CHAPTER- 5 MEASUREMENT OF LENGTH AND MOTION
NOTES
1. People use a ruler or a measuring tape to measure the length of an object.2. We always need such a unit so that the measurement of the same length taken by different people is the same.
3. Different systems of units developed over time in different parts of the world.
- The system of units used nowadays is known as the International System of Units, or SI (an abbreviation of International System of Units).
- The SI unit of length is the metre. Its symbol is m.
- A 1 cm length is further divided into 10 equal parts. The length of each of these smaller parts is called a millimetre (mm).
- 1 mm is equal to one-tenth of a centimetre (1 mm = 0.1 cm).
- To measure large lengths, we use the larger unit kilometre (km). 1 km = 1000 m 1 m = 100 cm 1 cm = 10 mm.
4. You cannot directly use a metre scale to measure the thickness of a tree trunk or the size of your chest. For such measurements, a flexible measuring tape like a tailor’s tape is more suitable.
5. Correct position of the eye while reading a scale-
- For example, if you are trying to measure a pencil by aligning it with a scale, your eye should be positioned directly above the tip of the pencil.
6. Measuring the length of a curved line– In the case of a curved line, a flexible measuring tape or a thread can be used for measurement. The thread can then be straightened, and its length can be measured using a metre scale.
7. Reference point– The distance of an object is determined relative to another object or point, and this point is called the reference point.
8. An object can move only when its position changes with time relative to a reference point.
- If the position of an object does not change with time relative to a reference point, then the object is stationary.
9. Types of motion-
- Rectilinear motion– When an object moves along a straight line, its motion is called rectilinear motion.
- Circular motion– When an object moves along a circular path, its motion is called circular motion.
- Oscillatory motion– When an object moves back and forth about a fixed position, its motion is called oscillatory motion.
LET US ENHANCE OUR LEARNING
Question- 1. Some lengths are given in Column 1 of Table 5.5 some units are given in Column 2. Match the lengths with the units suitable for measuring those lengths.
Answer-
Table 5.5
Question- 2. Read the following statements and Mark True (T) or False (F) against each.
(1) The motion of a car moving on a straight road is an example of linear motion. ( )
(2) Any object which is changing its position with respect to a reference point with time is said to be in motion. ( )
(3) 1 Km = 100 Cm ( )
Answer- (1) - True, (2) - True, (3) - False.
Question- 3. Which of the following is not a standard unit of measuring length?
(1) Millimetre (2) Centimetre (3) Kilometre (4) Handspan
Answer- (4) Handspan
Question- 4. Search for the different scales or measuring tapes at your home and school. Find out the smallest value that can be measured using each of these scales. Record your observations in a tabular form.
Answer- Measuring tape – 1 mm
- Meter ruler – 1 mm
- Small scale – 1 mm
Question- 5. Suppose the distance between your school and home is 1.5 km. Express it in metres.
Answer- 1 Kilometre = 1000 metre
So, 1.5 kilometre = 1.5×1000 = 1500 metre.
Question- 6. Take a tumbler or a bottle. Measure the length of the curved part of the base of glass or bottle and record it.
Answer- I took a glass and measured the curved part of its base. This measurement is 15 centimeters, therefore the length of the curved part of the glass's base = 15 cm.
Question- 7. Measure the height of your friend and express it in (1) metres (2) centimetres and (3) millimetres.
Answer- My friend height is-
- 1.8 m
- 180 cm
- 1800 mm
Question- 8. You are given a coin. Estimate how many coins are required to be placed one after the other length wise, without leaving any gap between them, to cover the whole length of the chosen side of a notebook. Verify your estimate by measuring the same side of the notebook and the size of the coin using a 15 cm scale.
Answer- The edge of the booklet is 30 cm and the diameter of the coin is 2 cm, so the estimated number will be 15. By measuring, the exact number is also 15.
Question- 9. Give two examples each for linear, circular and oscillatory motion.
Answer- Simple linear motion - A car moving on a straight road, a falling paper.
- Circular motion - A fan rotating in a circle, the moon revolving around the Earth.
- Oscillatory motion – The motion of a swinging swing, the motion of a pendulum.
Question- 10. Observe different objects around you. It is easier to express the lengths of some objects in mm, some in cm and some in m. Make a list of three objects in each category and enter them in The table 5.6.
Table 5.6: Sizes of objects around us
Answer-
Answer- In the roller coaster shown in the picture, the types of motion of the ball and the corresponding parts of the track are as follows:
Straight-line motion– From part ‘A' to ‘B’. This is the portion of the track where the ball moves straight downward. Here, the motion of the ball is called straight-line (rectilinear) motion.
**Circular motion– Parts ‘D’ and ‘F’. This is the section where the track forms a circular loop. Here, the ball is in circular motion because it moves along a circular path.
**Straight-line motion** – Part ‘F’. This is the section where the ball once again moves straight forward. Here too, the motion of the ball is called straight-line motion.
Thus, on this track, the motion of the ball can be identified as straight-line motion and circular motion.
Question- 12. Tasneem wants to make a metre scale by herself. She considers the following materials for it- plywood, paper, cloth, stretchable rubber and steel. Which of these should she not use and why?
Answer- Tasneem should not use elastic rubber or cloth. The reason is that elastic rubber and cloth can stretch, which may cause errors in measurement. For a measuring instrument, it is important that it is accurate and stable so that there is no variation in the measurement. The elastic nature of rubber makes it unsuitable as a precise measuring tool.
Question- 13. Think, design and develop a card game on conversion of units of length to play with your friends.
Answer- Game Plan:
- Different units (mm, cm, m, km) will be written on cards.
- Players have to arrange these cards in the correct order. Example: cm = 10 mm, 1 m = 100 cm, etc.
- In the game, the player who arranges the units in the correct order will earn points.
