Question

An empty plastic bottle floats, while a plastic bottle filled with sand sinks in water. Why?

Answer 1

Hint: To answer this, think about the Archimedes principle, it states that when a object is partially or completely immersed in a fluid, it experiences an apparent loss in weight (due to upthrust) that is equal to the weight of the fluid displaced by the immersed part of the object.

Complete step by step solution
An empty plastic bottle has air inside it which means that it reduces its density due to which it is able to float over water. But in the case of a plastic bottle filled with sand inside it, it has a higher density because of the sand present inside it and also due to some of the air present in it, that is why it sinks. A ship that is launched sinks into the ocean until the weight of the water it displaces is just equal to its own weight which is the one of the examples of Archimedes principle.
Hence we can say that the gravitational pull on the empty plastic bottle is less than the upthrust force exerted on it by the water. In other words, its density is less than the density of water due to which reason it floats. Whereas on the other hand, a plastic bottle filled with sand sinks as the gravitational pull or weight on it is more than the upthrust force of water.

Note: Even the plastic bottle filled with water also sinks as it is unable to displace that much volume of water. Also, we can notice that it is easier for an aluminium foil to float in water for the same reason a ship floats on the water as the air that is inside a ship is much less dense than water.