please explain me the applications of single fixed and single movable pulley and explain me the applications of gears

 

Single fixed pulley is used to lift up things. For example: pulling a bucket from well.

Single movable pulley is used when we need mechanical advantage to lift a heavy object. For example in the above diagram the force necessary to lift up the heavy object using the pulley is half that is necessary to lift without it.

Gears are used to control rotational speed that is make a system rotate fast and slow. It is also used to increase and decrease the mechanical advantage of the rotating body.

For example you have brought a motor that rotates at the rate of 2000 rpm. (means 2000 rotation per minute). Now you will use this motor in the hands of a robot which needs to turn only 8 rpm. How will you do this? You must use gears at this point to control the speed.