Chapter 8, Problem 8.94PAE

Interpretation Introduction

Interpretation:To explain the concept to choose a better lubricant having high viscosity.

Concept introduction:

• Viscosity: It is a common experience of daily

  life that different liquids flow with different speeds, for example, water flows in greater speed than glycerol. Obviously some sort of internal friction is operating which checks the flow of liquids and which varies from liquid to liquid. This internal friction in liquids is primarily due to intermolecular force of attraction between the molecules. If we have a laminar flow of liquid in a tube, then the velocity of the layer just in touch with the side of the tube is zero and it increases as we proceed towards the center of the tube as shown in the figure. Thus, there exists velocity gradient between the different layers of the liquids.

  Due to greater intermolecular attraction between the molecules of liquids, the molecules moving in any layer will tend to impede the movement of the molecules in the adjacent faster moving layer as a result the velocity of the molecules in the faster layer decreases. Unless this decrease is prevented by applying a force along the layer in the forward direction, the velocity of the faster moving layer will go on decreasing and ultimately it will become zero. At this stage the liquid will stop flowing.

• Lubricant: A lubricant is an organic substance having high molecular weight, which is needed to reduce the interactions between surfaces of two material and control the generation of heat due to the movement of the surfaces. The molecule must have high viscosity for the effective application. It has a wide application in field of motors in vehicle, pump and many a field.