Suppose we have an iron nail and an iron ball of same mass. When submerged in water, the iron ball may float but the nail always sinks. This is because

According to Archimedes' principle, the buoyant force acting on an object submerged in a fluid is equal to the weight of the fluid it displaces. While an iron nail and an iron ball of the same mass have the same weight, their shapes significantly influence the volume of water they can displace. A solid iron nail is compact and displaces a volume of water equal only to its small metal volume; since iron is denser than water, its weight exceeds the weight of the displaced water, causing it to sink. Conversely, an iron ball can be designed (e.g., hollowed) to occupy a much larger volume. By increasing the volume through its shape, the ball displaces a greater weight of water, resulting in a larger buoyant force that can equal or exceed its own weight, allowing it to float.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 5: Exploring Forces > Activity 5.13: Let us investigate > p. 76