Question

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

Answer 1

The density of an object:

The density of an object is expressed as the ratio of the mass of the object to its volume. The higher the density greater is the pull on the object towards the earth or the object is under higher gravitational force.

1. When any object is immersed in a water body, it may float on the surface of water or sink to the bottom of the water reservoir. This effect depends on the density of the object immersed. An object in water is under influence of two forces, which are downward gravitational force and upward buoyant force.
2. Heavier objects are pulled to the earth with more force hence gravitational force dominates over the buoyant force and the object sinks to the bottom of the water reservoir.
3. On another hand for lighter objects buoyant force dominates over the gravitational pull, hence they float on the surface of the water.

Explanation

1. When a block of plastic is released underwater, two forces namely upward buoyant force and downward gravitational forces act on it.
2. As the density of plastic is lesser than that of water, buoyant force dominates over the gravitational force and pushes the block to the surface of the water. Hence the block of plastic comes up to the surface of the water