Water is the most important substance needed for survival of living beings. Living beings cannot live long without water. Water is available in plenty on earth. More than three-fourth of the earth’s surface is covered with water in the form of seas, rivers and lakes. It is also found inside the earth’s crust Most of the water that we get from the wells comes from this source.
The sources of food are broadly classified into two groups - plants and animals. Humans consume everything from fruits, flowers and even the stem of some plants. Leaves and stem like lettuce, celery, etc; roots of some plants like carrots, beetroot, etc. and seeds like wheat, rice, etc. All food come from plants as even the animals depend on plants. Hence, the source of all food in this world are plants, directly or indirectly.
The passing down of similar characters generation after generation is termed heredity. Heredity is controlled by genes. Differences in gene combinations lead to variations or differences even among members of the same family. The science of heredity and variation is termed Genetics.
The nervous system and the endocrine system ensure that the body works in a controlled and coordinated manner. The nervous system includes the brain, spinal cord, sense organs and nerves while the endocrine system operates through certain chemicals called hormones which are produced by specialized glands and are secreted directly into the blood. The nervous system works with the endocrine system to communicate, integrate and coordinate the functions of various organs and systems in our body.
The activities by which living organisms take in food, derive energy, remove waste from their body and respond to changes in the environment are called life processes.
Biodiversity is the enormous variety of organisms is termed biodiversity (bios means life and diversity means variety). There is diversity in size among organisms and in complexity. For example, bacteria are simple single celled organisms and humans are made of a trillion cells and are highly complex.
Some objects such as sun, stars, burning candles, lamp, etc. which emit light by their own are called luminous objects. Some other objects may bounce back a part of the light falling on them from any luminous object. This bouncing back of light after falling on any surface is called reflection of light.
Temperature is a measure of the degree of hotness of an object. Thermometer is a device used for measuring temperatures. Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes, we use the laboratory thermometers. The range of these thermometers is usually from -10°C to 110°C. The normal temperature of the human body is 37°C.
A plastic comb when brought near a piece of paper does not pick up small pieces of a paper. But if you comb your dry hair and bring the comb close to a small piece of paper, you will notice that the bits of paper are attracted towards the comb. This happens because the comb gets charged or electrified when you comb your dry hair.
According to Newton’s first law of motion the application of an unbalanced force brings a change in the velocity of an object. Thus, the force can produce a change of momentum. Newton’s second law of motion establishes a relationship between force and change in momentum.
The force required to stop a moving body depends upon its mass. Suppose two balls of same mass are moving with different velocities. Which ball will need more force to stop? The faster moving ball require more force to stop it. Thus, the force required to stop a body also depends upon its velocity.
Consider the motion of an object along a circular track of radius R through four points A, B, C and D on the track. If object completes each round of motion in same time, then it covers equal distance in equal interval of time and its motion will be uniform motion.
Speed during a certain interval of time can not be used to determine total distance covered in given time of the journey and also the time taken to cover the total distance of journey. It is because a body does not always travel equal distance in equal interval of time.
For a moving object two points are significant. One is the point of start or origin where from the object starts its motion and the other is the point where it reaches after certain interval of time. Points of start and destination are connected by a path taken by the object during its motion.
If you observe a moving bus, you will notice that the position of bus is changing with time. This means that the bus is in motion. Now suppose you are sitting in a bus moving parallel to another bus moving in the same direction with same speed. You will observe that the position of the other bus with respect to your bus is not changing with time. In this case the other bus seems to be at rest with respect to your bus.