Why does a block of plastic released under water cone up to the surface of water ?

Q: Why does a block of plastic released under water cone up to the surface of water ?

Sol: Density of plastic block is less than the density of water. Thus when if floats, it is partially immersed in water such that its weight gets balanced with the buoyant force acting on it When it is put completely in water, buoyant force on it becomes more then its weight. Thus when it is released it comes up to the surface of the water due to a net upward force acting on it .