Explain how a bound system can have less mass than its components. Why is this not observed classically, say for a building made of bricks?

The quantity of matter in a physical body is referred to as its mass. It is also used to calculate the body's inertia or resistance to acceleration when a net force is applied. The mass of an object determines the strength of its gravitational attraction to other bodies. The kilogramme is the SI base unit of mass.

We know that the binding energy is the energy involved in a system's disassembly. Strong forces hold a bound system together. It takes a significant amount of energy to break it. Binding energy can disassemble the nucleus into neutrons and protons.

When neutrons and protons are separated and at rest, their rest mass is greater than the mass of the nucleus containing protons and neutrons. Mass defect occurs when the sum of the masses of the constituents of a nucleus exceeds the mass of the nucleus. There are no nuclear forces involved in the classical process.

Building made of rock, in which bricks are held together by cement, results in an increase in the mas of the system, and thus the mass of a building.