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Water
Water is a chemical substance with the chemical formula H2O. A molecule of water contains one oxygen and two hydrogen atoms connected by covalent bonds. Water exists in nature in all the three states. Water exists as vapour in air, as liquid in rivers, sea and under the ground and as solid in the polar regions and as snow on the mountains. Water covers about three-fourth of the earth's surface. Only 2.5% of the water available on the earth is fresh water and suitable for drinking, rest is in the form of oceans as salty water.
Water Sources
The various sources of water are:
Surface water: Water is available on the earth's surface in the form of oceans, rivers, lakes, etc.
Note: About 70.9% of earth's surface is covered by water.
Ground water: The rain water gets collected deep inside the layers of the earth as groundwater. Traditionally, wells were used for utilizing the ground water. Now a days groundwater is pumped with the help of power operated tube wells.
Rainwater: Rainfall is the primary source of water. It replenishes the surface water and groundwater..
Scarcity of Water
The reasons for the scarcity of water are:
Pattern of Rainfall: The rainfall occurs mainly in rainy season. Therefore, there is scarcity of water in the other seasons.
Loss of Water: All the rain water that falls on the ground cannot be utilized and is lost due to evaporation and runs off the land into streams.
Overuse: Overuse of water is another reason for its scarcity.
Pollution: Pollution of water bodies is one of the main causes of scarcity of water.
Deforestation: Deforestation of hill slopes and other areas leads to excessive runoff of water. In addition to this, negligence and non-proper maintenance of old water tanks, ponds and wells leads to water shortage
What is the chemical formula of water?
(a)
(b)
(c)
(d)
(e) None of these
Answer: (b)
In which one of the following states water is present on earth?
(a) Solid
(b) Liquid
(c) Gas
(d) All of these
(e) None of these
Answer: (d)
Water is a chemical substance with the chemical formula.
A water molecule consists of one oxygen and two hydrogen atoms.
Only 2.5% of the water available on the earth is fresh water.
Approximately 70% of the fresh water used by human beings is for agriculture.
Time
Time is used to measure the duration in which things occur, and interval between two occurrences. Time give us the idea of the duration of the occurrence of a certain event. For example, repetition of the day, night and seasons, phases of the Moon, appearance of certain star in the sky etc.
In ancient time, people observed stars and constellations for such estimations. Appearance of the certain star in the sky was the indicator of new season. But this way of estimation was very inconvenient as stars and constellations appear only at night and could not be seen at cloudy nights. Therefore, they started to develop new systems to measure the day and night durations.
Sand Clock
Sand clock is one of the instruments, which was used earlier to measure the time duration. Sand clock consists of two glass bulbs placed one above the other and are connected with the help of a narrow glass tube. Sand flows from the top to the bottom. When the top bulb gets empty, clock is inverted so that lower glass tube comes up and sand clock keeps on working. The sand takes a fixed duration of time to flow from one glass tube to other. In this way, earlier people used to measure time duration of an event.
In the modern time we use clocks to know the duration of time. Clock uses second as a unit; 60 second is equal to 1 minute and 60 minute to one hour.
Modern basis of measuring time duration
60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
365 days = 1 year
10 years = 1 decade
10 decades = 1 century
10 century’s = 1 millennium
Velocity
Velocity is a physical quantity that is used to show the direction in which object is moving, as well as speed of the object. Thus velocity gives more close information about the position of the object. Velocity is a vector quantity as it has both direction and magnitude.
Uniform Velocity
When an object moves on a straight path with a constant speed and does not change direction, the object is said to be in the uniform velocity. When an object moves on a circular path, it might have uniform speed but not uniform velocity, as an object has to change its direction continuously to move on a circular path.
In the above figure, the car is moving in east direction with a constant speed. Therefore, it is said that the car is moving with uniform velocity.
Non-uniform velocity
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Distance and Displacement
Distance and displacement are two different physical quantities. Let us study them in detail
Distance
When an object moves from one place to other, it covers some distance. Distance is the total length of the path on which the object move. Distance does not shows the direction. It just represents the length, which an object has traveled. Therefore, distance is a scalar quantity.
Displacement is the distance between initial point from where object started to move and the final point where the object stopped. Displacement shows finally the object in which direction and at how far from the initial place sifted. Therefore displacement shows direction as well as distance. And it is a vector quantity.
Speed
When a body moves, it covers some distance. How much distance does it cover in a given time or how long will it take to cover a particular distance? Such questions arise in our day to day life. To solve such queries a term "speed" is used. Speed is related to time and distance. With the help of speed we can find the distance or the required time to cover the particular distance. The standard unit of the time is second. The distance covered by an object in a unit time is called speed. Speed is a scalar quantity as it does not tell about the direction.
\[\text{Speed}\,\,\text{=}\,\,\frac{\text{Distance}}{\text{Time}}\]
Uniform Speed
If an object does not change its speed while moving, the object is said to be in uniform speed.
Look at the following figure
Speed of the car does not vary in different interval of time. Therefore speed of the car is uniform.
Graphical representation of uniform speed.
Time distance graph of a body, which has uniform, speed, is a straight line
Non-uniform Speed
If an object changes its speed while moving, the object is said to be in non uniform, speed.
In the above figure, speed of the car varies at different interval of time. Therefore, speed of the car is non-uniform.
Graphical representation of non-uniform speed.
Distance-time graph of a body, which has non-uniform speed, is a curved line.
Which one of the following is true for the uniform speed
(a) If speed of the object varies at different interval of time, the object has uniform speed
(b) If speed of the object is constant at different intervals of time, the object has uniform speed
(c) If a moving object suddenly comes to rest, the object has uniform speed
(d) All of these
(e) None of these
Answer: (b)
A cyclist covers 25 km in first hour, 30 km in the second hour and 35 km in the third hour. The cyclist is in _____.
(a) Uniform motion
(b) Non uniform motion
(c) Cannot be find that the body is in uniform motion or in non uniform motion
(d) All of these
(e) None of these
Answer: (b)
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