#### Transcript PHYSICS PROJECT ON PROPERTIES OF ELASTICITY

PHYSICS PROJECT WORK FOR ANNUAL EXAM 2009-10 MADE BY :1. 2. 3. 4. PRASAD AMIT KUMAR ARJUN VIJJAN AKASH THAPA APPU PROPERTIES OF ELASTICITY Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed. Greater forces may cause permanent changes of shape or size, called plastic deformation. In the modern statement of Hooke's law, the terms "stress" and "strain" have precise mathematical definitions. Stress is the applied force divided by the area the force acts on. Strain is the added length divided by the original length. Stress and Strain: Stress: is a quantity that is proportional to the force causing a deformation. Stress is the external force acting on an object per unit cross sectional area. Strain: is a measure of the degree of deformation. It is found that for sufficiently small stresses strain is proportional to stress. The constant of the proportionality depends on the material being deformed and on the nature of deformation We call this proportionality constant the elastic modulus. The elastic modulus is therefore the ratio of stress to the resulting strain. Elastic Modulus=Stress/Strain In a very real sense it is a comparison of what is done to a solid object (a force is applied) and how that object responds (it deforms to some extent) We consider three types of deformation and define an elastic modulus for each: Young’s Modulus: which measures the resistance of a solid to a change in its length 2. Shear Modulus: which measures the resistance to motion of the planes of a solid sliding past each other 3. Bulk Modulus: which measures the resistance of solids or liquids to changes in their volume 1. Young’s Modulus: Consider a long bar of cross sectional area A and initial length Li that is clamped at one end. When an external force is applied perpendicular to the cross section internal forces in the bar resist distortion “stretching” but the bar attains an equilibrium in which its length Lf is greater than Li and in which the external force is exactly balanced by internal forces. In such a situation the bar is said to be stressed. We define the tensile stress as the ratio of the magnitude of the external force F to the cross sectional area A. the tensile strain in this case is defines as the ratio of the change in length ΔL to the original length Li. Y=tensile stress/ tensile strain Y=(F/A)/(ΔL/Li) The Elastic Limit: The elastic limit of a substance is defined as the maximum stress that can be applied to the substance before it becomes permanently deformed. It is possible to exceed the elastic limit of a substance by applying sufficiently large stress, as seen in in the figure Initially a stress strain curve is a straight line. As the stress increases, however the curve is no longer a straight line. When the stress exceeds the elastic limit the object is permanently distorted and it does not return to its original shape after the stress is removed. What is Young’s modulus for the elastic solid whose stress strain curve is depicted in the figure ?? Young’s modulus is given by the ratio of stress to strain which is the slope of the elastic behavior section of the graph in slide 9 reading from the graph we note that a stress of approximately 3x10⁸N/m² results in a strain of 0.003. The slope, and hence Young’s modulus are therefore 10x10¹ºN/m². Shear Modulus: Another type of deformation occurs when an object is subjected to a force tangential to one of its faces while the opposite face is held fixed by another force. The stress in this case is called a shear stress. If the object is originally a rectangular block a shear stress results in a shape whose cross section is a parallelogram. To a first approximation (for small distortions) no change in volume occurs with this deformation. We define the shear stress as F/A, the ratio of the tangential to the area of A of the force being sheared. The shear strain is defined as the ratio ΔX/H where ΔX is the horizontal distance that the sheared force moves and H is the height of the object. In terms of these quantities the shear modulus is S= shear stress/ shear strain S= (F/A)/ (ΔX/H) Bulk Modulus: Bulk modulus characterizes the response of a substance to uniform squeezing or to a reduction in pressure when the object is placed in a partial vacuum. Suppose that the external forces acting on an object are at right angles to all its faces, and that they are distributed uniformly over all the faces. A uniform distribution of forces occur when an object is immersed in a fluid. An object subject to this type of deformation undergoes a change in volume but no change in shape. The volume stress is defined as the ratio of the magnitude of the normal force F to the area A. The quantity P=F/A is called the pressure. If the pressure on an object changes by an amount ΔP= ΔF/A the object will experience a volume change ΔV. The volume strain is equal to the change in volume ΔV divided by the initial volume Vi B= volume stress/volume strain B=-(ΔF/A)/(Δ V/Vi) B=- Δ P/(ΔV/Vi) When a solid is under uniform pressure it undergoes a change in volume but no change in shape. This cube is compressed on all sides by forces normal to its 6 faces. Viscosity: The term viscosity is commonly used in the description of fluid flow to characterize the degree of internal friction, or viscous force is associated with the resistance that two adjacent layers of fluid have to moving relative to each other. Viscosity causes part of the kinetic energy of a fluid to be converted to internal energy. Units of Measure Dynamic viscosity and absolute viscosity are synonymous. The IUPAC symbol for viscosity is the Greek symbol eta (η), and dynamic viscosity is also commonly referred to using the Greek symbol mu (μ). The SI physical unit of dynamic viscosity is the Pascal-second (Pa·s), which is identical to 1 kg·m−1·s−1. If a fluid with a viscosity of one Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one Pascal, it moves a distance equal to the thickness of the layer between the plates in one second. The name Poiseuille (Pl) was proposed for this unit (after Jean Louis Marie Poiseuille who formulated Poiseuille's law of viscous flow), but not accepted internationally. Care must be taken in not confusing the Poiseuille with the poise named after the same person. The cgs physical unit for dynamic viscosity is the poise (P), named after Jean Louis Marie Poiseuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). The centipoise is commonly used because water has a viscosity of 1.0020 cP (at 20 °C; the closeness to one is a convenient coincidence). 1 P = 1 g·cm−1·s−1 The relation between poise and Pascal-seconds is: 10 P = 1 kg·m−1·s−1 = 1 Pa·s 1 cP = 0.001 Pa·s = 1 mPa·s