Whether something is going to sink or float in water depends on the density of the object in water and the density of the water. Density is a measure of how much mass is packed into a certain unit of volume. In short density is mass per unit of volume.
The condition of sink and float is as follows:
- If an object is denser than water, it will sink. This means it has more mass than water in the same unit of volume. For example, metals like iron are denser than water, so they sink.
- Similarly if something is less dense than water , it will stay afloat on the water. For example, wood and plastic are less dense than water, so they float.
And that is why when you put on the life jacket, or ride on the raft, you will on top of the water because the overall density of yourself plus the raft or yourself plus the life jacket becomes less dense as compare to water.
Objects like ships and boats are designed to be less dense than water, so they can float. It’s like when you push a balloon under the water in a bathtub – the water that the balloon pushes out of the way creates an upward force that helps keep it afloat. That upward force is also known as buoyant force. The strength of the upward buoyant force is equal to the weight of the fluid displaced by the object.
This principle, which is also known as principle of floatation, was first discovered by Archimedes in 246 BC in one of the history’s most famous Eureka moment while Archimedes suddenly hit upon the idea of how to detect whether the King’s crown is made of impure gold.
Principle of floatation
“A body floating in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.”
Simple Experiment
The principle of floatation can be demonstrated using simple experiment. The experiment involve following things:
- Beaker filled with water
- Ping pong ball
- Quartz crystal ball
The respective densities of water, ping pong ball and quartz crystal ball are shown in below picture:
Drop the ping pong ball into the water and observe the outcome. Since the density of ping pong ball is significantly lower than that of water, the upward buoyant force acting on it would be much stronger than its weight. Hence, in this case, the ping pong ball will float on water surface.
Place the quartz crystal ball in the water. This time the density of quartz crystal ball is more than that of water. Consequently, the upward buoyant force is less than the weight of the quartz crystal ball which is acting in downward direction. Therefore quartz crystal ball will drown in the water.
