Explain the concepts of floating and sinking

Floating and Sinking

When we put some objects in water, they stay on top of the water. We say these objects float. Other objects go down into the water. We say these objects sink.

Why Do Objects Float or Sink?

The main reason an object floats or sinks is its density. Density tells us how heavy something is compared to its size. We say:

• If an object is less dense than water, it floats.
• If an object is denser than water, it sinks.

Think about a piece of wood and a stone. A piece of wood is light for its size, so it is less dense than water — it floats. A stone is heavy for its size, so it is denser than water — it sinks.

Objects That Float on Water

Objects that float have these characteristics:

• They are less dense than water
• They are often hollow or have air inside them
• They displace (push aside) enough water to support their weight

Examples of floating objects: wooden block, plastic bottle, canoe, leaf, rubber ball

Objects That Sink in Water

Objects that sink have these characteristics:

• They are denser than water
• They are usually solid with no air inside
• They cannot displace enough water to support their weight

Examples of sinking objects: stone, metal coin, glass marble, nail

Simple Activity

Imagine you have a piece of wood and a coin.

• Drop the piece of wood into water. It floats because wood is less dense than water.
• Drop the coin into water. It sinks because metal is denser than water.

This is why wooden boats can carry people and heavy goods on lakes and rivers — the boat is hollow and contains air, making its overall density less than water.

Real-life application

In Tanzania, fishermen use boats made of wood or fibreglass to fish on Lake Victoria and other lakes. These boats float because they contain air inside them, making them less dense than water. Even when the boat carries people and fish, it still floats because the total weight of the boat equals the weight of water it displaces. This is the same principle that keeps large ships floating in the ocean.