Question

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

Answer 1

Hint
This is due to Archimedes’ principles, which states that any object, fully or partially immersed in a fluid, is pushed up by a force equal to the weight of the fluid displaced by the object. In the case of plastic, the buoyant force is greater than the weight of the plastic object.

Complete step by step answer
To understand the concept of this we need to understand what buoyant force is. The buoyant force is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. If the buoyant force exerted by the liquid on a body is greater than its weight then the object floats on the surface. These objects have a density less than that of water. Plastic is one such substance that has a very less density compared to water which is why it comes up to the surface of the water with a great force when submerged. We have to keep in mind that the volume of the object submerged into the liquid and the volume of liquid displaced by the object are always equal. It is the buoyant force that is greater than the weight of the object and this is due to having a density lower than that of the liquid.
Hence a plastic block floats on the surface of the water because the buoyant force exerted by water is greater than its weight which also means its density is lesser than that of water.

Note
Few objects don’t sink or float they are in between this is because the weight of the object is almost as much as the buoyant force exerted on it. Although the weight has to be a little greater than the buoyant force to stay in between the surface and depth. There is another reason for this to happen, as we go deeper into a liquid the buoyant force keeps increasing. So, for objects that don’t float or sink, the buoyant force becomes equal to the weight of the object at a particular depth which is why they stay there.