The postman brings letters for the mother and father 

1.  Agriculture has changed life of human beings drasticaly.  Early men used to live nomadic life, they were hunter-gatherers but after the development of agriculture or farming, humans started forming settlements.
2.  In these settlements, people began farming, building permanent shelters  like mud houses or thatched houses, making better tools and clothing.
3.  The surplus production of food grains helped humans to concentrate on other activities like trade and commerce,craftsmanship,teaching,builders and administration etc.
4.  They also found important uses of animals. They started taming or domesticating sheep, dogs, goats, and cow etc. They used the animals for various purposes like to carry heavy burdens over long distances,for wool,milk and security purposes etc.
5.   Each of these changes made humans life more stable and comfortable.They started evolving into more civilised and advanced humans.

Ex 8.1 Class 8 Maths Question 1.
Find the ratio of the following:
(a) speed of a cycle 15 km per hour to the speed of scooter 30 km per hour.
(b) 5 m to 10 km
(c) 50 paise to ₹ 5
Solution:
(a) Speed of cycle : Speed of Scooter = 15 km per hour : 30 km per hour
= 1530 = 12
Hence, the ratio = 1 : 2
(b) 5 m to 10 km
= 5 m : 10 × 1000 m [∵ 1 km = 1000 m]
= 5 m : 10000 m
= 1 : 2000
Hence, the ratio = 1 : 2000
(c) 50 paise to ₹ 5
= 50 paise : 5 × 100 paise
= 50 paise : 500 paise
ratio = 1 : 10

Ex 8.1 Class 8 Maths Question 2.
Convert the following ratios to percentages:
(a) 3 : 4
(b) 2 : 3
Solution:

Ex 8.1 Class 8 Maths Question 3.
72% of 25 students are good in mathematics. How many are not good in mathematics?
Solution:
Number of students who are good in mathematics = 72% of 25

Number of students who are not good in mathematics = 25 – 18 = 7

For more click on the given links:

• <a href="http://mycbseguide.com/blog/ncert-solutions-class-8-maths-exercise-8-1/">Comparing Quantities Exercise 8.1</a>
• <a href="http://mycbseguide.com/blog/ncert-solutions-class-8-maths-exercise-8-2/">Comparing Quantities Exercise 8.2</a>
• <a href="http://mycbseguide.com/blog/ncert-solutions-class-8-maths-exercise-8-3/">Comparing Quantities Exercise 8.3</a>

The air in the atmosphere is mixture of many Gases

 The air around us is a mixture of gases, mainly nitrogen and oxygen, air also contains very smaller amounts of water vapor, argon, and carbon dioxide, and very small amounts of other gases, suspended dust, spores, and bacteria.

15.5% of Rs 3000 =
                            3000 x 15.5
                             100
                         = 465

Therefore, 15.5% of Rs 3000 = Rs 465

Roman Empire

• The ancient Roman empire which was spread across the three continents namely – Europe, Asia and Africa.
• To the North, the boundaries of the empire were formed by two great rivers – the Rhine and the Danube.
• To the South, by the huge expanse of desert called the Sahara.
• To the East river Euphrates and to the West Atlantic Ocean.
• This vast stretch of territory was the Roman Empire. That is why Roman Empire is called an Empire across Three Continents.
• The Mediterranean Sea is called the heart of Rome’s empire.

A journey by boat is always very pleasant. It has a charm of its own. Rowing a boat provides a lot of fun. Last summer I had the opportunity to go to Mauritius for about a week. We stayed in a hotel, which was within walking distance from the sea.


On the full moon day, my father hired a boat for two hours. The boat was quite spacious though not very big. All of us—my father, mother, sister, younger brother and myself, got into the boat.

It was dusk. The sun had just set. The scenery was very beautiful. There were not many boats in the sea. I wanted to row but my father was afraid to let me do that. The boatman came to my help. He said that he would take care that I did not cause any harm to the boat or to myself. In the end, my father .
I sat in the place of the boatman and had the joy of rowing for almost ten minutes. Imagine my thrill and pride when I was able to row the boat and keep it on course. Of course, its speed was reduced. My brother envied me.

Soon the moon was shining brightly. The tall coconut trees silhouetted against the sky made a beautiful scene. The lights of the nearby houses and of the bridge threw beams of silver in the sea. The sky was studded with stars. The view was breathtakingly beautiful.

Time flew and very soon the two hours were over. The boatman brought us back to the shore. We enjoyed every single minute of the boat ride. It was really wonderful. The memory still mesmerizes me.

A n s w e r : Socialists, Catholics, and Democrats supported the Weimar Republic.

Socialists, Catholics, and Democrats were the supporters of newly formed Weimar Republic. These supporters were also called 'November Criminals'.

Given polynomial: 

Product of zeroes of this polynomial = 4.

A polynomial is an expression of more than one or two algebraic term, may be a sum of those terms also.

We know, that the product of zeroes of a polynomial  can be given by =c/a

Hence, for the polynomial: , a = a, b = -6 and c = -6.

Product of zeroes of this polynomial = (-6)/a

4 = -6/a

a =  -6/4= -3/2

Double fertilization : It occurs when one male nucleus fertilizes (fuses) with egg cell to form zygote cell and other male nucleus fuses (fertilizes) with two polar nuclei to cause triple fusion. As these two types of fertilizations take place at the same time in the ovule of the plant, it is known as double fertilization.
After fertilization:
(i) Ovary develops into fruit.
(ii) Ovules develop into seeds.

Double fertilization : It occurs when one male nucleus fertilizes (fuses) with egg cell to form zygote cell and other male nucleus fuses (fertilizes) with two polar nuclei to cause triple fusion. As these two types of fertilizations take place at the same time in the ovule of the plant, it is known as double fertilization.
After fertilization:
(i) Ovary develops into fruit.
(ii) Ovules develop into seeds.

 Medium of exchange: The primary function of money is to act as a medium of exchange between two parties involved in a transaction. It avoids the practical problems of wastage of time and resources involved in the barter system of exchange and it improves the efficiency of the transaction. It promotes allocation efficiency in the trade and production of goods and services. In case of the barter system, the sale and purchase of goods occurs at the same time. Their sale and purchase value also remains equal at that point. A person can purchase or sell goods with cash without selling or purchasing any good at that point, after money came into existence. Thus, the act of purchase and sale has been separated. Thereby the medium of exchange facilitated sale and purchase very easily in terms of monetary value. For example, a fruit seller wants to sell his fruits to buy wheat. In the absence of money, he will have to look for some person who wants to sell wheat and buy fruits. This is not always easy and possible. However, with money as a medium of exchange, the fruit seller has to just find a buyer for his fruits. When fruits are exchanged for money, he can purchase wheat from the market.

(i) Change in colour: Reaction between lead nitrate solution and potassium iodide solution.

Pb(NO₃)₂(aq)+2KI → PbI₂(s)+2KNO₃(aq)

In this reaction, colour changes from colourless to yellow.

(ii)Change in temperature: Action of dilute sulphuric acid on zinc.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂

In this reaction, heat is evolved

(iii) Formation of precipitate: Action of barium chloride on sodium sulphate.

BaCl₂(aq) +Na₂SO₄(aq) →  BaSO₄(s) +2NaCl(aq)

BaSO₄(s) - ppt

In the History of Ideas, there are two major concepts of justice :

1. Numerical Concept of Justice: It gives an equal share to all. Bentham said, “Everyone is to count for one, nobody for more than one”. It means even unequal would be treated as equal. Modern liberal democracies are also based on this principle. Rich or poor, illiterate or literate, employed or unemployed all have the right of a single vote.
2. Geometric Concept of Justice: Plato and Aristotle favored this concept of justice. It is a concept of proportionate equality. It means an equal share to equals and unequal to unequal. According to Aristotle flutes should be distributed only among those who have the capacity for flute playing. Numerical justice is sometimes called democratic justice and geometric justice is equated with aristocratic justice.

Proportionate justice:

• This principle indicates rewarding people in proportion to the scale and quality of their effort.

• It is just to reward different jobs differently on the basis of efforts and skills required and the danger involved.

• Thus, proportionality provides balance to the principle of equal treatment.

• The reward and compensation for a surgeon and an architect varies according to the skill that is required in their job.

A n s w e r :

 Bleaching powder is formed.

(i) It is used as bleaching agent in paper and textile industries. 

(ii)It is used as disinfectant in purification of drinking water.

1. Length of conductor: The resistance (R) of a conductor is directly proportional to its length (l).
R∝l
If we increase the length of a conductor, its resistance increases since only less amount of current can pass through it. If we reduce the length of a conductor, the resistance decreases since more current can pass through it.
2. Area of cross-section: Area of the cross-section is inversely proportional to the resistance.
R∝1 / A
If we double the area of cross-section, the resistance will be half. So, electricity can. That's why we are using thick wires for the heavy electrical loads.
3. Nature of material: Conductors offer less resistance compared to insulators. That’s why we are using plastic materials for electrical insulation purposes like switches, coatings etc.
Effect of temperature: Resistance of pure metals are showing a direct relationship with temperature. But generally, alloys like nichrome won’t show that much change in resistance with respect to the temperature. The resistance of semiconductors decreases with increasing temperature.
From these conclusions, we can conclude that,
R∝l / A
The proportionality constant is known as resistivity (ρ).
R=ρ l / A
Resistivity is a measurable property that quantifies how strongly a given material opposes the electric current flow. It depends upon the dimension of the material. It is also known as specific electrical resistance. It is inverse to conductivity. Its unit is Ω
If we are considering unit measurements, we can say that; the electrical resistivity of a material is the resistance of the material per unit length and per unit cross-sectional area at a specified temperature.

 

A n s w e r:

The resistance of the conductor depends on the following factors:

• The temperature of the conductor
• The cross-sectional area of the conductor
• Length of the conductor
• Nature of the material of the conductor

What were the things being taken for granted by the people of Alsace?
(a) Teachers of the school
(b) Time and school
(c) People around
(d) Money and power

Answer: (b) Time and school

In Pakistan, General Pervez Musharraf led a military coup in October 1999. He overthrew a democratically elected government and declared himself the ‘Chief Executive’ of the country. Later he changed his designation to President and in 2002 held a referendum in the country that granted him a five year extension.