Which of these situations shown are in stable equilibrium?
Stable equilibrium is the position where the particle has lowest potential energy.
If you displace the particle from this position then it will have a tendency to come back to the original position as a restoring force would have started acting on it.
If I displace the ball slightly to left or right it will have a tendency to go towards its equilibrium position.
Also the ball has minimum potential energy there.
Stable equilibrium
The ball has highest potential energy at this position. If displaced slightly will fall away from the starting point.
So Unstable Equilibrium
Since spring in natural length so potential energy is 0.
Also if the block is displaced little to the left, the left spring will get compressed and push to the right, towards the starting position and at the same time the right spring will get elongated and pull towards the right i.e., towards the starting point.
So Stable equilibrium
The wood half submerged and half floating. Its weight is equal to the buoyant force water applies on it.
If I push it little more in water the buoyant force will increase and try to push it up towards the starting position. If I try to pull it out a little it has a tendency to go back down to the initial position because of its own weight.
Stable Equilibrium