A physical balance is an example of:

Concept: A simple machine is a basic mechanical device that changes the magnitude or direction of an applied force. A lever is a rigid bar that turns about a fixed point called the fulcrum; it works on the principle of moments, where the load and effort are balanced about that fulcrum. In a first-class lever, the fulcrum lies between the load and the effort.

Application: A physical balance is a rigid beam pivoted at a central knife-edge (the fulcrum), with two pans hung at equal distances on either side. The object being weighed acts as the load on one pan and standard weights act as the effort on the other. Because the fulcrum sits between the load and the effort, it is a first-class lever.

Lever law: At balance, the anticlockwise and clockwise moments about the fulcrum are equal:

1. Load × (distance of load from fulcrum) = Effort × (distance of effort from fulcrum).

2. With equal arm lengths, this reduces to Load = Effort, so the standard weights directly read the mass of the object.

Cross-check: It is not an inclined plane (no sloping surface), not a wedge (it does not cut or split), and not a composite machine (it is a single simple machine, not a combination). Hence a physical balance is a lever.