Understanding how objects float - Archimedes principle

When you throw a piece of dry wood into a body of water like a stream, it floats. But when you throw a much smaller stone into the same water, it quickly sinks to the bottom. It has been observed that objects behave differently when immersed in a liquid like water. Even if you try to push a piece of rubber cork to the bottom of water in a basin, it quickly returns to the top and floats. It is very important to understand the principles that determine why some objects float and others sink.

A mathematician known as Archimedes developed a principle which has been used over the years to study and understand what makes some objects to sink or float in a liquid. This is called Archimedes principle. We will take

Archimedes Principle and floatation

In order to fully understand how objects float in a liquid, we will start by stating Archimedes principle. Here is what is states:

Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. source

This principle helps us understand how objects float. When an object is completely or partially immersed in a liquid, there is an upward force that acts on the object. This force that tends to push the object up towards the surface of the liquid is called upthrust. Archimedes principle helps us to understand the size of upthrust that which the object experiences. Basically, the size of upthrust or upward forces acting on the object is equal to the weight of the liquid displaced by the object.

For a body to experience bigger upthrust when placed in a liquid, it must displace an equally bigger volume of the liquid it is immersed into. This principle has been used to explain why a stone would sink to the bottom of a bucket while a cork simply floats.

In essence, if an object has an upthrust that is smaller than the weight of that object, it will simply sink. That happens because the object would have displaced a smaller volume of liquid compared to its own weight. As the weight of the object continues to displace less water, the object continues to go down until it reaches the bottom.

Objects however will float when the upthrust or volume of liquid they displace is equal to the weight of the object. Deriving from archimedes principle, there is another principle called the Principle of floatation. It tends to explain clearer why objects sink, although it is basically a more concise derivation from Archimedes principle. So here is what it states:

The floatation principle states that when an object floats in a liquid, the buoyant force acting on the object is equal to the object's weight. source

So for any object to float in liquid, the upthrust acting on the body must be equal or more than the weight of the object. Objects that float in a liquid are those that displaces their own weight in the liquid. Hence, this make it easy to understand why a large ship would float and remain on top of the water while a small stone will quickly sink to the bottom.

The upthrust experienced by the ship is equal to the weight of the ship or less than it. The ship will continue to remain at the surface of the water. However, an object like a piece of metal will quickly sink to the bottom because the upthrust it experiences is smaller than the weight of the metal.

Basically, there are two fundamental forces acting on a body immersed in a fluid. The first force is acting downwards which is the force exerted by the weight of that object on the fluid. The second force is acting upwards. It is the force (called Upthrust) exerted by the displaced fluid on the object. For the object to float, the two forces must be equal and cancel each other out. If the upthrust is bigger than the weight of the object, it will not float. The object will continue to sink until it finally comes to rest at the bottom of the liquid.

Finally

The Floatation and Archimedes principles explains why a ship usually floats. It has a large surface arear primarily designed to displace a large volume of water bigger than its entire weight. Thus it floats easily. But objects with smaller surface areas relative to their weight struggle to float in a fluid. An example is a stone, a piece of metal of other heavy objects with small surface areas.