Class 6 Science Chapter 7 Important Questions

We have to take certain precautions while measuring length by using a metre scale or ruler. These precautions are necessary to get the correct reading of length. The precautions to be taken while using a metre scale to measure length are as follows:
1.The scale should be placed parallel to the side of the object be measured. It should also be in touch with the object being measured.
2.In some scales, the ends may be broken. You may not be able to see the zero mark clearly. In such cases, you should avoid taking measurements from the zero mark of the scale. You can use any other full mark of the scale, say, 1.0 cm. Then you must subtract the reading of this mark from the reading at the other end. For example, the reading at one end is 1.0 cm and at the other end it is 14.3 cm. Therefore, the length of the object is (14.3-1.0) cm = 13.3 cm.
3.Correct position of the eye is also important for taking measurement. Your eye must be exactly in front of the point where the measurement is to be taken. Straight position is the correct position of the eye.

Note: From straight position the reading is 7.5 cm. And if we take diagonal or slantwise reading the readings may be different.

The various types of devices which are commonly used for measuring length are metre scale and measuring tape. Metre rod is made of metal and cannot bend at all. The rulers are made of plastic, wood or metal and also cannot bend. The measuring tape is, however, made of a flexible material which can bend easily around the object to be measured.
If the object is straight, we can use either a metre scale or a measuring tape to measure its length. As cloth merchant uses a metre rod for measuring cloth.
If the object is round, then we have to use a measuring tape for making measurement of length.

We cannot use an elastic measuring tape to measure length because an elastic measuring tape can stretch easily while taking measurement and hence giving wrong value or greater value of length of an object then its actual length. So, though a measuring tape is flexible but it is made of non-stretchable material.

The girth of a tree can be measured only by using a measuring tape because a measuring tape can be bent round the tree. The tailor also measures our chest and waist by using a measuring tape. The girth of tree, and chest and waist of a person cannot be measured by using a metre scale or meter rod. Remember, if the object is round then we have to use a measuring tape for making measurement of length.

We cannot measure the length of a curved line directly by using a meter scale. We can use a thread to measure the length of a curved line.
Use a thread to measure the length of the curved line AB. Put a knot on the thread near one of its ends. Place this knot on the point A. Now, place a small portion of the thread along the line, keeping it taut using your fingers and thumb. Hold the thread at this end point with one hand. Using the other hand, stretch a little more portion of the thread along the curved line. Go on repeating this process till the other end B of the curved line is reached. Make a mark on the thread where it touches the end B. Now, stretch the thread along a metre scale. Measure the length between the knot in the beginning and the final mark on the thread. This gives the length of the curved line AB.

An object is said to be at rest when it does not change its position with the time. While an object is said to be in motion when it changes its position with time.
For example, when a car is stationary or not moving it is said to be at rest but when the car is moving it is said to be in motion.

When an object moves along a straight line or path it is called a rectilinear motion. For example, when a bullet is fired from a gun, the bullet moves in a straight line or path. So, the movement of a bullet fired from a gun is an example of rectilinear motion. The march past of soldiers in a parade is rectilinear motion and the motion of a sprinter or short distance runner running on a straight track is also rectilinear motion.

A round path having the shape of a circle is called circular path. 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. The moon is a natural satellite of earth. The movement of artificial satellite or man-made satellite around the earth is also circular motion. Not only that, when the earth moves around the sun in an almost circular path. So, the movement of earth around the sun is also an example of circular motion.

The electric fan or the clock by themselves are not moving from one place to another. But, the blades of the fan rotate and so do the hands of a clock. If we mark a point anywhere on the blades of a fan or on the hands of a clock, the distance of this point from the centre of the fan or the clock, will remain the same as they rotate. In some cases, an object repeats its motion after some time. This type of motion is called periodic motion. Therefore, the motion which repeats itself after regular intervals of time is called periodic motion.

When an object turns or spins about a fixed axis, it is called rotational motion. For example, a spinning top move round and round on its axis, so the motion of a spinning top is an example of rotational motion. Our earth also turns around and around on its axis like a spinning top. So, the spinning of earth on its axis is an example of rotational motion.

Differences between wheel of moving bicycle and mark on the blade of electric fan are as given:
Wheel of Moving Bicycle:
(1) It executes rectilinear as well as circular motion because the wheel as a whole will move forward and its point or particles around the rim will execute circular motion.
(2) It changes position while executing circular motion.
(3) It can cover some distances in any time interval.
(4) Wheel around its ‘axle’ execute rotational motion.
Mark on the Blade of Electric Fan
(1) It executes circular motion because the because the mark on the blade of a fan will move such that its distance from the centre of the fan will remain same.
(2) It cannot change its position.
(3) It cannot cover any distance.
(4) It cannot execute rotational motion.

(i)Rectilinear motion: The handle of bicycle will always move in rectilinear path because it cannot execute circular or rotatory motion.
(ii) Circular motion: The pedals of bicycle will always move circularly around its chain fixing system because they cannot move in forward direction without the whole chain system.
(iii)Both Rectilinear and Circular motion: The Wheels of bicycle will move in rectilinear as well as in circular motion because the wheel as a whole will move forward and its point or particles around the rim will execute circular motion.

While travelling, trees near the tracks seem to be moving back (i.e. opposite to the direction of movement of the train) because there is a relative motion between outside trees and the moving train. While in case of co-passengers, the relative motion between us and co-passengers, is zero, so they appear to be stationary.

A pace or a footstep not be used as a standard unit of length because the size of pace and food step vary from person to person. This will lead to confusion while measuring the lengths by different persons. We should use standard units like International System of Units (SI Units).

Examples of objects which undergo combination of different types of motion can be given as:
(a) Drawing water from well: In this case, the pulley on which road is put had circular motion and bucket has linear motion.
(b) Sewing Machine: Needle of the sewing machine oscillates and the wheel rotates in circular motion.

The reasons may be as follows:
(i)Their scale of measurement may not be standard or they may be using different scales of measurement.
(ii) The length of the scale may not be proper, i.e. the length of scale may be shorter than the length they want to measure.
(iii)There may be some errors in the scale, which they are using or they may not be using the correct method of observing the scale properly.