1.3.4BehaviourofSpringsandMaterials.ppt.pptVIP

* Summary All materials show elastic behaviour up to the elastic limit Brittle materials break at the elastic limit Ductile materials become permanently deformed beyond the elastic limit Polymeric materials can show either characteristics, depending on the molecular structure and temperature Questions Physics 1 – Chapter 8 SAQ 1 to 9 End of Chapter questions 1 to 4 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Stowmarket Physics 1.3.4 Behaviour of Springs and Materials Objective Describe how deformation is caused by a force in one direction and can be tensile or comprehensive Deformation Can be caused by tensile or compressive forces Tensile cause tension stretching forces Compressive cause compression squeezing forces Deformation two equal and opposite tensile forces stretching a wire two equal and opposite compressive forces squeezing a spring Objective Describe the behaviour of springs and wires in terms of force, extension, elastic limit, Hooke’s Law and the force constant – i.e. force per unit extension or compression Definitions Force (F) applied to a spring or wire in tension or compression Extension (x) the change in length of a material when subjected to a tension, measured in metres Elastic Limit the point at which elastic deformation becomes plastic deformation Definitions Elastic Deformation when the deforming force is removed, the object will return to it’s original shape eg rubber band, spring (usually) Plastic Deformation when the deforming force is removed, the object will not return to it’s original shape eg Plasticine, Blutack Hooke’s Law When tension is plotted against extension, a straight line graph denotes elastic deformation This is summarised by Hooke’s Law: ‘The extension of a body is proportional to the force that causes it’ or as a formula: where F = Force F = kx x = extension k = force/spring constant Hooke’s Law F x Extension /mm Tension /N Force/Spring Constant F = kx Expr