Explanation:

When a fluid flows through a sudden contraction, the flow area decreases abruptly. This results in a sudden increase in velocity of the fluid. According to the principle of conservation of mass, the mass flow rate remains constant. Therefore, the increase in velocity leads to a decrease in pressure, as per Bernoulli's equation.

1. Flow Contraction:
The flow contraction itself does not cause any loss. It is simply a change in the flow area. The sudden contraction causes the fluid to accelerate, resulting in an increase in velocity.

2. Expansion of Flow after Sudden Contraction:
After the sudden contraction, the fluid expands to fill the larger downstream area. This expansion causes a decrease in velocity. The fluid undergoes a deceleration process, and the kinetic energy is converted back into pressure. This decrease in velocity and subsequent increase in pressure is known as the expansion loss.

3. Boundary Friction:
Boundary friction refers to the frictional losses that occur due to the interaction between the fluid and the walls of the pipe or conduit. In the case of sudden contraction, the boundary friction contributes to the overall minor loss, but it is not the main reason for the loss.

4. Cavitation:
Cavitation is the formation and subsequent collapse of vapor bubbles in a flowing liquid due to a decrease in pressure. It typically occurs when the pressure drops below the vapor pressure of the fluid. While cavitation can occur in sudden contractions, it is not the primary cause of minor loss in this case.

Conclusion:
The minor loss due to sudden contraction is primarily caused by the expansion of flow after the sudden contraction. This expansion leads to a decrease in velocity and an increase in pressure, resulting in the loss. The other factors such as flow contraction, boundary friction, and cavitation may contribute to the overall minor loss, but they are not the main reasons for the loss.