Its density is greater than that of water.
Explanation
The phenomenon of sinking or floating is governed by Archimedes' Principle and the concept of Relative Density. Whether an object sinks or floats depends on the relationship between the density of the object and the density of the fluid it is placed in, which determines the balance between the object's weight and the buoyant force (upthrust) acting upon it.
Detailed Analysis
- Its weight is less than the upthrust. is Incorrect: If the weight of an object is less than the upthrust (buoyant force) exerted by the liquid, the object will float on the surface. Sinking occurs only when the weight of the object exceeds the upthrust.
- Its density is less than that of water. is Incorrect: An object with a density less than that of water will displace a weight of water equal to its own weight while only partially submerged. Consequently, it will float.
- Its density is greater than that of water. is Correct: An iron nail sinks because the density of iron is greater than the density of water. This means that the weight of the iron nail is greater than the weight of the water it displaces (the upthrust). As a result, the net force acting on the nail is downward, causing it to sink.
- It has a pointed tip. is Incorrect: While shape can influence how an object interacts with the surface tension of water (e.g., a carefully placed needle) or how a ship floats (due to being hollow), a solid iron nail sinks primarily due to its material density relative to water, regardless of its pointed tip.
Key Takeaway
Law of Floatation: An object will sink in a fluid if its density is greater than the density of the fluid. Conversely, it will float if its density is less than that of the fluid.