WHY OBJECTS FLOAT OR SINK WHEN PLACED ON THE SURFACE OF WATER

Have you ever had a swim in a pool and felt lighter? Have you ever drawn water from a well and felt that the bucket of water is heavier when it is out of the water? These questions can be answered by considering buoyancy.

Buoyancy refers to the upward force that a fluid (like water) exerts on an object that is immersed in it. This force is what makes objects feel lighter in water compared to when they are in air. Essentially, when an object is placed in water, it experiences two forces: gravitational force pulling it downward and buoyant force pushing it upward. If the buoyant force is greater than the weight of the object, it floats; if it is less, it sinks.

Imagine you are on a diving board. When you leap into the water, you feel a 'push' from the water supporting your body. This is buoyancy at work, making you feel lighter as you swim. The design of ships, which are made of heavy steel yet float, takes advantage of this upward force.

Activity ______________9.4

• Take an empty plastic bottle. Close the mouth of the bottle with an airtight stopper. Put it in a bucket filled with water. You see that the bottle floats.
• Push the bottle into the water. You feel an upward push. Try to push it further down. You will find it difficult to push deeper and deeper. This indicates that water exerts a force on the bottle in the upward direction. The upward force exerted by the water goes on increasing as the bottle is pushed deeper until it is completely immersed.
• Release the bottle. It bounces back to the surface. Why doesn’t the bottle stay immersed in water after it is released?

In this activity, the empty plastic bottle floats because the buoyant force acting on it (upward force from the water) is greater than the weight of the bottle itself. When pressure is exerted to push it deeper, we can feel the resistance from the water, illustrating how water generates a more considerable upthrust against the object the more it is submerged. Upon release, the bottle rises back to the surface due to the dominant buoyant force.

Think of a beach ball in the water—when you try to push it under, it pushes back at you. This interaction is similar to what you experience with the bottle; the water is essentially trying to 'help' the bottle stay afloat.

Activity ______________9.5

• Take a beaker filled with water.
• Take an iron nail and place it on the surface of the water.
• Observe what happens. The nail sinks. This is because the force due to the gravitational attraction of the earth on the iron nail pulls it downwards. There is an upthrust of water on the nail, which pushes it upwards. But the downward force acting on the nail is greater than the upthrust of water on the nail.

In this activity, the iron nail sinks because its weight is greater than the buoyant force that the water can exert. The buoyant force is not sufficient to overcome the gravitational force acting downward due to the nail’s higher density compared to water. This illustrates the principle that objects will sink if they are denser than the fluid they are placed in.

Picture throwing a rock into a pond. The rock is denser than water, and since it cannot be buoyed up by the water, it sinks to the bottom. This principle is true for many objects, such as metals, which typically have a higher density than water and will therefore sink when placed in it.

Activity ______________9.6

• Take a beaker filled with water.
• Take a piece of cork and an iron nail of equal mass.
• Place them on the surface of water. The cork floats while the nail sinks. This occurs because of the difference in their densities.

Here, the cork floats because it has a lower density than water. When the cork is placed on the surface, the buoyant force exerted by the water is greater than the weight of the cork, causing it to float. This is directly linked to the concept of density—the mass per unit volume. If an object has a lower density than the fluid it is placed in, it will float; if its density is higher, it will sink.

Consider how boats are designed. Even though they are made from heavy material, their shape allows them to displace enough water to create a buoyant force that exceeds their weight. Just like cork floating, so do ships—it’s all about achieving the right balance of density and volume!

Therefore, objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid.

The overall conclusion is based on comparing the density of an object to the density of the fluid it is in. If the object's density is lower than that of the liquid, the buoyant force is sufficient to keep it afloat; if it’s higher, the object will sink. This relationship between an object's density and buoyancy is crucial for understanding many phenomena in fluid mechanics.

An everyday analogy is observing various fruits in a bowl of water. A peeled orange may float, while a grapefruit sinks because of their respective densities. This observation can help students relate the scientific concept of buoyancy to real-life experiences they encounter daily.