Question

Question 20
Why does a block of plastic released under water come up to the surface of water?

Answer 1

When plastic is immersed in water, the following two forces act on it:
$\to$ Gravitational force or its weight in vertically downward direction
$\to$ Buoyant force in vertically upward direction
As the density of plastic is less than that of water, the buoyant force is greater than its weight. This explains why the plastic block comes up to the surface of the water as soon as it is released under water.