When a solid object is immersed in water, there is a loss in its weight. This loss is

1. Understanding Pressure in Fluids (basic)

To understand fluids, we must first define pressure. In its simplest form, pressure is the force acting per unit area (P = F/A). As noted in Science, Class VIII (NCERT 2025), Chapter 5, p.83, when the area over which a force is applied is smaller, the resulting pressure is higher. This is why a sharp knife cuts better than a blunt one. While solids primarily exert pressure downward due to gravity, fluids (liquids and gases) behave differently because their particles are free to move. They don't just push down; they exert pressure in all directions—sideways against the walls of a container and even upwards.

If you puncture a plastic bottle filled with water, you will notice the liquid spurting out perpendicular to the surface. This observation confirms that liquids exert pressure on the walls of their container Science, Class VIII (NCERT 2025), Chapter 6, p.84. This is also why leaking water pipes spurt like fountains; the internal pressure is constantly pushing against every square inch of the pipe's interior surface. Interestingly, while pressure significantly compresses gases, it has a very small effect on the density of liquids because they are nearly incompressible Science, Class VIII (NCERT 2025), Chapter 11, p.148.

Crucially, fluid pressure is not uniform; it increases with depth. The deeper you go in a bucket of water or the deeper you descend into the Earth's interior, the greater the pressure becomes Fundamentals of Physical Geography, Class XI (NCERT 2025), Chapter 2, p.19. This happens because the layers of fluid at the bottom must support the weight of all the fluid layers above them. This is the fundamental reason why overhead water tanks are always placed at a height—to use gravity to create enough pressure to push water through the pipes of a house Science, Class VIII (NCERT 2025), Chapter 6, p.83.

Sources: Science, Class VIII (NCERT 2025), Exploring Forces, p.83; Science, Class VIII (NCERT 2025), Pressure, Winds, Storms, and Cyclones, p.84-85; Fundamentals of Physical Geography, Class XI (NCERT 2025), The Origin and Evolution of the Earth, p.19; Science, Class VIII (NCERT 2025), The Amazing World of Solutes, Solvents, and Solutions, p.148

2. Density and Relative Density (basic)

To understand why a massive iron ship floats while a tiny pebble sinks, we must look at the concept of Density. At its most fundamental level, density is a measure of how much "stuff" (mass) is packed into a specific amount of space (volume). Mathematically, we define it as Density = Mass / Volume. It is an intrinsic property of a substance, meaning it remains the same whether you have a small block of gold or a giant gold bar, though it can change slightly with temperature and pressure Science, Class VIII. NCERT (2025), The Amazing World of Solutes, Solvents, and Solutions, p.140. In the study of the Earth, we even use variations in density to understand the crust; "gravity anomalies" occur because the uneven distribution of mass within the Earth's interior influences gravitational pull Physical Geography by PMF IAS, Earths Interior, p.58.

While density tells us about a single substance, Relative Density (also called specific gravity) tells us how that substance compares to water. It is the ratio of the density of a substance to the density of water at the same temperature. Because it is a ratio of two similar quantities, it is a pure number without any units Science, Class VIII. NCERT (2025), The Amazing World of Solutes, Solvents, and Solutions, p.141. For example, if a piece of wood has a relative density of 0.6, it is 60% as dense as water.

This comparison is the secret to floating and sinking. When an object is placed in a fluid, it experiences an upward buoyant force. According to Archimedes' Principle, this upward force is exactly equal to the weight of the fluid that the object displaces. If the object is denser than the fluid, its weight will be greater than the buoyant force, and it will sink. If it is less dense than the fluid, it will float Science, Class VIII. NCERT (2025), Chapter 5: Exploring Forces, p. 76. This is why an object with a relative density greater than 1 sinks in water, while one with a relative density less than 1 floats.

Feature	Density	Relative Density
Definition	Mass per unit volume.	Ratio of substance density to water density.
Units	kg/m³ or g/cm³	Unitless (Dimensionless)
Dependence	Changes with temperature/pressure.	Comparative; used to predict floating.

Sources: Science, Class VIII. NCERT (2025), The Amazing World of Solutes, Solvents, and Solutions, p.140-141; Science, Class VIII. NCERT (2025), Chapter 5: Exploring Forces, p.76; Physical Geography by PMF IAS, Earths Interior, p.58

3. Concept of Buoyancy and Upthrust (intermediate)

Have you ever noticed how much lighter you feel when you are in a swimming pool, or how a heavy bucket of water suddenly feels much heavier the moment it leaves the water's surface? This isn't an illusion; it is the result of a fundamental physical force called buoyancy or upthrust.

When an object is placed in a liquid, it experiences two opposing forces. The gravitational force (or weight) pulls the object downward toward the center of the Earth Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.72. Simultaneously, the liquid exerts an upward force on the object. This upward push is known as upthrust or the buoyant force Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.76. You can test this by pushing an empty plastic bottle into a bucket of water; the resistance you feel, and the way the bottle bounces back to the surface, is a direct demonstration of upthrust in action.

The magnitude of this force is governed by Archimedes' Principle. It states that the upward buoyant force exerted on a body immersed in a fluid (whether fully or partially) is exactly equal to the weight of the fluid that the body displaces. This creates a state of apparent weight loss. If you weigh a stone in air and then weigh it while it is submerged in water, it will appear lighter. The difference between these two weights is precisely equal to the weight of the water the stone pushed aside.

Whether an object sinks or floats is determined by the "tug-of-war" between these two forces:

Scenario	Force Comparison	Result
Sinking	Gravitational Force > Buoyant Force	The object moves downward through the liquid.
Floating	Gravitational Force = Buoyant Force	The object stays at the surface or suspended within the liquid.

It is important to remember that the buoyant force is not fixed for every liquid; it depends heavily on the density of the liquid Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.76. This is why it is much easier to float in the highly salty (and thus more dense) water of the Dead Sea than in a freshwater lake!

Sources: Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.72, 76, 77

4. Surface Tension and Capillarity (intermediate)

Imagine the surface of a pond. Have you ever noticed how some insects can walk on water without sinking? This isn't magic; it is Surface Tension. At the molecular level, particles in a liquid exert interparticle forces of attraction on one another Science, Class VIII, Particulate Nature of Matter, p.104. While a molecule deep inside the liquid is pulled equally in all directions, a molecule at the surface has no neighbors above it. This results in a net inward pull, causing the surface to contract and behave like a stretched elastic membrane. This is why raindrops are spherical—the sphere is the shape with the minimum surface area for a given volume.

We can manipulate this "skin" of the liquid to our advantage. For instance, water alone often has too much surface tension to penetrate deep into the fibers of dirty clothes. When we add soap, the soap particles reduce the surface tension of the water. This allows the water to spread out more effectively, surrounding and lifting oil or dirt particles away from the fabric Science, Class VIII, Particulate Nature of Matter, p.111. This ability of a liquid to spread and "wet" a surface is a direct consequence of lowering its surface tension.

Capillarity (or Capillary Action) is a closely related phenomenon where a liquid spontaneously rises or falls in a narrow tube. It is a "tug-of-war" between two forces: Cohesion (attraction between like molecules, like water-to-water) and Adhesion (attraction between unlike molecules, like water-to-glass). If adhesion is stronger, the liquid climbs the walls of the tube. This principle is vital in nature; for example, the thinnest blood vessels in our bodies are called capillaries because of their hair-like diameter Science, Class X, Life Processes, p.93. While biological transport involves complex pumping, the physical principle of capillarity assists in moving fluids through microscopic spaces, such as moving lymph through intercellular gaps Science, Class X, Life Processes, p.94.

Concept	Primary Cause	Common Example
Surface Tension	Inward Cohesive forces at the surface.	Formation of spherical water droplets.
Capillarity	Balance between Adhesion and Cohesion.	Water rising in a thin glass tube or a sponge.

Sources: Science, Class VIII, Particulate Nature of Matter, p.104; Science, Class VIII, Particulate Nature of Matter, p.111; Science, Class X, Life Processes, p.93; Science, Class X, Life Processes, p.94

5. Pascal's Law and Hydraulic Systems (intermediate)

To understand Pascal’s Law, we must first revisit the fundamental definition of pressure. Pressure is defined as the force acting per unit area (P = F/A). Its SI unit is the pascal (Pa), named in honor of Blaise Pascal Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.82. While solids exert pressure downward due to gravity, fluids (liquids and gases) are unique because they exert pressure in all directions on the walls of their container Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.94. Pascal’s Law takes this a step further: it states that in a confined, incompressible fluid, any change in pressure applied at any point is transmitted undiminished to every other point in the fluid and to the walls of the container.

This principle is the secret behind hydraulic systems, which act as "force multipliers." Imagine two pistons connected by a U-shaped tube filled with oil. One piston is small (small area, A₁), and the other is large (large area, A₂). When you apply a small force (F₁) to the small piston, it creates a specific pressure. According to Pascal’s Law, this exact same pressure (P) reaches the large piston. Since Force = Pressure × Area, the larger area of the second piston results in a much larger output force (F₂). This allows a person to lift a heavy car using only a small amount of effort at a hydraulic jack.

However, nature doesn't give us a "free lunch." While force is multiplied, the distance the pistons move is different. To lift the heavy load by just 1 centimeter, you might have to push the small piston down by 10 centimeters. This ensures that the work done (Force × Distance) remains constant, adhering to the law of conservation of energy. This mechanical advantage is why hydraulic systems are indispensable in heavy machinery, such as the cranes used in factories and scrap yards to move massive metal items Science, Class VIII, Electricity: Magnetic and Heating Effects, p.52.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p.82, 94; Science, Class VIII . NCERT(Revised ed 2025), Electricity: Magnetic and Heating Effects, p.52

6. Archimedes' Principle Explained (exam-level)

Have you ever noticed how you feel lighter while swimming in a pool, or why a heavy iron ship floats while a tiny pebble sinks? This is governed by Archimedes' Principle. At its core, the principle states that when an object is fully or partially immersed in a fluid (liquid or gas), it experiences an upward force called buoyancy or upthrust. The breakthrough discovery made by Archimedes was that this upward force is exactly equal to the weight of the fluid displaced by the object Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p. 76.

To understand this, imagine the "weight" of an object as the Earth's gravity pulling it down. When you submerge that object, the liquid pushes back up. This upward force creates an apparent loss of weight. If you weigh an object in the air and then weigh it while it is submerged in water, the difference between these two readings is the weight of the water that the object pushed out of its way. This is why the SI unit for both weight and buoyant force is the Newton (N) Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p. 77.

Whether an object sinks or floats depends on the "tug-of-war" between gravity pulling down and buoyancy pushing up. We can summarize the outcomes as follows:

Scenario	Condition	Result
Sinking	Weight of object > Weight of displaced liquid	The object falls to the bottom.
Floating	Weight of object = Weight of displaced liquid	The object stays at the surface or suspended.

One critical takeaway for your exams is that the buoyant force depends on the density of the liquid and the volume of the object submerged, not the total volume of the liquid in the container Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p. 76. This is why it is easier to float in the highly dense salt water of the Dead Sea than in a freshwater swimming pool.

Sources: Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p.76; Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p.77

7. Laws of Floatation and Applications (exam-level)

Have you ever noticed how a heavy bucket of water feels remarkably lighter as long as it is still submerged in the well? Or why a tiny iron nail sinks in a pond, while a massive iron ship sails across the ocean? This happens because of a phenomenon called buoyancy. When any object is placed in a fluid (liquid or gas), the fluid exerts an upward force on it, opposing the downward pull of gravity. This upward force is known as the buoyant force or upthrust Science, Class VIII, NCERT (Revised ed 2025), Chapter 5, p. 76.

To understand exactly how much force is pushing upward, we look at Archimedes' Principle. It states that when an object is fully or partially immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the object. This explains the "apparent loss of weight" you feel in a swimming pool; your "apparent weight" is simply your actual weight minus the weight of the water you have pushed out of the way.

The Law of Floatation is a specific application of this principle. For an object to float, it must displace a weight of fluid equal to its own weight. This is why density is a critical factor Science, Class VIII, NCERT (Revised ed 2025), Chapter 9, p. 140. We can categorize the behavior of objects based on the relationship between the density of the object (ρ_obj) and the density of the fluid (ρ_fluid):

Condition	Density Comparison	Result
Weight > Buoyant Force	ρ_obj > ρ_fluid	Sinks to the bottom (e.g., a stone).
Weight = Buoyant Force	ρ_obj = ρ_fluid	Floats fully submerged (neutral buoyancy).
Weight < Potential Buoyant Force	ρ_obj < ρ_fluid	Floats with a portion above the surface (e.g., wood or ice).

In practical UPSC terms, these laws explain how submarines operate: they use ballast tanks to take in or release water, changing their average density to sink or rise. Similarly, hydrometers (used to measure the density of liquids like milk or battery acid) work on the principle that they will sink deeper in a less dense liquid than in a more dense one.

Sources: Science, Class VIII, NCERT (Revised ed 2025), Exploring Forces, p.76; Science, Class VIII, NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.140

8. Solving the Original PYQ (exam-level)

You have just mastered the building blocks of fluid pressure and gravitational pull; this question is the perfect application of Archimedes' Principle. When a solid object is immersed, it doesn't actually lose its physical mass; rather, it experiences an upward buoyant force that counteracts gravity. This interaction results in an apparent weight loss. To solve this, you must recall the fundamental link you just learned: the upward force (buoyancy) is mathematically identical to the weight of the liquid the object pushes aside. Therefore, the reduction in weight is equal to the weight of the water displaced, making Option (A) the correct choice.

As an aspirant, you must be careful not to fall for the traps in Options (B) and (C). These are common distractors designed to confuse the loss of weight with the conditions for floating. While it is true that an object sinks if its total weight is greater than the displaced water, the specific magnitude of weight loss remains strictly equal to the displaced fluid's weight, regardless of whether the object sinks or floats. UPSC often uses these relative terms ("less than" or "greater than") to see if you can distinguish between a general principle and a specific state of equilibrium, as explained in Science, Class VIII. NCERT (Revised ed 2025).