Physics · Mechanical Properties Of Fluids · NEET
Gas is also a fluid. In physics the word fluid means anything that can flow. Both liquids and gases can flow, so both are fluids. Many students think fluid means only liquid, but that is wrong. Water, air, oil, and steam are all fluids. So the group 'fluids' is bigger than the group 'liquids' and includes gases too.
Because neither one can hold its own shape. A fluid offers almost no resistance to shear stress (a sideways sliding force). Even a very small shear force makes its layers slide over each other, so it flows. Liquids and gases both do this. NCERT states the shearing stress a fluid can resist is about a million times smaller than that of a solid. That shared property is why both are grouped as fluids.
A liquid is one type of fluid. A fluid is the full family that includes both liquids and gases. A liquid has a fixed volume but no fixed shape. A gas has neither a fixed volume nor a fixed shape (it fills the whole container). So all liquids are fluids, but not all fluids are liquids because gases are fluids as well.
A fluid cannot resist shear stress. When you tilt or pour it, the sideways force makes its layers slip past each other with almost no resistance, so it keeps flowing until it matches the container's shape. A solid resists this shear, so it keeps its shape. This 'no resistance to shear' is the single key idea that defines a fluid.
A solid resists shear stress and keeps a definite shape; a fluid cannot resist shear stress and flows to fit its container. Solids and liquids both have nearly fixed volume (low compressibility), but a gas is easily compressed. So the deciding test is shear: if a small sideways force makes it flow, it is a fluid.
Over very long times some solids, like glass or thick tar, can slowly deform, but for NEET a solid is treated as a substance that resists shear and keeps its shape. The clean physics definition is: a fluid flows because it cannot resist shear stress, and a solid does not flow because it can. Do not overthink the exceptions in the exam.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A fluid is any substance that can flow and take the shape of its container because it cannot resist shear stress. Both liquids and gases are fluids.
Yes. Gases flow just like liquids, so gases are fluids. A gas also fills the entire volume of its container, unlike a liquid which has a fixed volume.
Fluids offer almost no resistance to shear stress, so their layers slide easily and they flow. Solids strongly resist shear, so they keep their shape and do not flow.
No. Both are fluids, but a liquid has a fixed volume with low compressibility, while a gas has no fixed volume and is easily compressed to fill its container.
Yes, one key difference is compressibility. Liquids barely compress, so their volume is nearly fixed. Gases compress a lot, so their volume changes easily with pressure.
