Скачать или смотреть 7 Motion & Measurement of Distances CBSE Class 6 Science

Motion & Measurement of Distances
An object is said to be in motion or moving, when it changes its position with time. For example, a moving car at position A in front of a house at a particular time, will be at position B after 5 seconds which is little far from the house.
If an object moves fairy fast, then its movement or motion can be observed easily. But if it moves very slowly, then it becomes difficult to observe its movement immediately. For example, if we see a wristwatch, it has seconds hand, minutes hand & hour hand. Now, the seconds hand moves quickly, so we can observe the movement of seconds hand easily. We will have to keep observing the position of minutes hand and hours hand for quite some time to find out their movement.
In, science, when an object is stationary or not moving, it is said to be at rest. Thus, a house and tree for example are said to be at rest.
Types of motion
The motion of all the objects are not of the same type. Different objects show different types of motions. Some important types of motion are:
i. Rectilinear motion
ii. Circular motion
iii. Rotational motion and
iv. Periodic motion
Rectilinear motion
Motion in a straight line is called rectilinear motion. For example, when a bullet is fired from a gun, the bullet moves in a straight line. So, the movement of a bullet fired from a gun is an example of rectilinear motion. The motion of a cyclist running on a straight road is rectilinear motion. Motion of a train on a straight track is also a rectilinear motion. The motion of a falling stone is also a rectilinear motion.
Rectilinear motion takes place in a fixed direction.
Circular motion
A round path having the shape of circle is called circular path. When an object moves along a circular path, it is called circular motion. For example, the moon moves around the earth in a circular path, so the movement of moon around the earth is an example of circular motion. Similarly, the movement of earth around sun, and also, the movement of the other planets around sun is also in a circular motion.
Rotational motion
When an object turns or spins about a fixed axis is called rotational motion. For example, a spinning top moves round and round on its axis, so top is an example of rotational motion. Our earth also spins along its axis, so earth is an example of rotational motion. The other example of rotational motion is rotation of windmill, turning of a potter’s wheel, turning of a bicycle wheel.
Here it is important to note that the difference between circular motion and rotational motion is that in circular motion an object as whole travels along a circular path but in rotational motion, the object spins on its axis.
Periodic motion
The motion which repeats itself after regular intervals of time is called periodic motion. The seconds hand of a watch takes the same time to complete each round. So, the motion of seconds hand of watch is an example of periodic motion.
Similarly, the revolution of earth around the sun is a periodic motion. Because the earth always takes the same amount of time i.e. 1 year to complete one round around the sun.
Also, revolution of moon around the earth is also a periodic motion.
A pendulum consists of a small metal ball called bob suspended by a long thread from a rigid support, such that the bob is free to swing back & forth. The movement of a pendulum from 1 point to another and back is also a periodic movement. Similarly, a swing also shows periodic motion.
The musical instruments such as sitar or guitar have stretched strings which produce sound when plucked. Also, at the same time the string produces a to & fro vibration and produce a periodic motion.

Objects having more than one type of motion
Friends some of the objects may have combination of 2 or more different type of motion. This will become clear from the following examples.
i. Earth – earth moves around the sun, so it has circular motion and earth repeats its motion around the sun after regular intervals of time, so it has periodic motion too. Also, earth rotates on its axis, so it has rotational motion. In this way earth has 3 types of motions at the same time.
ii. Merry-go-round - If we consider merry-go-round as a whole, then it shows rotational motion because it turns on an axle. But the outer part of merry-go-round shows circular motion. In this way merry-go-round has 2 types of motions at the same time.
iii. Bicycle – suppose a bicycle is moving on a straight road. The wheel of bicycle is rotating on its axle and also, moving forward in a straight line. So, a bicycle has rotational motion and rectilinear motion at the same time.
iv. Sewing machine – The sewing machine has a wheel which rotates on an axle. So, the wheel of a sewing machine shows rotational motion. The sewing machine also has a needle which moves up and down continuously. The needle of a sewing machine undergoes a periodic motion. In this way sewing machine has 2 types of motions at the same time.