First slide
Concept of potential energy
Question

In which case does the potential energy decrease

Moderate
Solution

In compression or extension of a spring work is done against restoring force. 
In moving a body against gravity work is done against gravitational force of attraction.
It means in all three cases potential energy of the system increases. 
But when the bubble rises in the direction of upthrust force then system works so the potential energy of the system decreases.

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