Hi .
lets 1st imagine that u r that body. u r standing on a smooth someone just touches u. u will start moving in plane in direction of applied force.since the person touches u so the force is negligible y equal to zero.
lets assume if a force acting on u then u will get accelerated or deaccelerated.then u will either moves more fastly and your spped increases with time or u will stop and start moving in negative direction.
####  but actually no force is acting:-  a)frictional force is zero.b)gravitation force is equl to normal force.  and there is no other force acting on u eg.electrostatic,magnetic,tension,nuclear etc.
only 2 forces r acting on u.
1)gravity            2)normal
both r opposite in direction and equal in magnitude.
so net force is zero.