Answer: The knob of a door always has a larger radius than the shaft so that the distance over which the force is applied is greater in order to reduce the effort force needed to open the door.
Step-by-step explanation:
A machine makes our work easier by reducing our effort. A door knob is designed in such a way that it reduces the effort force to open the door. A door knob is a simple machine with two parts similar to wheel and axle. The force required to turn the shaft would be greater as compared to turn the knob. The distance over which the force applied increases which reduces the required effort force.