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

A block of plastic released underwater comes up to the surface because of the buoyant force exerted by the water. This can be explained using the principles of buoyancy:

1. **Buoyant Force**: When an object is submerged in a fluid (like water), it experiences an upward force called the buoyant force. This force is equal to the weight of the fluid displaced by the object.

2. **Density**: Plastic is less dense than water. Density is defined as mass per unit volume, and since the density of plastic is lower, it displaces a volume of water that weighs more than the plastic itself when submerged.

3. **Net Force**: The block experiences two forces: the gravitational force (weight) pulling it downward and the buoyant force pushing it upward. For plastic underwater, the buoyant force is greater than the gravitational force because the displaced water weighs more than the plastic block. As a result, there is a net upward force.

4. **Floating**: Because of this net upward force, the plastic block moves upward until it reaches the surface, where it will float.

This is a manifestation of Archimedes' principle, which states that the buoyant force on a submerged object is equal to the weight of the fluid that the object displaces. Since the buoyant force exceeds the weight of the plastic block, it rises to the surface.