Physics · Mechanical Properties Of Fluids · NEET
It is very similar. Viscosity is like an internal friction inside a fluid. When a solid slides on a surface, friction resists it. In a fluid, different layers move at different speeds, and these layers rub against each other. This rubbing between fluid layers is viscosity. The big difference: solid friction does not depend much on speed, but viscous force depends on how fast the layers slide past each other (the strain rate).
Honey has stronger forces between its molecules, so its layers hold on to each other more tightly. To make one layer slide over another, you need a bigger force. That is why honey pours slowly and water pours fast. In numbers, the coefficient of viscosity (eta) of honey is much larger than that of water. NCERT notes that thick liquids like coal tar, blood, and glycerine are more viscous than thin liquids like water and alcohol.
It is a single number that tells you how thick or sticky a fluid is. Formally, eta = shearing stress divided by the strain rate (the rate at which layers slide). A large eta means the fluid strongly resists flow (honey, glycerine). A small eta means it flows easily (water). Its SI unit is the poiseuille (Pl), also written as pascal-second (Pa s) or N s per m squared.
The coefficient of viscosity eta itself is a property of the fluid at a given temperature, so it does not change with speed for ordinary (Newtonian) fluids. But the viscous force DOES depend on speed. If the top layer moves faster, the velocity gradient increases, and the viscous force increases. So eta stays fixed, but the drag force grows with the speed difference between layers.
No. Viscosity is a property of the fluid, described by the number eta. The actual force is called the viscous force. Do not confuse the two. Viscosity (eta) tells you how sticky the fluid is; the viscous force is the actual pull that slows the layers down and depends on eta, the area, and the velocity gradient.
When you heat a liquid, its molecules move faster and the attractive forces between them weaken. The layers can then slide over each other more easily, so eta drops. That is why warm honey pours faster than cold honey. Important trap: for GASES it is the opposite. The viscosity of a gas INCREASES with temperature. NCERT states this clearly.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Viscosity is the property of a fluid by which it resists the relative motion of its layers. It acts like internal friction between adjacent layers of the fluid that move at different speeds.
eta = shearing stress / strain rate. In terms of the two-plate model, eta = (F/A) / (v/l), where F is the tangential force, A is the layer area, v is the speed of the top layer, and l is the separation between the plates.
The SI unit is the poiseuille (Pl), which is the same as pascal-second (Pa s) or newton-second per square metre (N s per m squared). Its dimensional formula is [M L^-1 T^-1].
Blood is more viscous than water. NCERT points out that blood is thicker (more viscous) than water, which is one reason it flows more slowly through narrow vessels.
An ideal fluid is assumed to have zero viscosity. In reality no fluid has exactly zero viscosity, but treating a fluid as ideal (non-viscous) is a useful approximation, for example in deriving Bernoulli's equation.
