Explanation:

When an object is immersed in a fluid, it experiences an upward buoyant force due to the displaced fluid. The point at which this buoyant force acts is called the centre of buoyancy (B). The point at which the weight of the object appears to act is called the centre of gravity (G).

If the centre of gravity (G) of a submarine lies above the centre of buoyancy (B), the submarine is in an unstable equilibrium. This means that any disturbance or change in the state of the submarine will cause it to move away from its original position.

Reasons for unstable equilibrium:

The following reasons explain why a submarine is in unstable equilibrium if its centre of gravity lies above the centre of buoyancy:

1. Unstable restoring moment: When a submarine is tilted or disturbed, the buoyant force acting on it will shift from one side to the other. This shift will create a moment that tends to restore the submarine to its original position. However, in the case of an unstable equilibrium, the restoring moment is negative, which means that it tends to increase the tilt or disturbance rather than reducing it.

2. High metacentric height: The metacentric height is the distance between the centre of gravity and the metacentre. The metacentre is the point at which the buoyancy force acts when the submarine is tilted. A high metacentric height means that the distance between the centre of gravity and the metacentre is large. This makes the submarine more unstable as it tends to roll more when disturbed.

Conclusion:

In conclusion, if the centre of gravity (G) of a submarine lies above the centre of buoyancy (B), the submarine is in an unstable equilibrium. This means that any disturbance or change in the state of the submarine will cause it to move away from its original position.

B