Question

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

Answer 1

Answer:

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The buoyant force acting on the block of plastic is more than its weight. As a result of this, it comes up when released under water. The cause of this larger buoyant force on the block of plastic is due to its density being smaller than that of water.

Answer 2

Answer:

The upward buoyant force that is exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.

When a plastic material is released to the water then the  volume of water displaced by the plastic material  is greater than its weight. So, buoyancy force is more than gravity for materials with density smaller than water which holds for plastic and hence it comes up to the surface of water.

Explanation: