Question

Why does a block of plastic released underwater come up to the surface of water

Answer 1

Answer:

The density of plastic is less than that of water, so the force of buoyancy on plastic block will be greater than the weight of plastic block displaced. Hence, the acceleration of plastic block will he in upward direction, and comes up to the surface of water.

Answer 2

under water come up to the surface of water?

Answer:

For an object immersed in water two forces act on it:

Gravitational force which tends to pull object in downward direction

Buoyant force that pushes the object in upward direction.

Here, in this case buoyant force is greater than the gravitational pull on the plastic block. This is the reason the plastic block comes up to the surface of the water as soon as it is released under water.