1-why does not a ship go down into the sea? 2-why is not a bubble like a cube?

Answer 1

Because the density of the ship is less than that of the water (there is air in the ship, and it's possibly made of wood).

A bubble is not like a cube because areas of varying pressure would cause the corners to even out, turning the cube into sphere.

Answer 2

1. Because its buoyancy (B) is equal to its weight (W) when it floats. In which case W - B = ma = 0; where m is the mass of the ship and a = 0 is its acceleration (up/down). Since acceleration is zero when the buoyancy is equal to the weight, its sinking stops...it floats. When W > B, it sinks until W = B; when B > W it rises (like when a submarine surfaces) until W = B.

2. Minimum energy. The energy of the surface tension in a bubble is less than that in a cube. Nature is lazy and will always seek the lowest energy level when it can. When it comes to bubbles, the sphere is that lowest energy level. All other shapes require more energy than the sphere.