The buoyant force exerted by the water is greater than the weight of the block.
Explanation
The phenomenon of an object rising or sinking in a fluid is governed by Archimedes' Principle and the laws of flotation. When an object is submerged in a fluid, it experiences two primary vertical forces:
1. Gravitational Force (Weight, $W$): Acting downwards.
2. Buoyant Force (Upthrust, $F_B$): Acting upwards, equal to the weight of the fluid displaced by the object.
Detailed Analysis
The motion of the submerged block is determined by the net force acting on it ($F_{net} = F_B - W$).
- The density of plastic is greater than that of water. is Incorrect: If the density of the plastic were greater than that of water, the weight of the block would exceed the buoyant force ($W > F_B$), causing it to sink. The block rises precisely because the density of plastic is less than that of water.
- The buoyant force exerted by the water is greater than the weight of the block. is Correct: Since the plastic is less dense than water, the weight of the water displaced (which determines the buoyant force) is greater than the weight of the plastic block itself. Consequently, the upward buoyant force exceeds the downward gravitational force ($F_B > W$). This net upward force causes the block to accelerate towards the surface.
- The gravitational force does not act on objects submerged in water. is Incorrect: Gravitational force acts on all objects with mass, regardless of whether they are in a vacuum, air, or submerged in water. The object rises not because gravity is absent, but because the buoyant force overcomes it.
- The viscosity of water pushes the block upwards. is Incorrect: Viscosity is the property of a fluid that resists the relative motion of objects moving through it (fluid friction). While viscosity acts on the moving block, it opposes the motion (drag) rather than causing the upward movement.
Key Takeaway
An object released underwater will rise to the surface only if the buoyant force exerted by the fluid is greater than the weight of the object, which occurs when the object's density is lower than that of the fluid.