force on it perpendicular to the surface of the door, there will be production of torque in the door and it will open (x is 90 and sine becomes 1, producing maximum torque).
But if you held the knob and apply force parallel to the surface of the door, there won't be any torque and door won't open or move (x becomes 0 causing sine to be 0. And hence there will be 0 torque in the door)