Drawing Basic Shapes
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Compass, scale, sets square ( 30 – 60, 45-45), protractor are used to draw basic geometrical shapes.
We can use compass and scale to draw triangle. Various steps to draw triangles are shown in the figure. Sum of all three angles of triangle is 1800. Area of Triangle = base ´ height / 2
Quadrilateral has four sides. Sum of angles of quadrilateral is 3600. Square has all four sides of same length. Rectangle has two opposite sides of same length. Square /rectangle is drawn as follows:
3) Regular Pentagon
Following are the steps in drawing pentagon of five sides of equal length. For e.g lets draw a pentagon of 5 cm in length: 1. Draw a line AB of 5 cm. 2. Draw a circle of 5cm radius by taking A as a center. Then draw another circle with B as a center. Both circle will cross each other at point X and Z. Join length XZ. 3. Now take distance ZA in compass and draw circle with Z as a center. The circle will cut other circles at point S and R. 4. Now draw line RC and line SE passing through point Y. 5. Using compass ,mark arc of length 5 cm from point E and C. The arc will cross each other at point D. 6. Now join all points A B C D E, to form a pentagon.
Hexagon has six sides. Draw hexagon of equal side.
1. Draw a circle of radius equal to side of hexagon. 2. Put the compass anywhere at the edge of a circle, call that point as A. 3. Draw arc of circle cutting circle at point B and F. Now put compass at point B and mark point C using compass. Similarly point D and E are marked. 4. Join all points using scale to get a hexagon.
Area of geometric shapes
Area of regular shape is calculated using standard mathematical formula.
Use following formula to calculate area of triangle. Area = ½ ´ base ´ height
Area of quadrilateral = Length ´ breadth
Area of circle = p ´ R 2 ( R = Radius )
Area of irregular shape
1) Graphical Method
In regular practice, we need to measure irregular shapes of land, construction area, material etc. Graphical method is used to calculate area of such irregular shapes. Complete square ——— 71 ´ 1 = 71 75% square ——— 10 ´ 0.75 = 7.5 50% square ——— 5 ´ 0.5 = 2.5 25% square ——— 8 ´ 0.25 = 2.0 Total square is = 83 This means total area of land on the graph is 83 Sq cm. If we multiply it with the scale we will get actual area of land. 2) We can also calculate the area of irregular shape by drawing many big size squares and triangles into the drawing. We can add area of all these squares and triangles to get area of the irregular shape.
i) Match the following: A B Area of triangle Length ´ breadth Area of rectangle p ´ R 2 Area of circle ½ ´ base ´ height
What you have learnt
In this lesson, We read about standard symbols and lines used in engineering drawing. Now we are able to select appropriate scale for drawing. We have also learned to draw different geometrical shapes and areas Calculation, of regular and irregular geometrical shape aslo.
1. If an MS bar of 2 mm thickness has ‘L’ shaped cross-section as shown in the figure. Calculate its cross-sectional area in cm2.
2 Draw the following i) Equilateral triangle of 10 cm length ii) Square of 15 cm in length iii) Rectangle of length = 12 cm , breadth = 10 cm iv) Circle with radius = 7 cm 3 Draw symbols for the following i) River ii) Road iii) Rail iv) Hidden line v) Center line vi) Electric switch
Answer to intext Questions
1.1 Scale = 1 cm : 3 m 1.2 i) ½ ´ base ´ height ii) Length ´ breadth iii) P ´ R 2
1) On A3 size (11.693 ´ 16.535 inches) drawing sheet - Draw the typical symbols used in all four forms of drawing – Engineering Drawing, Geographical Maps, Contours & Graphs and Line types. 2) Find area of a field/garden. 3) Find top surface area of a percolation tank.
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