Bcoz the upthrust force acting on the coin is less n the gravitational force is greater.... this is the reason 4 sinking of coin in water.

Introduction:
When a coin is placed on the surface of water, it sinks rather than floating. This phenomenon is due to several factors, including the coin's density, surface tension of water, and buoyancy.

Density:
One of the main reasons why a coin sinks in water is its density. Density is defined as the mass of an object divided by its volume. If an object is denser than the liquid it is placed in, it will sink. Coins are typically made of metals such as copper or nickel, which have higher densities compared to water. Therefore, when a coin is placed on the water's surface, its density causes it to sink.

Surface Tension:
Another factor contributing to the sinking of a coin is the surface tension of water. Surface tension is the cohesive force between water molecules at the surface. It creates a "skin-like" layer on the surface of water that allows certain objects to float. However, the surface tension of water is not strong enough to support the weight of a coin, especially if it is made of metal and has a relatively larger surface area in contact with the water. As a result, the coin breaks through the surface tension and sinks.

Buoyancy:
Buoyancy is the upward force exerted on an object submerged in a fluid. It is determined by the weight of the fluid displaced by the object. In the case of a coin, since it sinks below the water's surface, the water it displaces is less than its own weight. As a result, the buoyant force acting on the coin is less than its weight, causing it to sink.

Conclusion:
In conclusion, a coin sinks when placed on the surface of water due to its density being greater than that of water, the inability of surface tension to support its weight, and the resulting buoyant force being less than its weight. Understanding these factors helps explain why certain objects sink in water while others float.