Whether an object floats or sinks when placed on water depends on its density compared to the density of water.

Here’s the science behind it:

• Density: Density is the amount of mass per unit volume of a substance. Water has a density of around 1000 kg/m³.
• Buoyant force: When an object is submerged in a fluid (like water), it experiences an upward force called the buoyant force. This force is equal to the weight of the fluid displaced by the object.
• Float or sink: If the object’s density is less than the density of water, the buoyant force will be greater than the object’s weight, pushing it upwards and making it float. Think of a cork or an inflatable beach ball.
• Sink: Conversely, if the object’s density is greater than the density of water, the buoyant force will be smaller than the object’s weight, and the object will be pulled downwards by gravity, causing it to sink. Think of a rock or a metal bar.

Additional factors:

• Shape and size: An object’s shape and size can also influence its buoyancy. For example, a wide, flat object (like a raft) displaces more water, generating a larger buoyant force and making it more likely to float.
• Surface tension: Water’s surface tension can also play a role, especially for small objects. This thin elastic membrane can provide temporary support for objects with densities close to that of water, causing them to “bob” on the surface for a short time.

Remember:

• The key factor determining whether an object floats or sinks is its density relative to the density of the fluid it’s submerged in.
• Buoyant force plays a crucial role in this phenomenon, pushing objects upwards with a force equal to the weight of the displaced fluid.