Question

why does a block of plastic released under water come up to the surface of water?

Answer 1

The density of plastic is less than water which results in a upward buoyant force ,thus pushing the plastic block to the surface of the water.

Answer 2

a block of plastic released under water comes up to the surface of water
because of the following factors:

1. Gravitational force is acting on it
2. buyount force is acting in vertically upward direction
3. The density of the plastic is less than the density of water