 Hello everyone, my name is Saurabh Deshmukh, working as an assistant professor in Walsh and Institute of Technology, Sallapur. In this video, we are going to learn the viscosity along with its definition, units and a problem, the learning outcome. At the end of this session, the learner will able to explain viscosity along with its units and will able to solve the problem on viscosity. The contents will start with the definition along with the Newton's law of viscosity, then the units of viscosity, effect of temperature on viscosity and problem on viscosity. So, we will start with the definition of viscosity first. So, viscosity is defined as the property of a fluid which offers resistance to the movement of one layer of the fluid over another layer of fluid. So, I will write the definition here. Viscosity is defined as the property of fluid which offers resistance to the movement of one layer of fluid over another adjacent layer. So, what is the cause of this resistance to the movement? Actually, what happens? The top layer causes the shear stress on the adjacent lower layer of fluid. So, I will just draw here a velocity variation diagram. So, as the distance of the layer increases from the solid boundary, the velocity of that layer increases. So, I will just draw here a velocity diagram. So, this will be our velocity U and this will be our height of the layer from the solid boundary. Suppose this is a solid boundary. So, as the height of the layer increases, the velocity also increases. So, suppose at the distance y, the velocity will be U and at distance y plus dy, the velocity will be U plus du. So, here we get the height difference and here we will get the difference in velocity. So, what happens when the fluid flows? The top layer causes the shear stress. This is the top layer. This top layer causes the shear stress on the adjacent lower level and the lower level causes the shear stress on the adjacent top layer. Now, we will see the Newton's law of viscosity first. So, the Newton's law of viscosity states that the shear stress that is the developing between the adjacent two layers, we denote it by the Greek letter tau. This shear stress on the fluid element layer is directly proportional to the rate of shear strain and what is the rate of shear strain? Actually, it is the ratio of change in velocity upon change in distance. So, this is the Newton's law of viscosity. What it states? It states that the shear stress on the fluid layer is directly proportional to rate of shear strain. So, now we will remove this proportionality sign and we will replace it by the equality sign. So, tau equals to mu into du by dy and this mu is called coefficient of viscosity and this mu is called the coefficient of viscosity. So, what is the formula of coefficient of viscosity now? Mu equals to tau upon du by dy. Now, we will see the units of viscosity. So, what is the formula of viscosity? It is mu equals to tau upon du by dy. So, we will substitute the units of tau that is shear stress upon the du by dy. So, mu equals to what is the formula of shear stress? It is Newton per meter square upon du is change in velocity. So, its unit will be meter per second into dy is the change in height of the layer. So, its units will be 1 upon meter. So, meter meter will get cancelled. So, what is the unit of mu now? Mu equals to Newton second per meter square. This is the SI unit. Also in CGS unit it is called as poise or it is also called as din second per centimeter square. Now, we need the correlation between the SI unit and CGS unit. So, 1 poise equals to 1 by 10 Newton second per meter square or 10 poise equals to 1 Newton second per meter square. Now, we will see the kinematic viscosity. So, the kinematic viscosity is defined as the ratio of dynamic viscosity and the density of the fluid. So, the kinematic viscosity is denoted by the Greek letter mu. So, mu equals to what is the definition of kinematic viscosity? It is the ratio of the dynamic viscosity and the density of the fluid. So, the dynamic viscosity will represent it by the mu upon density it is rho. Now, we will see the unit of the kinematic viscosity mu equals to what is the unit of mu here that is dynamic viscosity it is Newton second per meter square upon what is the unit of rho that is density it is kg per meter cube. We will convert the Newton into kg kg into meter per second square into second by meter square upon kg per meter cube. Now, after canceling the similar terms what we will get it is meter square by second. So, this is the unit of kinematic viscosity. In next video, we will see the effect of temperature on viscosity and problems on viscosity. These are the references. Thank you.