![]() These resistive forces affect the way the fluid flows through the pipe. Friction also occurs between the different layers of fluid. For example, a fluid flowing through a pipe is subject to resistance, a type of friction, between the fluid and the walls. In this section, we introduce the forces of friction that act on fluids in motion. We explained that at low speeds, the drag is proportional to the velocity, whereas at high speeds, drag is proportional to the velocity squared. We also discussed drag and air resistance in that same chapter. Friction depends on the types of materials in contact and is proportional to the normal force. In Applications of Newton’s Laws, which introduced the concept of friction, we saw that an object sliding across the floor with an initial velocity and no applied force comes to rest due to the force of friction. Describe the conditions under which an object has a terminal speed.Use the Reynolds number for a system to determine whether it is laminar or turbulent.Calculate the Reynolds number for an object moving through a fluid.Explain how pressure drops due to resistance.Calculate flow and resistance with Poiseuille's law.The viscosity of water is taken as 1 centipoise at 25 oC.By the end of this section, you will be able to: Water is usually chosen as a reference liquid. And viscosity is calculated by using the formula: For this purpose time for the flow of reference and test liquid between two marks, a & b made on the viscometer is noted. In this method relative viscosity is measured. Viscosity is usually measured by an Oswald viscometer (See Figure given below). The coefficient of viscosity is defined as the force required to maintain a difference of velocity of 1m/s between two parallel layers of liquid one metre apart. Viscosity is measured in terms of the coefficient of viscosity (η). Sulphur molecules are ring-shaped at 140 oC and offer less resistance to each other’s flow while the molecules at 190 oC are tangled and offer more resistance to flow. viscosity of liquid Sulphur is higher at 190 oC than that at 140 oC. Irregular-shaped molecules offer more resistance to each other’s flow as compared to regular-shaped molecules e.g. viscosity of sulphuric acid is higher than water due to the higher density of sulphuric acid. Higher is the density high will be the viscosity and vice versa e.g. mobile oil, grease etc have high viscosity, and they consist of larger hydrocarbon molecules. Molecules having bigger sizes or more molecular mass offer more resistance to each other’s flow, so more will be the viscosity e.g. In the refrigerator due to the low-temperature viscosity of honey was high. ![]() it is easy to pour honey from a bottle placed outside as compared to a bottle placed in the refrigerator. Higher is the temperature low will be the viscosity and vice versa e.g. the viscosity of glycerin is higher than in water because hydrogen bonding in glycerin is stronger than in water. Stronger intermolecular forces among molecules of liquid higher will be the viscosity and vice versa e.g. This internal resistance that one layer offers to the flow of the other layer is called viscosity. ![]() Liquid molecule’s layers slide one past the other and they offer resistance to each other. It means liquid flows in the form of layers. Water flowing in the river runs fast in the middle but slowly along the bank. ![]()
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