Title TOPIC 6 MECHANICAL PROPERTIES OF MATTER - WazaElimu.com.pdf ✅

47 TOPIC 6: MECHANICAL PROPERTIES OF MATTER CONCEPT OF ELASTICITY ELASTICITY  This is the ability of a body to return to its original shape and size when the deforming force is removed. NOTE: Elastic objects  These are objects that undergo elasticity. Brittle materials  These are materials that break when a force is applied to them. OR These are materials which do not undergo elastic deformation. Examples: Glass, block They break just when elastic limit is reached. Plastic material  These are materials which do not return to its original shape and size after removing the deforming force. Ductile materials  These are materials which undergo permanent deformation without breaking. Example: Copper, Iron, Brass. RELATIONSHIP BETWEEN TENSION AND EXTENSION OF A LOADED ELASTIC MATERIAL Consider the graph below which show the relationship between tension and extension Certain observation can be made from the graph as follows: At OA: The tension (force) is directly proportional to the extension of the spring. This is accordance with Hooke’s law. Therefore: OA = Proportionality limit.
48 Proportionality limit: This is the limit where material return to its original shape and size after the deforming force is removed. Hooke’s law: This law states that: “Within elastic limit, extension is directly proportional to tension (force)” OR “Within elastic limit, the extension is directly proportional to tension, provided that the elastic limit is not exceeded”. Elastic limit  This is the limit at which beyond this Hooke’s law does not obeyed and the material cannot return to its original shape and size. Example at point AY At point B = Plastic deformation Here the materials undergo permanent deformation under the action of external force. At point C = Breaking point At this point, when external force is applied the materials will break. NOTE: From Hooke’s law Force applied (F) α Extension (X) of the spring F α X F = KX Therefore; Where by K = Elastic force constant and its SI-Unit is N/m F = Tension (Force) and its SI-Unit is Newton (N) X = Extension and its SI-Unit is metre (m) APPLICATION OF ELASTICITY IN REAL LIFE  At home: Elasticity is applied in:-  Rubber gasket that seal the refrigerator door  Clothing  Spring in furniture  Rubber bands that hold things together  Toys like balloons and balls.  In transport: Elasticity is applied in:-  Rubber tyres, hoses, belts and shock-absorbing springs for cars and trucks.  Airplane wings  Support cables for bridges K = F X
49  In industry: Elasticity is applied in:-  Steel beams used in construction  Conveyor belts  Measuring weight  Insulation against vibration and sound  Mechanical control devices 1. ADHESION AND COHESION Adhesion (adhesive force)  This is the force of attraction between molecules of different substances. Example: Water to glass molecules Cohesion (cohesive force)  This is the force of attraction between molecules of the same substance. Example: Water to water molecules  Definite shape of solid is due to strong cohesive force among the molecules. So water molecules can experience cohesion among themselves while water molecule and the glass molecules will experience adhesion. Consider the figures below which show the shapes of meniscus of liquids. When reading the volume of the above liquids;  For mercury, the top of the meniscus is read.  For water, the bottom of the meniscus is read. The formation of the meniscus in a liquid is due to forces of cohesion between the liquid and the wall of the container.  The meniscus of water curves upward forming a concave shape and the meniscus of mercury curves downward forming a convex shape  A drop of mercury does not wet glass because the cohesive force is greater than the adhesive force.  A drop of water or methanol wet grass because the adhesive force is greater than the cohesive force between the molecules.
50 APPLICATION OF ADHESION AND COHESION a) To stick two different objects together. Here we use the adhesive effects of tape or glue. b) Adhesion can also be used to remove harmful materials such as bacteria from drinking water. c) Cohesion assists in transport of water in plants and animals by allowing one molecule to pull others along with it. d) The bodies of plants and animals also use the cohesion of tissue to repair damage. e) Ink sticks on paper because of adhesive force between the paper and ink. 2. SURFACE TENSION  This is the ability of the surface of the liquid to behave like a fully stretched elastic skin. Consider figure below which show surface tension supporting an object. NOTE:  Cohesive forces are responsible for the surface tension of the liquid. Surface tension enables insects such as water striders and mosquitoes to walk on water; it also allows small objects, even metallic ones such as needles and razor blades to float on the surface of water.  When detergent is added to the water, the metal object sinks. This is because the detergent decreases the surface tension of the water.  Detergents are examples of surfactants. Surfactant  This is the substance that reduces the surface tension of a liquid. NOTE: The term surfactant is an acronym for surface active agent. FACTORS AFFECTING THE SURFACE TENSION OF ANY LIQUIDS These factors are: a) Nature of the liquid Different liquids have different surface tension. For example, mercury has a higher surface tension than water. b) Contamination Impurities in a liquid lower the surface tension.