Fluid at rest

Volume is simply the space a fluid takes up, measured in cubic metres or litres. Density tells you how tightly packed the particles of a fluid are—it's the mass per unit volume, calculated as mass divided by volume. Water has a density of 1000 kg/m³, which serves as our standard reference.

Relative density compares how dense a substance is compared to water. When you pour palm oil into water, it floats because its relative density is less than one, meaning it's lighter than water. If a liquid sinks like kerosene in water, its relative density exceeds one. Think of petrol stations in Lagos—the fuel they sell has a lower relative density than water, which is why it separates when accidentally mixed with water.

Relative density has no units because you're comparing two densities. Understanding these three concepts helps explain why some fluids float and others sink.

Pressure in fluids is the force pushing on every part of the fluid, including the walls of its container. When you fill a bucket with water, the water pushes down on the bottom and sides equally in all directions. This happens because fluid particles move randomly and collide with surfaces constantly.

The deeper you go in water, the greater the pressure becomes. Think about swimming in a dam or deep well in Nigeria—your ears hurt more when you dive deeper because water pressure increases. This is why the base of a water tank experiences stronger pressure than the top. Pressure in fluids depends on the depth, the density of the fluid, and gravitational pull.

An important principle is that pressure acts perpendicular to any surface it touches. When the Lagoon floods streets in Lagos, water pushes equally against buildings from all sides, not just from above.

When an object is placed in a fluid like water or oil, it experiences two main forces: its weight pushing downward and the upward push from the fluid called buoyancy. The body reaches equilibrium when these forces balance perfectly, meaning the object neither sinks nor rises but stays suspended at one level.

Think of a fishing net in the Lagos lagoon. When fishermen load it with just the right amount of weights and floats, it hangs at a specific depth where the weight of the net and catch equals the buoyant force of the water pushing upward. This equilibrium helps them fish at their preferred depth.

For equilibrium to occur, the weight of the body must equal the buoyant force. This principle applies whether the object floats on the surface, remains completely submerged, or hovers inside the fluid.

When you push an object into water, the water pushes back on it. Archimedes' principle states that the upward force (called buoyant force) acting on any object submerged in a fluid equals the weight of the fluid displaced by that object. Think of it this way: if a stone displaces 2 litres of water, the water pushes up with a force equal to the weight of 2 litres of water.

A perfect Nigerian example is a canoe floating on a river. The canoe sinks into the water and displaces some volume. The upward push from the water exactly balances the weight of the canoe and everything inside it, which is why it floats rather than sinking.

This principle explains why ships made of heavy steel can float—they displace enough water to support their weight. Understanding this concept is crucial for solving buoyancy problems.

The law of flotation states that a floating object displaces a volume of fluid equal in weight to the object itself. Think about it this way: when you place something in water, the water pushes up against it. That upward push, called upthrust or buoyancy, must equal the weight of the object for it to float peacefully.

Consider a wooden canoe on the Lagos lagoon. The canoe floats because the weight of water it pushes aside equals the canoe's weight. If you load the canoe with too much cargo, it sinks deeper into the water, displacing more water until the upthrust again balances the total weight. The canoe stays afloat, but lower in the water.

This principle explains why ships made of steel can float despite being heavy. Their hollow shape displaces enough water to support their weight. The moment the upthrust becomes less than the object's weight, it sinks completely.