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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.


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.