Question

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

Answer 1

A block of plastic released under water comes up to the surface of water due to boyancy. As Soon as the the plastic is released under water, water starts exerting boyant force on block which forces it to come on the surface.
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In other words,

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 be in upward direction, and comes up to the surface of water

Answer 2

$\bf{\underline{\underline{Question:}}}$

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

$\bf{\underline{\underline{Answer:}}}$

Two forces act on an object immersed in water. One is the gravitational force, which pulls the object downwards, and the other is the buoyant force, which pushes the object upwards. If the upward buoyant force is greater than the downward gravitational force, then the object comes up to the surface of the water as soon as it is released within water. Due to this reason, a block of plastic released under water comes up to the surface of the water.