Whether an object or a person sinks or floats in water depends on the relationship between the density of the object and the density of the water.
Density is a measure of how much mass is packed into a given volume. It is calculated by dividing the mass of an object by its volume. If the object's density is greater than the density of water, it will sink. If the object's density is less than the density of water, it will float.
When an object is denser than water, it means that it has more mass packed into a given volume compared to water. The gravitational force acting on the object is greater than the buoyant force exerted by the water, causing it to sink. This is why heavy objects, such as a solid metal ball, sink in water.
On the other hand, when an object is less dense than water, it means that it has less mass packed into a given volume compared to water. In this case, the buoyant force exerted by the water is greater than the gravitational force acting on the object. This upward buoyant force counteracts the force of gravity, allowing the object to float on the water's surface. Materials that are less dense than water, like wood or some plastics, are examples of objects that float.
It's important to note that the shape and size of an object also play a role in determining whether it will sink or float. The density of an object determines its tendency to sink or float, but the shape and volume affect how much water the object displaces and, therefore, the magnitude of the buoyant force acting on it.