Measurement of Length and Motion Class 6 Notes Science Chapter 5

Class 6 Science Chapter 5 Notes Measurement of Length and Motion

→ Travelling is the act of going from one place to another using different modes of transport.

→ Physical quantity: Anything that can be measured. Basic physical quantities include length, mass, temperature, and time.

→ Measurement: The comparison of an unknown quantity with some known quantity of the same kind.

→ In ancient times, people used the length of a foot, the width of a finger, and the distance of a step as units of measurement. This confused me, and a need to develop a uniform system of measurement arose.

→ Measurement of an object consists of The number of units and the unit of measurement of the object.

→ Example: 66.4 cm has 66.4 as the number of units and cm as the unit of measurement.

→ International Standard Units of Measurement: These are units to measure any quantity completely and uniformly acceptable universally. We use the International System of Units (SI Units), accepted all over the world.

→ SI Unit (Standard International Unit) for length is meter (m), for mass is the kilogram (kg), for time is second (s), and for temperature is Kelvin (K). SI units were widely accepted in 1960 by all the scientists.

→ Rules for writing units: Some rules followed for writing the units are stated as follows:

• Units must be written in small letters unless derived from a scientist’s name. Example: cm, kg, K.
• Units must be written in singular form. Example: km and not kms, cm and not cms.
• There should be no full stop after the unit unless it is at the end of the sentence. For example, the length of my pencil is 10 cm, and Ram is 1.5 m tall.

→ Precautions to be taken while measuring the length of any object: Precautions for accurate measurement are:

• Place the scale in contact with an object along its length and align the zero mark of the scale with the starting point of the object.
• The eye must be placed vertically above the point where the measurement has to be taken, otherwise, there will be an error in the reading which is taken due to parallax.
• If the scale is damaged from the edge or it is not smooth, or if the zero mark on the scale is damaged, start the measurement from another full mark on the scale and then subtract the reading of this mark from the reading taken at the right end.

→ Motion: An object is said to be in motion if its position changes concerning the reference point with time.

→ Rest: If an object does not change its position concerning a fixed object, it is said to be at rest.

→ Reference point: It is a stationary or fixed point that serves as a basis for observing and describing the position, and velocity of the object in consideration.

→ Types of motion: The motion of objects is classified in the following ways:

→ Linear or translatory motion: The motion where all parts of an object move equal distances. It is further classified into two categories:

• Rectilinear motion: When an object on the whole moves along a straight path. Example: a group of ants moving in a line, soldiers moving in a parade, a bus moving on a straight highway, an athlete running a 100 m race, and a batsman taking a run between the wickets.
• Curvilinear motion: When an object on the whole moves along a curved path. Example: a car moving along a curved road and a ball is thrown up at an angle.

→ Random or non-periodic motion: When an object moves from one position to another and irregularly changes direction. Examples: butterfly flies randomly in the garden, the motion of a flying bird, the motion of smoke coming out of the fire, the motion of a football player on the ground, etc.

→ Circular motion: An object moves circularly about its axis or around a fixed center. The object remains at the same distance from a fixed point which is the centre of the path of the motion. There are two types of circular motion:

• Revolutionary motion: An object moves as a whole around a fixed center. Example: the Earth revolving around the Sun in a definite orbit, a toy train moving on a circular track, the moon revolving around the Earth, etc.
• Rotatory motion: Rotatory motion is the movement of any object about an axis. Examples: blades of a moving fan and windmill, a spinning top, hands of a clock, etc.

→ Oscillatory motion: When an object moves to and fro about a fixed point or along the same path is called oscillatory motion. The fixed point is called the mean position. In this motion, the object oscillates to and fro along the same path again and again and with the same speed. Example: a child sitting on a rocking chair, the pendulum of a clock moving left and right, the piston of the engine of an automobile, etc.

→ Periodic motion: When the motion of an object repeats itself after regular time intervals, it is called a periodic motion. Example: the pendulum of a clock, the motion of the Earth revolving around the Sun, the motion of the moon around the Earth, etc.

→ Uniform motion: When a body covers equal distances in equal time intervals.

→ Non-uniform motion: When a body covers unequal distances in equal intervals of time.

→ The International System of Units (SI units) has been adopted by countries as standard units of measurement.

→ The SI unit of length is a meter. Its symbol is m.

→ 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm.

→ When distance is stated to a field object or point, then this point is called a reference point.

→ An object is said to be in motion if its position changes for a reference point with time.

→ When an object moves along a straight line, its motion is called linear motion.

→ When an object moves along a circular path, its motion is called circular motion.

→ When any object moves to and fro about any field position, its motion is called oscillatory motion.

Deepa, a curious eleven-year-old girl, lives in a town in the state of Haryana. The new school year has started. Deepa needs a new uniform since she has grown taller. Her mother takes her to a clothes shop. She asks for a two-metre cloth piece. The shopkeeper measures the cloth using a metal measuring rod. Then, the tailor takes her measurements using a flexible measuring tape. Her mother instructs the tailor to increase the length of her uniform by char angula (four figures width).

Deepa shares her experience with her school friends Anish, Hardeep, Padma, and Tasneem and this leads to a discussion among them.

How do we Measure?

Hardeep says, “I have seen my grandmother measuring cloth by the length of her arm.”
“Have you ever seen how a farmer measures length to divide his field into beds? He walks and counts the number of his strides,” says Padma.

“Oh, not just the length of the strides—sometimes they also use the length of their feet to measure,” adds Anish.
Deepa says excitedly, “Measuring length using body parts must be so much fun! Let us also measure something using a body part.”
“What should we measure? Okay, let us measure the length of the table in our classroom,” says Tasneem.
Padma adds, “And which body part should we use to measure it?”
Deepa says, “Let us use our handspan. I will show you how to use it. I have seen my mother using it. She calls it balisht.”
Hardeep adds, “Okay. Let us also note down our measurements.”

Padma says, “Oh, the number of handspans is different for all of us. So, what can we say about the length of the table?”
“But why should the number be different?” Hardeep asked thoughtfully.
Tasneem says, “I can guess. Our handspans are of different sizes.”
Anish gives an idea, “Let us check this.”
So, all five of them put their handspans along each other and conclude that the lengths of their handspans are different.
Deepa says thoughtfully, “No wonder people use scales and measuring tapes.”

Deepa and her friends compare the length of the table with the length of their handspans. The length of the table is expressed in terms of their handspans. Here, the handspan used for measurement is an example of a unit. The length is expressed in two parts, a number and a unit. For example, if the length of the table is found to be 13 handspans, then 13 is the number, and ‘handspan’ is the unit selected for the measurement. However, handspans and other similar units, such as length of hand, foot, fit, or figures, differ from person to person. Thus, there is a need for such a unit for which measurements of the same length made by different people do not differ.

India has a rich history of measurement systems dating back to ancient times. Angula (finger width), multiples of angula, dhanusa, and yojana are some of the units mentioned in ancient Indian literature and used in measuring artifacts, architecture, and town planning. The angula is still used by traditional craftspeople like carpenters and tailors. Several objects with ruled markings which could be scales have been excavated from sites of the Harappan Civilisation.

Standard Units

Several systems of units evolved with time in different parts of the world. However, when people started traveling from one place to another, it created a lot of confusion. This led to the different countries coming together and adopting a set of standard units of measurement. The system of units now used is known as the ‘International System of Units’ or SI units. The SI unit of length is a meter. Its symbol is m. A meter scale is shown in Figure.

One metre (m) is divided into 100 equal divisions. Each division is called a centimetre (cm). You may be familiar with a smaller part of the metre scale, typically 15 cm long, shown in Figure.

Look carefully at the 15-cm scale. It has markings (in cm) from 0 to 15. The length of any section between two consecutive big marks, such as between 1 and 2 or between 5 and 6, is 1 cm. Observe that these sections of 1 cm length are further divided into 10 equal parts. The length of one of these smaller parts is called a millimetre (mm). 1 mm is the smallest value of length that you can measure using this scale. 1 mm is equal to one-tenth of a centimeter (1 mm = 0.1 cm). For measuring larger lengths, we use a larger unit called a kilometre (km) which is equal to 1000 metres. And for measuring smaller lengths, we use units such as centimetre or millimetre.

In some scales, you might have noticed another scale marking. This scale marking is in inches, where 1 inch = 2.54 cm. In earlier days, units, such as inch and foot, were used to measure length. These units are still used by some people.

Correct Way of Measuring Length

For measuring any length, we need an appropriate scale. For example, if you want to measure the length of your pencil, you may use a 15-cm scale. Similarly, if the height of a room is to be measured, you may need a metre scale or a measuring tape. You cannot directly measure the girth of a tree or the size of your chest using a metre scale. For such measurements, flexible measuring tape, such as a tailor’s tape is more suitable. While measuring lengths, we need to take care of some points.

What is the correct way to place the scale?
Place the scale in contact with the object along its length as shown in Figure.

What is the correct position of the eye while reading the scale?
For example, if you are trying to measure the length of a pencil by aligning it with a scale, the position of your eye should be directly above the tip of the pencil (Figure).

How to measure the length if the ends of the scale are broken?
If the ends of the scale are broken or the zero marking is not clear, it can still be used for measurement. With such a scale, use any other full mark of the scale, say, 1.0 cm (Figure). Then you must subtract the reading of this mark from the reading at the other end. For example, in Figure, the reading at one end is 1.0 cm, and at the other end, it is 10.4 cm. Therefore, the length of the object is 10.4 cm – 1.0 cm = 9.4 cm.

How do visually challenged students measure lengths? They use scales with raised markings that can be felt by touching them.

Activity 5.1 Let Us Measure
Select some objects around you, such as a comb, a pen, a pencil, and an eraser to measure their lengths. Measure their lengths one by one using a meter scale and note down the measurements in Table.

While writing the length, do not forget to write the unit also. Thus, your result will consist of two parts—one part is a number and the other part is the unit of measurement. Some of your friends in the class would have measured the length of the same objects. Compare the lengths measured by you with that of your friends. Are the measured lengths the same or slightly different? If not the same, discuss the possible reasons for the differences.

Units of length, such as kilometer, meter, centimeter, and millimeter, begin with a lowercase letter, except at the beginning of a sentence. Their symbols km, m, cm, and mm are also written in lowercase letters and are never followed by ‘s’ for the plural. Note that a full stop is not written after the symbol, except at the end of a sentence. While writing the length, always leave a space between the number and the unit.

Measuring the Length of a Curved Line

Anish and his parents found electric string lights on the arches of the verandah of their house, as shown in Figure, for a celebration at home. How would they have measured the required length of string lights?

In the case of a curved line, measurements can be made with the help of a flexible measuring tape or by using a thread as shown in Figure.

The thread can then be straightened and its length can be measured using a meter scale.

Describing Position

One day the teacher informs her students that she has planned an educational visit to a nearby garden. She asks the students to get there directly in the morning. Deepa and her friends start discussing whether the garden would be closer than their school or farther. Tasneem and Padma say that the garden would be closer, while Deepa and Anish feel that the school would be closer, Hardeep thinks that both would be almost at an equal distance (Figure).

Who do you think is correct? All of them are correct (Figure). Then, why are their observations different? They are locating the distances of the school and garden from their houses. If, instead, each of them had thought of distances from the same object or point, say, the bus stand, then their observations would have been the same.

When distance is stated concerning a field object or point, then this point is called a reference point. A few days later, Hardeep tells his friends excitedly, “Let us all go to the playground. The sports teacher wants us to help her to draw lines with chuna powder (limestone powder) for making the Kabaddi court for the sports day.”
Padma: “We will need a longer measuring tape. Let us take it from the sports room.” (Figure)

Deepa: “Let us first decide the point on the ground from which we will measure the distances to start drawing the lines. Let us call this our reference point.” (Figure)

After a few days, Padma was traveling by bus to visit her grandparents in Delhi. She was eager to reach Delhi and was reading the kilometer stones on the side of the road. On one of the kilometer stones, it was written ‘Delhi 70 km’ (Figure).

Further on, the next kilometer stone read ‘Delhi 60 km’. Each kilometer stone indicated to her that she was getting closer to her grandparents’ house. These kilometer stones indicated her distance from Delhi. So, Delhi is the reference point in this situation.

If the kilometer stone reads ‘Delhi 70 km’ as shown in Figure, we can say that the position of Padma is 70 km from Delhi. When the kilometer stone reads ‘Delhi 60 km’, the position of Padma is 60 km from Delhi.

Moving Things

Activity 5.2 Let Us Explore
Look around and prepare a list of fie objects that are in motion and fie objects that are at rest. Record your observations in Table. Think about how you decided whether an object was in motion or at rest. Write your explanation (justification) in Table.

Compare and analyze your justifications. How can one decide if an object is in motion or at rest? An object is said to be in motion if its position changes to the reference point with time. If an object is not changing its position concerning the reference point with time, it is said to be at rest.

Deepa looked around her in the bus and noticed that all the passengers were seated. She looked around again after a minute and found them still occupying their seats. She wondered, ‘Are they moving?’ She concluded that the position of the passengers was not changing with time. Therefore, they were certainly at rest. However, when she looked outside, she felt they were in motion as their positions were changing to things outside. The reference point is important in deciding whether an object is at rest or in motion. If Deepa considered herself (or the bus) as the reference point, then passengers were at rest. However, if she considered any object outside the bus (say a building) as the reference point, then the passengers (and the bus) were in motion.

Types of Motion

Activity 5.3 Let Us Explore
Take an eraser and drop it from a certain height. Observe its motion. Does it move along a straight line? When an orange drops from the tree, does it move in a straight line? Have you seen the Republic Day parade? Recall the march- past of students during the parade. Do they move on a straight-line path? When a heavy box is pushed, it may also move along a straight line (Figure).

When an object moves along a straight line, its motion is called linear motion. Identify such linear motion in your surroundings. But do things always move along a straight line? You might have enjoyed playing on swings and merry-go-rounds. Are these types of motion also linear motion?

Activity 5.4 Let us Investigate
Tie an eraser (or a potato) at one end of a thread. Hold the other end of the thread with your hand and whirl it (Figure).

Observe its motion. Is the motion of the eraser the same as that of a merry-go-round? When an object moves along a circular path, its motion is called circular motion.

Activity 5.5: Let us Investigate
Tie an eraser (or a potato) at one end of a thread. Hang the eraser by holding the other end of the thread (Figure). Keep your hand steady. Using the other hand, take the eraser slightly to one side and then release it (Figure).

Does it start moving to and fro? Is its motion similar to the motion of a swing? When an object moves to and fro about some fixed position, its motion is called oscillatory motion.

Activity 5.6 Let us Investigate
Take a thin metal strip of about 50 cm long. Hold its one end pressed to a table. You may use a few books or a brick to hold it (Figure).

Press the free end of the strip slightly and let it go. Observe the motion of this end of the strip. Does it move up and down? This is also an example of oscillatory motion.

If an object repeats its path after a fixed interval of time, its motion is said to be periodic. When an object is in circular motion, it moves along the circular path again and again. An object in oscillatory motion also repeats its motion while moving to and fro. Both circular and oscillatory motions are periodic.

Activity 5.7 Let us Identify
Look at the picture of a children’s park (Figure) or visit a children’s park.

Observe different kinds of motions. Classify them as linear, circular, or oscillatory motion. List them in Table.

Give your justification for why you put each in a certain category.

Class 6 Science Notes

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