Current Affairs 8th Class

  Synthetic Fibres and Plastics   ·                     Fibres that are obtained by chemical processing of petrochemicals are called synthetic fibres. Like natural fibres, these fibres can also be woven into fabrics. ·                     Synthetic fibres and plastics are made of very large units called polymers. ·                     Polymers are made up of many smaller units called monomers. ·                     Synthetic fibres have various uses in our day to day life. They are used in household articles like ropes, buckets, furniture, containers, etc. Apart from items of household uses, they are also used in aircrafts, ships, spacecraft?s, healthcare, etc. ·                     Rayon, nylon, polyester and acrylic are some of the main types of synthetic fibres. ·                     The properties of rayon are similar to that of silk. Due to this rayon is also called as artificial silk. ·                     The different types of fibres differ from one another in their strength, water absorbing capacity, nature of burning, cost, durability, etc. ·                     The material which gets decomposed through natural processes, such as action by bacteria, is called biodegradable. ·                     The material which is not easily decomposed by natural processes is termed as non-biodegradable. ·                     Plastic becomes one of the most important things of our life. We can see lots of things made up of plastics at home, or outside, everywhere. ·                     Plastics can be classified mainly into two types ?

(i) Thermoplastics and

(ii) Thermosetting plastics.

·                     Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Polyethylene, polypropylene, polystyrene and polyvinyl chloride are some of the examples of thermoplastics. ·                     Thermosetting plastics are the plastics that can melt and take shape once; after they have solidified, they stay solid. They undergo chemical change in their composition when heated and cannot be molded again and again. Polystyrene, polyisoprene, rubber are examples of thermosetting polymers. ·                     Plastics release poisonous gases on burning. On dumping in the ground they may take years to get decomposed. They are non-biodegradable in nature. ·                     We need to use synthetic fibres and plastics in such a manner that we can enjoy their good qualities and at the same time minimise their environmental hazards. ·                     Be a responsible citizen and always remember the 4 R principle - Reduce, Reuse, Recycle and Recover to minimise the pollution which more...

  Crop Production and Management   ·                     Crop production and management plays a key role in our country's economy. By adopting advanced and latest technologies in agriculture, enough yields of crops can be grown to provide food to our growing population. ·                     The plants of same kind grown and cultivated at one place on a large scale are called a crop. ·                     There are mainly two types of crops grown in India - rabi and kharif crops. ·                     Rabi crops are the agricultural crops that are sown in winter and harvested in the spring. Wheat, Gram, Pea, Mustard, Linseed, Barley, Peas are the examples of rabi crop. ·                     Kharif crops are referred to those crops that are sown in the rainy (monsoon) season. These crops are planted for autumn harvest and may also be called the summer or monsoon crop. Paddy, cotton, maize, sugarcane are some examples of the kharif crops. ·                     For growing a good crop, it is necessary to prepare soil by tilling and livelong before sowing the seeds. Ploughs and levellers are used for this purpose. ·                     Selection of good quality seeds is another important step of growing a good crop. The seeds should be sown at appropriate depth and at enough distance from one another to avoid competition for nutrients. ·                     The use of organic manure and fertilizers helps in replenishing the quality of soil. Use of chemical fertilizers is commonly used in present time. The use of chemical fertilizers in excessive amount is also harmful for environment. ·                     Water is important for proper growth and development of plants. The supply of water to crops at different intervals is called irrigation. Plant roots absorb water and mineral from the soil. Along with water, minerals and fertilisers are also absorbed. The time and frequency of irrigation varies from crop to crop, soil to soil and season to season. ·                     Moat, Chain pump, Rahat and Dhekli are the traditional methods of irrigation. At present irrigation is done by pumps and tube wells run on electricity, by sprinkler system and by drip system. ·                     Sprinkler system and drip irrigation methods also minimise the loss of water. ·                     Some unwanted plants called weeds also need to be removed from the field from time to time as they compete with the main crops for nutrients. The practice of removing of weeds is also called weeding. It can be done by using weedicides like 2, 4 -D. ·                     The cutting of the mature crop manually or by machines is called harvesting. ·                     The process of separating out grains more...

  Number System   Learning Objectives

• Properties of Rational Numbers
• Square and Square Roots
• Cube and Cube Roots
• Playing with Numbers
• Divisibility Test

  Properties of Rational Numbers Rational numbers are the numbers that can be expressed in the form \[\frac{p}{q}\], where p and q are integers and \[q\ne ~0\] . The collection of rational numbers is denoted by Q. These rational numbers satisfy following laws or properties

•  Rational numbers are closed under addition, subtraction and multiplication. If a, b are any two rational numbers, then the sum, difference and product of these rational numbers is also a rational number, thus we can say that rational numbers satisfy the closure law.
• Rational numbers are commutative under addition and multiplication. If a, b are rational numbers, then:
• Commutative law under addition: \[\mathbf{a}+\mathbf{b}=\mathbf{b}+\mathbf{a}\]
• Commutative law under multiplication: \[\mathbf{a}\times \mathbf{b}=\mathbf{b}\times \mathbf{a}\]

 

• Rational numbers are associative under addition and multiplication. If a, b, c are rational numbers, then:
• Associative law under addition: \[\mathbf{a}+\left( \mathbf{b}+\mathbf{c} \right)=\left( \mathbf{a}+\mathbf{b} \right)+\mathbf{c}\]
• Associative law under multiplication: \[\mathbf{a}\left( \mathbf{bc} \right)=\left( \mathbf{ab} \right)\mathbf{c}\]
• 0 is the additive identity for rational numbers.
• 1 is the multiplicative identity for rational numbers.

 

• The additive inverse of a rational number \[\frac{p}{q}\] is \[\left( -\frac{p}{q} \right)\], and the additive inverse of \[\left( -\frac{p}{q} \right)\] is\[\frac{p}{q}\].
• If \[\frac{a}{b}\times \frac{p}{q}=1\], then \[\frac{a}{b}\] is the reciprocal or multiplicative inverse of \[\frac{p}{q}\] and vice versa.
• For all rational numbers p, q and r; p (q + r) = pq + pr and p (q - r) = pq - pr is known as the distributive property.

  Example 1. Find the multiplicative identity of the rational number \[\frac{\mathbf{455}}{\mathbf{1024}}\] (a) \[\frac{1024}{455}\]                (b) \[\frac{1}{455}\] (c) \[\frac{1}{1024}\]                (d) 1 (e) None of these Answer: (a)   2. Which one of the following rational numbers lies between \[\frac{\mathbf{45}}{\mathbf{78}}\] and \[\frac{\mathbf{26}}{\mathbf{52}}\]? (a) \[\frac{75}{156}\]               (b) \[\frac{84}{156}\] (c) \[\frac{95}{156}\]               (d) \[\frac{105}{156}\] (e) None of these Answer: (b) Explanation: On equating the denominator, the given rational number reduce to \[\frac{90}{156}\] and \[\frac{78}{156}\] and the rational number lying between these two is \[\frac{84}{156}\].   Square and Square Roots If a natural number m can be expressed as \[{{n}^{2}}\], where n is also a natural number, then m is called the square root of a square number, \[{{n}^{2}}\]. For example, 1, 4, 9, 16, 25 are the square numbers. Some interesting properties of square numbers are as follows:

• All square numbers end with 0, 1, 4, 5, 6 or 9 at unit's place.
• No square number ends with 2, 3, 7 or 8 at unit's place.
• When a square number ends with 6 at unit's place, the number whose square it is, will have either 4 or 6 in unit's place.
• There are 2n non perfect square numbers between the squares of the numbers n and (n + 1).
• If a natural number cannot be expressed as a sum of successive odd natural numbers starting with 1, then it is not a more...

  Geometry   Learning Objectives  

• Understanding Polygons
• Parallelogram
• Rhombus
• Trapezium
• Kite
• Rectangle
• Square
• Practical Geometry

  Understanding Polygons  

• A polygon is a simple closed curve made up of only line segments.
• A line segment that connects the two non-consecutive vertices of a polygon is called a diagonal.
• A convex polygon is defined as a polygon with all its interior angles less than \[180{}^\circ \]. This means that all the vertices of the polygon will point outwards, away from the interior of the shape.

 

• A concave polygon is defined as a polygon with one or more interior angles greater than \[180{}^\circ \]. It looks like a vertex has been 'pushed in' towards the inside of the polygon.

 

• A regular polygon is both 'equiangular' and ?equilateral?. For example, a square has sides of equal length and angles of equal measure and thus it is a regular polygon. A rectangle is equiangular but not equilateral, so it is a irregular polygon.

  Properties of Polygon  

• The sum of the measures of the external angles of any polygon is \[360{}^\circ \].
• For a regular polygon,
• Number of sides \[\left( n \right)=360{}^\circ \]/ (measure of an exterior angle)

Or  

• Number of sides \[\left( n \right)=360{}^\circ \]/ (\[180{}^\circ \]- measure of an interior angle)
• For a regular polygon,
• Measure of interior angle \[(\theta )=180{}^\circ -\left( 360{}^\circ /n \right)\]

Or

• Measure of interior angle \[(\theta )=180{}^\circ \left( \left( n-2 \right)/n \right)\]
• A polygon having three sides is called a triangle. Similarly a polygon having four sides is called quadrilateral, a polygon having five sides is called pentagon, a polygon having six sides is called hexagon.
• A triangle can never be concave.
• The sum of all interior angles of a triangle is \[180{}^\circ \].
• The sum of all interior angles of a quadrilateral is \[360{}^\circ \].

  Parallelogram A parallelogram is a quadrilateral whose opposite sides are parallel. Properties of a parallelogram

• Opposite angles of a parallelogram are equal.
• Adjacent angles of a parallelogram are supplementary i.e. their sum is equal to \[180{}^\circ \].
•  The sum of all interior angles of a parallelogram is \[360{}^\circ \].
• Opposite sides of a parallelogram are parallel (by definition) and so will never intersect.
• Diagonals of a parallelogram bisect each other.

  Rhombus A rhombus is a quadrilateral whose all sides are equal and opposite sides are parallel.   Properties of a Rhombus  

•  Opposite angles of a rhombus are equal.
•  Adjacent angles of a rhombus are supplementary i.e. their sum is more...

  Mensuration   Learning Objectives  

• Mensuration
• Visualizing Solid Shapes

  Mensuration   Mensuration is the branch of mathematics which deal with the study of geometric shapes, their area, volume and related parameters. Some important formulae of area and volume are listed below.   Area of some plane figures:   more...

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Figure	Shape	Area
Rectangle		\[a\times b\]
Square		\[\begin{align}   & {{a}^{2}} \\  & \frac{1}{2}{{d}_{1}}{{d}_{2}} \\ \end{align}\] Where \[{{d}_{1}}={{d}_{2}}=\sqrt{2}a\]
Triangle		\[\frac{1}{2}\times b\times h\]
Parallelogram		\[b\times h\]