Ch 2Mechanical Properties of Fluids — Class 12 Physics, concept-first.

In XIth Std we studied how a solid behaves under the action of a force — it nearly keeps its own shape and volume no matter how large a force is applied to it.

Any substance that can flow is called a fluid: formally, a fluid is a substance that deforms continually under the action of an external force, and this definition is broad enough to include liquids,…

Hydrostatics is the branch of physics that studies the properties of fluids at rest — as opposed to hydrodynamics, which studies fluids in motion (taken up starting in section 2.5 of this chapter).

A fluid at rest exerts a force on any surface it is in contact with — this surface might be a container's wall, or the bottom of an open vessel holding the fluid.

To find the pressure due to a liquid at some depth below its free surface, imagine a vertical cylinder of the liquid, of cross-sectional area A, extending down to a depth h inside a vessel filled with…

The Earth's entire atmosphere is itself a fluid, made of air, and — exactly like the liquid column of the previous section — it exerts a downward force on anything beneath it because of its own weight…

To find how pressure varies between two different points inside a fluid at rest, consider a tank of water with an imaginary vertical cylinder of horizontal cross-sectional area A marked inside it, wit…

Consider several interconnected vessels of very different shapes — some narrow, some wide, some tapering — all joined together at their base and open at the top.

Pascal's law states that the pressure applied at any point of an enclosed fluid at rest is transmitted equally and undiminished to every point of the fluid, and also to the walls of the container hold…

Instruments used to measure pressure are known generally as pressure meters, pressure gauges, or (for pressures below atmospheric) vacuum gauges.

A liquid at rest shows a distinctive and interesting property called surface tension. It is responsible for a whole range of everyday observations: a water spider is able to walk across the surface of…

To understand surface tension, we first need a small set of terms from the molecular theory of liquids that describe how molecules behave near a liquid's surface.

a) Surface tension. As just seen, the free surface of a liquid behaves like a stretched membrane, and every molecule within its surface film experiences a stretching force.

When a liquid's surface comes into contact with a solid surface (for example, the wall of a narrow tube dipped into the liquid), it forms a meniscus — a curved surface that can be either convex (bulgi…

a) Effect of impurities. (i) When a soluble substance, such as ordinary common salt (sodium chloride), is dissolved in water, the surface tension of the water increases.

Every molecule sitting on a liquid's surface experiences a force due to surface tension that is tangential to the (static) liquid surface at that point; the direction of the net, resultant surface-ten…

Free liquid drops, and small bubbles, are always observed to be spherical in shape. This is because, at these small scales, the forces of surface tension dominate completely over gravitational forces,…

A tube with a very fine bore (of order about 1 mm) and open at both ends is called a capillary tube. If one end of a capillary tube is dipped into a liquid that wets the tube's surface, partially or c…

Moving fluids are part of everyday life: water flowing out of a tap, cooking gas flowing through a tube, or water flowing through a river or a canal are all examples that can be understood using the c…

The flow of a fluid, whether streamline or turbulent, is distinguished on the basis of the velocity of the flow.

When water is poured out of a glass, it flows freely and quickly; but when syrup or honey is poured, it flows sluggishly and clings to the sides of the container.

According to Newton's law of viscosity, for a streamline flow, the viscous force f acting on any given layer of a fluid is directly proportional both to the area A of that layer and to the local veloc…

In 1845, Sir George Gabriel Stokes (1819-1903) stated the law that gives the viscous force acting on a small spherical object as it falls through a viscous medium.

Consider a spherical object falling under gravity through a viscous fluid. Three separate forces act on it during this downward fall: (1) the viscous force , directed upward, whose magnitude keeps inc…

Consider the steady flow of an incompressible fluid through a flow tube whose cross-sectional area varies along its length.

Observing a river, one notices that the speed of the water decreases in a wider stretch of the river and increases in a narrower stretch.

Sample papers and previous-year board questions for this subject.