Class 6 Science Chapter 5 Notes Measurement of Length and Motion
This chapter deals with the concept of measurement of length, its standard units, and the correct way of measuring length. It also includes an understanding of motion and its types.
Measurement
Measurement is the method of comparison of an unknown quantity with a given known quantity. It is expressed in two parts: a number and a unit. For example, You measure the length of your study table to be 2 meters. In this example, the first part, i.e. 2 is the number and the second part, i.e. meter is the unit of measurement.
Ancient Methods of Measurement
In ancient days, people used to have the following methods or units to measure length or distance.
→ Hand span is the length between the tip of the thumb and the little finger of our hand when we stretch it as shown in Figure (a).
→ Yard is the length between the tip of the middle finger and the chin of an outstretched arm as shown in Figure (b). It was mainly used to measure the length of clothes and wires.
→ Footstep is the length between the big toe and the heel of the foot as shown in figure (c). This method was used to measure the distance from one point to another.
→ Mutthi (fist) refers to the quantity that a person’s mutthi or a fist can hold as shown in Figure (d). It is used to measure small quantities of grains and pulses like rice, grams, wheat, etc.
→ Angul (finger) is the width of the finger as shown in Figure (e). It was used to measure very small distances. You may have heard this from your grandparents that a teacup has a height of “chaar angul”, i.e. the width of four fingers.
→ Dhanusa is an ancient unit of measuring height used in Jain literature.
→ Yojana is a measuring of distance used in ancient India.
In ancient times units like Dhanusa, and Yojana were used in measuring artifacts, architecture, and town planning. The Angola is still used by traditional craftspeople like carpenters and tailors. However, the ancient methods of measurement are really interesting but they differ from person to person.
Standard Unit of Measurement
Since, the ancient methods of measurement were inaccurate, to have a fixed value of measurement, a set of standard units of measurement is adopted. The system of units now used is known as the ‘International System of Units’ or SI units.
→ The SI unit for measuring length is meter (m).
- 1 m = 100 cm
- 1 cm = 10 mm
→ For measuring smaller lengths, we use units such as centimeters or millimeters.
- 1 mm = 0.1 cm
→ For long distances, the meter is not a suitable unit, hence we define, a larger unit of length that is a kilometer (1cm).
- 1 km = 1000 m
In some scales, you might have noticed another scale marking. This scale marking is in inches, where 1 inch = 2.54 cm. These units are used to measure one’s height.
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 door is to be measured, you may need a meter scale or a measuring tape. You cannot directly measure the size of your chest using a meter scale. For such measurements, flexible measuring tape, such as a tailor’s tape is more suitable. So, for measurement, the usage of the correct measuring device and way of measuring the lengths play a vital role.
Precautions while Taking Measurements
For taking measurements, we must keep the following points in mind.
→ To take measurements of the length of the object, we must put the scale along its length as shown in the figures.
→ If the end of the scale is broken and we are not able to see a zero mark, then take any full mark of the scale and subtract the reading of this mark from the reading at the other end as shown in the figures.
→ During measurement, the position of the eye is also very important. So, we must keep our eyes in such a way that it is exactly in front of the point where the measurement is to be taken as shown in the figure.
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
We cannot measure the length of a curved line directly by using a meter scale. We can use a flexible measuring tape or a thread to measure the length of a curved line. If we carefully move the thread along the curved line while keeping it stretched tightly and then measure the used part of the thread by a meter scale, we get the length of the curved line.
Visually challenged students use scales with raised markings that can be felt by touching them.
Describing Position
Position is nothing but the, location of an object. When the distance of an object is stated with respect to a fixed point or position, then this point is called a reference point. So, we can say that distance is the measure of how far is something from a reference point. For example, While traveling, the kilometer board that we see indicates our distance from our destination point.
Motion
If the position of an object changes concerning the reference point with time, then the object is said to be in motion. If an object is not changing its position concerning the reference point with time, it is said to be at rest.
The reference point is important in deciding whether an object is in motion or at rest. If you are on a bus and consider yourself as a reference point, then other passengers on the bus are at rest as there is no change in their position concerning you. But for another reference point outside the bus (maybe a tree, a pole, a building), the passengers inside the bus are in motion because their position is changing concerning the reference point.
Types of Motion
When we see around us, we experience a lot of things moving around, for example, vehicles on a straight road, rotation of a fan, a moving bicycle, etc. Different objects show different types of motion. Some of them are stated below
(i) Linear Motion:
When an object moves along a straight line, its motion is called linear motion.
e.g. Vehicle running on a straight road, the motion of a bullet when fired, sprinters in a 100 m race, march-past of soldiers, etc.
(ii) Circular Motion:
When an object moves along a circular path, its motion is called circular motion.
e.g. Hands of a clock, motion of stone in the following figure, etc.
(iii) Oscillatory Motion:
When an object moves to and fro about some fixed position, its motion is called oscillatory motion.
e.g. Motion of a simple pendulum, a garden swing, etc.
(iv) Periodic Motion:
If an object repeats its motion after a fixed interval of time, its motion is said to be periodic.
e.g. The motion of the minute hand of a clock, the revolution of the earth around the sun, etc.
All oscillatory motions are periodic but periodic motions are not always oscillatory.
Example: Motion of the minute hand of the clock (it exhibits periodic motion but not oscillatory).
→ Measurement: The method of comparing any unknown quantity with a known quantity is called measurement.
→ S.I. units: It is a universally accepted metric system of units.
→ Length: Length is the size of something from one end to the other.
→ Distance: The total length of the path from one point to another is called distance.
→ Reference: Point A reference point is a fixed point concerning which something is observed or measured.
→ Motion: An object is said to be in motion if its position changes concerning the reference point with time.
→ Linear Motion: Linear motion is when an object moves along a straight line.
→ Circular Motion: Circular motion is when an object moves along a circular path.
→ Oscillatory Motion: Oscillatory motion is when an object moves to and fro about its mean position.
→ Periodic Motion: Periodic motion is when an object repeats its motion after a fixed interval of time.