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why does a block of plastic released under water comes up to the surface of water
There are 2 forces which act on an object that is in a liquid. The buoyant force which is a net unbalanced upward force, which results from higher pressure as one goes deeper into the liquid. The other force is offcourse the force of gravity, which is a downward force.
  If the force of gravity is larger than the buoyant force the object will sink to the surface of the liquid, and if the buoyant force is greater than the gravitational force, the object will float on the liquid. For a plastic, the buoyant force is greater than the weight  (gravitational downward force), and so it floats.


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