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Practical Geometry
We have learnt the technique of measuring line segments and comparing two line segments by observation, by tracing and by using a divider.
In practical geometry, we study geometrical constructions.
The word 'construction' in geometry is used for drawing a correct and accurate figure from the given measurements. In this chapter, we shall learn how to draw some plane figures using the instruments; ruler, set-square, protractor, compass.
Let us first know something about these geometrical instruments.
I: Ruler/Straight-Edge/Scale:
The straight-edge is graduated into centimeters along one edge and inches along the other edge.
It is used to draw line segments and measure their length.
II: Set-Squares:
A pair of set-squares has two instruments (triangular in shape) in the geometry-box.
In one, the angles are 30°, 90°, 60° and in the other, the angles are 45°, 45°, 90°.
III: Protractor:
To measure the number of degrees in an angle, an instrument called protractor is used.
It is graduated into 180 equal divisions; each division represents one unit measure of an angle, called a degree. Thus, the protractor is marked from 0 degree up to 180 degrees (i.e. 0° to 180°) in both directions and the straight edge of the protractor lies along the line segment joining the 0° mark and 180° mark.
IV: Compass:
A compass has a pointer (i.e. metal-end) on one end and a pencil holder on the other.
It is used to mark off equal lengths, but does not measure them. It is also used to draw arcs and circles.
V: Divider:
A divider has a pair of pointers. It is used to compare lengths.
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