A door knob is a simple machine that can be considered to be a type of wheel and axle system. When the door knob is rotated a force has to be applied that results in a spring loaded latch that holds the door closed retracting. If a door knob system were not used a large force would have to be applied to the latch in an arrangement that is would make it very uncomfortable.
Here, the door knob has an IMA or ideal mechanical advantage of 8.5. The diameter of the door knob is 8 cm or its radius is 4 cm. The radius of the shaft that the door knob is connected to is equal to 4/8.5 cm or approximately 0.47 cm. The required radius of the shaft that the door knob is connected to is 0.47 cm.