A metal block of mass `0.5` kg is placed on a plane inclined to the horizontal at an angle of `30^(@)`. If the coefficient of friction is `0.2`, what force must be applied (i) to just prevent the block from sliding down the inclined plane (ii) to just move the block up the inclined plane and (iii) to move it up the inclined plane with an acceleration of `20cms^(-2)`?