Why does an object float or sink when placed on the surface of water?

An object's ability to float or sink in water depends on its density relative to the density of the water. Here's a breakdown:

1. **Density**: Density is the mass of an object divided by its volume. Water has a density of about 1 gram per cubic centimeter (g/cm³). If an object's density is less than the density of water, it will float. If its density is greater, it will sink.

2. **Buoyant Force**: When an object is placed in water, it experiences an upward force called the buoyant force. This force is equal to the weight of the water displaced by the object. For an object to float, the buoyant force must equal the object's weight.

3. **Floating**: If an object displaces a volume of water whose weight is equal to the weight of the object, the buoyant force will counteract gravity, and the object will float.

4. **Sinking**: If the buoyant force is less than the object's weight (i.e., the object displaces less water than its own weight), the object will sink.

So, whether an object floats or sinks is determined by comparing its density to that of water and the balance between the buoyant force and the object's weight.