Measurement of Length and Motion – Long Answer Type Questions
CBSE Class 6 Science — Chapter 5: Measurement of Length and Motion
30 Long Answer Type Questions with exam-ready answers — NCERT-aligned for CBSE Class 6.
Long Answer Questions — Topic-wise (30 Questions)
Answers are concise but detailed — suitable for school examinations and revisions.
Units & Instruments (Questions 1–8)
1. Explain what is meant by the term "unit of length" and why units are important in measurement.
A unit of length is a fixed standard used to measure how long or short an object is, for example the metre (m), centimetre (cm) and kilometre (km). Units provide a common reference so that measurements made by different people at different times can be compared and understood. Without units, a number has no clear meaning — saying "20" tells nothing unless we know whether it means 20 cm, 20 m, or 20 km. Units also allow conversions between scales (e.g., 1 m = 100 cm), which is essential for calculations and scientific communication.
2. Describe three instruments used to measure length and explain when each should be used.
Common instruments include the ruler (for short straight lengths up to about 30 cm), the measuring tape (flexible and used for long or curved surfaces like cloth or ropes), and the metre scale or metre stick (for longer straight measurements up to 1 m). A ruler is best for pencils and small objects, a tape is ideal for measuring around objects or long distances that are not straight, and a metre scale is useful for classroom measurements of furniture or room dimensions. Each instrument must be used correctly—for example, the tape should be kept taut and the ruler must start at zero for accurate readings.
3. Explain the relationship between metres, centimetres and millimetres with an example of converting units.
The relationships are: 1 metre = 100 centimetres and 1 metre = 1000 millimetres; therefore 1 centimetre = 10 millimetres. For example, to convert 2.35 metres to centimetres, multiply by 100: 2.35 m = 235 cm. To convert 78 mm to cm, divide by 10: 78 mm = 7.8 cm. Using these relationships helps ensure consistent units in measurements and calculations, avoiding mistakes when solving problems such as speed = distance/time.
4. A student measures the length of a pencil and records 12.7 cm. Explain how this measurement could be obtained and what the digits represent.
To obtain 12.7 cm the student places the pencil along a ruler starting at the zero mark and reads the mark at the other end. The digits mean 12 full centimetres and 7 tenths of a centimetre, which equals 7 millimetres (since 0.1 cm = 1 mm). Recording as 12.7 cm shows an estimated reading between marks to the nearest millimetre. It is important to note the unit (cm) and that the reading includes the estimated fraction for greater precision.
5. Explain why it is necessary to keep the measuring instrument straight and aligned with the object being measured.
Keeping the instrument straight and aligned ensures the measurement follows the true length of the object without added error. If the ruler or tape is at an angle, it can record a longer distance than the actual length. Similarly, a sagging tape on a curved path will give a larger reading than the true straight-line length. Proper alignment reduces parallax error (wrong reading due to viewing angle) and ensures consistency; this is why measuring procedures stress placing zero at one end and keeping the tool parallel to the object.
6. What is the purpose of using a trundle wheel and how is it used to measure long distances?
A trundle wheel (measuring wheel) is used to measure long ground distances quickly and accurately. It has a wheel of known circumference; each full turn corresponds to that circumference in distance. The user rolls the wheel along the ground from start to end while counting revolutions; the total distance equals the number of turns multiplied by the wheel's circumference, with partial turns converted proportionally. It is ideal for measuring paths, playgrounds or long outdoor stretches where tapes or metre sticks are impractical.
7. How would you measure the length of a curved line drawn on paper? Describe a simple method.
A simple method is to use a thread: lay the thread carefully along the curve from start to end, mark or cut the thread at the end point, then straighten the thread and measure it with a ruler. This converts the curved length into a straight measurement. Alternatively, for small curves, the curve can be approximated by measuring several short straight segments along the curve and adding them, but the thread method is more accurate and easier for school experiments.
8. Discuss why recording units is a crucial part of any measurement report.
Recording units clarifies what the numerical value represents — without units the number is ambiguous. Units ensure others can interpret, compare and use the measurement correctly, for example 5 could be 5 mm, 5 cm or 5 m, each very different. Units also guide necessary conversions when performing calculations (e.g., speed calculations require consistent distance and time units). In scientific reporting, omission of units is considered a serious error because it renders data meaningless.
Measuring Techniques & Accuracy (Questions 9–16)
9. Explain what is meant by an 'estimated reading' on a measuring scale and give an example.
An estimated reading is an approximation taken when the object end falls between marked divisions on the scale. For instance, if the ruler divisions are millimetres and the object ends between the 12 mm and 13 mm marks, the reader might estimate 12.4 mm based on visual judgment. This estimate improves precision beyond the smallest marked division; it should be recorded clearly and consistently, and repeated measurements can help confirm the estimate.
10. Describe how repeating measurements and taking an average helps improve accuracy.
Repeating measurements reduces the effect of random errors due to slight misplacement or parallax and gives multiple values that can be averaged. The average (mean) smooths out small variations, providing a more reliable estimate of the true value. For example, measuring the same length three times and averaging the results reduces the influence of any one erroneous reading and increases confidence in the reported value.
11. A student measures a board as 1.02 m, 1.01 m and 1.03 m in three trials. Calculate the average length and explain its significance.
Average = (1.02 + 1.01 + 1.03) m / 3 = 3.06 m / 3 = 1.02 m. The average value is taken as a better estimate of the true length because it accounts for small variations across trials. It reduces the impact of random errors and gives a single representative measurement to record in the experiment.
12. What is parallax error and how can it be avoided when reading a scale?
Parallax error occurs when the observer's eye is not directly above the measurement mark and thus reads a shifted value. It can be avoided by ensuring the eye is perpendicular to the scale mark when reading and by using instruments with clear aligned pointers. Keeping the ruler flat and viewing at right angle to the scale minimizes parallax, improving measurement accuracy.
13. Explain with an example how to measure an object longer than your ruler.
Measure the object in parts: align the ruler's zero with one end and note the reading at the ruler's end (say 30 cm). Then mark that point on the object, reposition the ruler with zero at the mark, and measure the remaining length (say 15 cm). Add the parts: 30 + 15 = 45 cm. This method, when done carefully without overlap or gaps, yields the full length using a shorter ruler.
14. Discuss reasons why two measurements of the same object might differ slightly.
Differences can be due to human error (parallax, misalignment), instrument precision (coarser divisions), slight changes in object position, or environmental factors (temperature causing expansion). Different observers may place the instrument slightly differently. Repeating and averaging measurements, using better instruments and following careful technique reduces such discrepancies.
15. Explain how a metre scale is different from a measuring tape in construction and use.
A metre scale is typically a rigid straight rod or stick marked in centimetres and millimetres, ideal for straight measurements; it resists bending. A measuring tape is flexible, often made of cloth or metal, allowing it to follow curves and measure around objects. The tape is better for garments and curved surfaces, while a metre scale provides rigidity and straightness for accurate linear measurements.
16. How would you measure the circumference of a circular object like a jar using simple school tools?
Use a flexible measuring tape to wrap around the jar at its widest part and read the length directly as circumference. If a tape is unavailable, use a thread to wrap around the jar, mark where it overlaps, then straighten and measure the thread with a ruler. These simple methods give a practical circumference measurement suitable for school activities.
Motion Basics (Questions 17–22)
17. Define motion and distinguish between uniform and non-uniform motion with examples.
Motion is the change in position of an object with time. Uniform motion occurs when an object covers equal distances in equal intervals of time (e.g., a toy car moving at constant speed), while non-uniform motion means the distances covered in equal time intervals are different (e.g., a car slowing down or speeding up in traffic). Uniform motion has constant speed, whereas non-uniform motion involves changing speed.
18. What is distance and how is it different from displacement (introduce concept simply)?
Distance is the total path length travelled by an object, irrespective of direction. Displacement is the straight-line distance from the starting point to the end point, along with direction. For Class 6, understanding distance as the whole path and displacement as the straight separation gives learners the basic difference without complex vector treatment.
19. A child walks 3 km east and then 1 km west. What is the total distance travelled and what is the simple idea of displacement here?
Total distance = 3 km + 1 km = 4 km. Simple displacement = final position relative to start = 3 km east − 1 km west = 2 km east (straight-line from start to end). This example shows distance accounts for the actual path, while displacement considers net change in position.
20. How is time measured in experiments and why must it be measured accurately?
Time is measured using stopwatches or clocks, recorded in seconds, minutes or hours depending on the experiment. Accurate time measurement is crucial because quantities like speed depend on time; errors in time directly affect calculated speed and lead to incorrect conclusions. Using precise timing and clear start/stop signals improves the reliability of motion experiments.
21. Explain how to conduct a simple experiment to show motion and record observations.
Set up a fixed distance on the floor (e.g., 5 m). Let a toy car or student walk from start to end while another student uses a stopwatch to measure time taken. Repeat several trials, record distances and times in a table, and calculate speed = distance/time for each trial. Observations may include variations in times, average time, and whether motion appeared uniform or non-uniform.
22. Why is it important to state both distance and time when describing motion?
Distance alone shows how far an object moved but not how quickly; time alone shows duration but not how far. Together they allow calculation of speed, which quantifies how fast the object moved. Clear reporting of both distance and time with units enables complete description and comparison of motions.
Speed Calculations & Units (Questions 23–28)
23. Define speed and write its formula. Explain the importance of consistent units with an example.
Speed is the distance travelled per unit time. Formula: speed = distance / time (v = d / t). Consistent units are essential: for example, if distance is 100 m and time is 20 s, speed = 100/20 = 5 m/s. If time were in minutes the numerical result would differ unless converted; 20 s = 0.333... min, so using inconsistent units gives wrong interpretations. Hence convert units to match desired speed units (m/s or km/h).
24. A bicycle travels 15 km in 1.5 hours. Calculate its speed and state the unit.
Speed = distance/time = 15 km ÷ 1.5 h = 10 km/h. The unit is kilometres per hour (km/h). This shows the cyclist travels 10 kilometres each hour at that average speed.
25. A student times a toy car covering 2 m in 0.5 s. Find its speed in m/s and explain the calculation steps.
Speed = distance/time = 2 m ÷ 0.5 s = 4 m/s. Steps: record distance (2 m), record time (0.5 s), divide distance by time to obtain speed. The unit m/s indicates metres per second.
26. Explain how to convert speed from m/s to km/h and give an example converting 5 m/s.
To convert m/s to km/h multiply by 3.6 because 1 m/s = 3.6 km/h (1 m/s = 3600 m/h = 3.6 km/h). Example: 5 m/s × 3.6 = 18 km/h. This conversion helps compare speeds measured in different unit systems.
27. If two students travel the same distance but one takes twice the time, compare their speeds.
If student A covers distance d in time t and student B covers the same d in time 2t, speeds: v_A = d/t, v_B = d/(2t) = v_A/2. Thus student B moves at half the speed of student A. This demonstrates inverse relation between time and speed for the same distance.
28. How do you calculate average speed for a journey with multiple parts? Illustrate with numbers.
Average speed = total distance / total time. Example: travel 30 km in 1 h, then 20 km in 0.5 h. Total distance = 50 km, total time = 1.5 h. Average speed = 50 ÷ 1.5 ≈ 33.33 km/h. Use totals rather than averaging the two speeds directly, because time spent in each part affects the average.
Practical Considerations & Errors (Questions 29–30)
29. Discuss three common sources of error in length and motion experiments and how to minimise them.
Common errors: parallax error when reading scales (avoid by viewing perpendicular to the mark), instrument alignment errors (keep rulers straight and tapes taut), and timing errors (start/stop stopwatch precisely and repeat trials). Minimisation: use proper technique, repeat measurements and average results, calibrate instruments, and ensure consistent procedures across trials to reduce random and systematic errors.
30. Explain how mastering measurement and motion concepts in this chapter helps in further studies of science.
Mastering measurement and motion builds a foundation for physics and other sciences: accurate measurement is essential for experiments, and understanding motion leads to topics like forces, velocity and acceleration in higher classes. It teaches careful observation, unit use, data recording and simple calculations—skills critical for scientific thinking. Good habits developed here—like precision, repeatability and clear reporting—are used throughout scientific study and real-world problem solving.