First-screen answer: Motion becomes measurable when you record distance and the time interval used to cover it. Speed is the total distance covered divided by total time taken. Its SI unit is metre per second (m/s), while kilometre per hour (km/h) is useful for road journeys. A pendulum helps us study periodic motion because one oscillation repeats after a nearly equal interval when its length and location stay the same.
The Class 7 NCERT book Curiosity builds this chapter from timekeeping to speed and then to uniform and non-uniform linear motion. The current official chapter teaches equal-interval motion tables rather than a graph topic, so this lesson follows that same boundary. Every numerical example below shows the formula, substitution, unit and a quick reasonableness check.
Your Game Plan
- Separate a clock reading from a duration.
- Use repeating events to understand timekeeping and periodic motion.
- Measure several oscillations before finding one time period.
- Choose one unit system before using a speed formula.
- Use equal-time tables to decide whether motion is uniform.
Instants and Time Intervals
An instant is a particular moment, such as 8:10 AM. A time interval is the duration between two instants, such as the 25 minutes between 8:10 AM and 8:35 AM. A clock can show both start and finish readings, but subtraction gives the interval. That difference matters in races, cooking, travel and experiments.
The SI unit of time is the second, symbol s. Sixty seconds make one minute, and sixty minutes make one hour. Unit symbols are written in the singular: 12 s, not 12 secs; 3 h, not 3 hrs. Leave a space between number and unit. These small conventions make scientific records easy for anyone to read.
A library activity begins at 10:18 AM and ends at 10:47 AM.
Calculation: 47 min − 18 min = 29 min.
Answer: The interval is 29 min.
Check: The end time is less than half an hour after the start, so 29 min is reasonable.
A bus leaves at 2:52 PM and arrives at 3:17 PM. From 2:52 to 3:00 is 8 min; from 3:00 to 3:17 is 17 min. Total interval = 8 min + 17 min = 25 min. Counting through the hour prevents the incorrect subtraction 17 − 52.
Why it works: separating start, finish and interval stops you from attaching the wrong unit or subtracting minutes without considering the hour.
How Time Is Measured
People first used repeating natural events. Sunrise and sunset helped mark days; lunar phases helped organise longer cycles; seasons helped connect time with farming. To divide a day into smaller intervals, communities developed devices such as sundials, water clocks, hourglasses and marked candles. Each device depended on a changing but observable process.
A sundial follows the changing position of a shadow and works only when sunlight is suitable. A water clock uses the flow or collection of water, so the size of the opening and changing water level affect it. An hourglass uses flowing sand and must be turned after its set interval. A candle clock uses the progress of burning, but wind and candle thickness can reduce reliability. These limitations encouraged more regular mechanical, quartz and atomic timekeepers.
For a one-minute classroom task, a phone timer or stopwatch is more suitable than a sundial. The task is short, may happen indoors and needs seconds. The choice is based on range, smallest division and working conditions.
Why it works: every timekeeper turns a regular change into counted intervals. Accuracy improves when the repeating process is stable and easy to count.
Periodic Motion and a Simple Pendulum
Motion is periodic when it repeats after equal or nearly equal intervals. A simple pendulum has a small bob suspended by a light thread from a rigid support. At rest, the bob hangs at the mean position. When moved slightly and released without a push, it swings to one side, through the mean position, to the other side and back.
One oscillation must return to an equivalent starting state. If you begin at extreme A, count A to B and back to A. If you begin at the mean O moving toward A, count O to A to B and back to O moving in the same direction. The time for one complete oscillation is the time period. At a given place, pendulums of the same length have nearly the same time period under the small, controlled swings used in this chapter. Length matters; the bob’s mass does not determine the time period in this simplified investigation.
A pendulum takes 18.6 s for 10 oscillations.
Formula: time period = total time ÷ number of oscillations.
Substitution: T = 18.6 s ÷ 10 = 1.86 s.
Answer: Time period = 1.86 s.
Check: Ten swings take about 19 s, so one should take about 1.9 s.
Why it works: timing many oscillations makes the effect of a small reaction-time error smaller when the total is divided.
Reliable Measurement and Fair Tests
A good measurement can be repeated. Use the same definition of one oscillation, the same pendulum length, a small starting displacement and the same timing method. Start the watch at a clear reference crossing, count aloud or use tally marks, and stop at the corresponding final crossing. Repeat the trial three or four times.
If the readings differ slightly, that does not automatically mean the experiment failed. Human reaction time, air movement and counting can cause small variation. Compare the readings, look for an obvious mistake, and calculate a representative value only after the method is consistent. A fair test changes one planned factor at a time. To test length, keep bob and location the same. To test bob mass, keep length and release method the same.
Three timings for 10 oscillations are 19.0 s, 18.8 s and 19.2 s. Their mean is (19.0 + 18.8 + 19.2) s ÷ 3 = 57.0 s ÷ 3 = 19.0 s. Estimated time period = 19.0 s ÷ 10 = 1.90 s. The tight group supports a consistent method.
Why it works: repeated measurements reveal random variation and help you notice an outlier caused by a missed count or late stopwatch press.
Speed, Distance and Time
Speed compares distance with time. For the chapter’s journeys, use speed = total distance covered ÷ total time taken. Rearranging gives distance = speed × time and time = distance ÷ speed. In real journeys the object may speed up and slow down, so the total-distance calculation gives average speed. The book often shortens this to “speed” after explaining the idea.
Before substituting, check that distance and time units match the required speed unit. If the answer is in m/s, use metres and seconds. If it is in km/h, use kilometres and hours. Write units in every line. Units are not decoration: they show whether multiplication or division makes sense.
A student walks 180 m in 120 s.
Formula: speed = distance ÷ time.
Substitution: speed = 180 m ÷ 120 s = 1.5 m/s.
Answer: 1.5 m/s.
Check: At this speed, 60 s gives 90 m and 120 s gives 180 m.
A bicycle moves at 12 km/h for 1.5 h.
Formula: distance = speed × time.
Substitution: distance = 12 km/h × 1.5 h = 18 km.
Answer: 18 km.
Check: One hour gives 12 km and half an hour gives 6 km; total 18 km.
A school van covers 72 km at an average speed of 48 km/h.
Formula: time = distance ÷ speed.
Substitution: time = 72 km ÷ 48 km/h = 1.5 h.
Answer: 1.5 h, or 1 h 30 min.
Check: 48 km in one hour plus 24 km in half an hour gives 72 km.
Why it works: the three formulas express one relationship. A reasonableness check by reversing the operation catches many arithmetic and unit errors.
Revise the prerequisite ideas in Class 6 measurement and motion foundations.
Units and Careful Conversions
The two common speed units are m/s and km/h. Because 1 km = 1000 m and 1 h = 3600 s, a conversion must change both distance and time. To change m/s to km/h, multiply by 3.6. To change km/h to m/s, divide by 3.6. You can also show the full unit fraction if that feels safer.
For example, 5 m/s = 5 × 3600 m/h = 18,000 m/h = 18 km/h. The factor is not a rule to memorise blindly; it comes from the two unit definitions. Estimation helps: a value in km/h should be numerically larger than the same speed in m/s because one km/h is a smaller speed unit than one m/s.
Convert 7.5 m/s.
Calculation: 7.5 × 3.6 = 27.0.
Answer: 7.5 m/s = 27 km/h.
Check: The km/h number is larger, as expected.
Convert 54 km/h.
Calculation: 54 ÷ 3.6 = 15.
Answer: 54 km/h = 15 m/s.
Inverse check: 15 × 3.6 = 54.
Why it works: unit conversion is multiplication by a form of one, so the physical speed does not change—only its numerical description does.
Uniform and Non-uniform Linear Motion
Linear motion follows a straight path. It is uniform linear motion when equal distances are covered in equal time intervals. It is non-uniform linear motion when the distance covered in equal intervals changes. A train may move non-uniformly while leaving a station, nearly uniformly on an open straight stretch, and non-uniformly again while slowing to stop.
Uniform motion is an idealisation over a chosen interval. Everyday traffic, turns, slopes and stops make perfectly constant speed uncommon. To decide from data, do not compare total positions alone. Subtract consecutive positions to find the distance covered in each equal interval. Equal changes support uniform motion; unequal changes show non-uniform motion. This evidence-first habit also appears across the Class 7 Science notes collection, where each conclusion should follow from an observation or record.
At 0, 10, 20 and 30 s, an object is at 0, 25, 50 and 75 m. Consecutive distances are 25 m, 25 m and 25 m in equal 10 s intervals. The motion is uniform. Speed = 75 m ÷ 30 s = 2.5 m/s.
At 0, 10, 20 and 30 s, a cart is at 0, 12, 29 and 45 m. Consecutive distances are 12 m, 17 m and 16 m. Because these are unequal for equal 10 s intervals, the motion is non-uniform. Average speed for the whole record = 45 m ÷ 30 s = 1.5 m/s.
Why it works: differences between consecutive positions describe what happened during each interval instead of hiding the changes inside one total.
Reading Motion Tables
A motion table should name each column and unit. Check whether the first column shows clock time or elapsed time, and whether the second shows total position, total distance or interval distance. If it shows position at equal times, calculate interval distances by subtraction. If it already shows interval distance, compare those entries directly.
The current official chapter teaches this comparison through tables. A different visual representation may be studied in another course or later lesson, but it should not be presented here as compulsory scope. For this chapter, master the table logic: equal time steps, consecutive distance changes and total-distance average speed.
Times are 0, 5, 10, 15 and 20 s. A toy moves uniformly at 3 m/s. Use distance = speed × time. Positions are 0, 15, 30, 45 and 60 m. Each 5 s interval adds 15 m, confirming uniform motion.
Why it works: the table keeps measured values and derived interval values separate, so your conclusion can be checked line by line.
Mini Investigation Lab
Science becomes clearer when you design a fair measurement yourself. This mini lab uses simple classroom materials and does not copy the textbook activities. Choose one investigation: compare the walking speeds of the same person over two marked paths, or compare the time periods of two pendulums of different lengths. Work with adult or teacher supervision when hanging a pendulum, keep the bob light, and keep the swinging area clear.
Question and prediction. Write one testable question. For walking, ask: “Does the same person have nearly the same average speed over 3 m and 6 m when asked to walk normally?” For pendulums, ask: “How does increasing thread length affect the measured time period?” A prediction is not a fact; it is what you expect and will test. Write a reason without pretending you already know the result.
Variables. The independent variable is what you deliberately change: path length or pendulum length. The dependent variable is what you measure: total walking time or time period. Controlled variables are kept as steady as practical. For walking, use the same person, surface, start method and timing person. For pendulums, use the same bob, support, location, small release angle, oscillation count and stopwatch operator.
Measurement plan. Mark the path length with a metre tape. Agree that the timer starts when the leading foot crosses the start line and stops when it crosses the finish line. Run three walking trials for each path. For the pendulum, measure from the point of suspension to the centre of the bob, release without pushing, time 10 oscillations and repeat three times. Never change the counting rule halfway through.
Recording table. Give every column a unit. A walking table may have path distance (m), trial, time (s) and calculated speed (m/s). A pendulum table may have length (cm), time for 10 oscillations (s) and time period (s). Record readings immediately, including an unusual result. Do not erase a reading merely because it is inconvenient; add a note if someone stumbled or the count was lost.
For a 6 m path, three times are 4.8 s, 5.0 s and 5.2 s. Mean time = (4.8 + 5.0 + 5.2) s ÷ 3 = 15.0 s ÷ 3 = 5.0 s. Average speed based on the mean = 6 m ÷ 5.0 s = 1.2 m/s. Check: at 1.2 m/s, five seconds gives 6 m.
For one length, times for 10 oscillations are 16.1 s, 16.3 s and 16.2 s. Mean = 48.6 s ÷ 3 = 16.2 s. Time period = 16.2 s ÷ 10 = 1.62 s. The spread is only 0.2 s, so the trials are reasonably close for hand timing.
Interpreting variation. If a walking trial is slower, ask whether the person changed pace, the timer reacted late or the floor condition differed. If a pendulum reading is far away from the others, check for a miscount or a push at release. Do not automatically delete it. Repeat the trial under the written method and report both the original issue and replacement reading.
Making a conclusion. A conclusion answers the question using the recorded values. It does not simply repeat the prediction. For example: “The longer pendulum had the longer measured time period in all three trials under our conditions.” Avoid the unlimited claim “long pendulums are always slow everywhere”. Your experiment used particular lengths, one location and a hand-operated timer.
Evaluating the method. Suggest one improvement linked to an error source. A longer timed interval can reduce the relative effect of stopwatch reaction time. A fixed release marker can make the starting displacement more consistent. A second observer can count oscillations while the first operates the timer. Repeating the whole investigation on another day tests whether the pattern is stable.
Why it works: planning variables, units, repeats and checks before calculating keeps the result connected to real measurements. It also teaches you to distinguish data from explanation.
Common Mistakes and Quick Fixes
Mistake 1: counting half an oscillation as a full one. Fix: return to the same starting state. Mistake 2: timing only one swing. Fix: time 10 or 20 and divide. Mistake 3: using minutes with metres but writing m/s. Fix: convert minutes to seconds first. Mistake 4: averaging two speeds by simply adding and dividing by two. Fix: use total distance divided by total time unless the conditions justify a simple average.
Mistake 5: calling any straight-line motion uniform. Fix: compare equal-time distances. Mistake 6: treating a speedometer as a distance meter. Fix: speedometer shows speed; odometer records distance travelled. Mistake 7: claiming pendulum period never changes. Fix: state the controlled chapter condition: same length and given location, with a small release.
Why it works: these fixes target the reasoning step where the error begins, instead of merely correcting the final number.
Rapid Revision: Measure, Calculate, Check
Time: an instant is a clock reading; an interval is the duration between two readings. The SI unit is second, symbol s. Use 60 s = 1 min and 60 min = 1 h. When a duration crosses an hour, count to the hour and then onward, or convert both readings consistently. Never subtract minute numbers alone when their hours differ.
Periodic motion: it repeats after equal or nearly equal intervals. A pendulum’s time period is the time for one complete oscillation. Define the start state and return to it. Time many oscillations and divide. Keep length, location, release method and counting rule controlled. At the chapter level, length affects the period while changing bob mass does not produce the controlling change.
Speed relationship: speed = total distance ÷ total time; distance = speed × time; time = distance ÷ speed. Write the target unit first. Use metres and seconds for m/s, kilometres and hours for km/h. The total-distance calculation describes average speed when the journey contains faster and slower parts. It does not tell you the speed at every instant.
Conversion: multiply m/s by 3.6 to obtain km/h; divide km/h by 3.6 to obtain m/s. The factor comes from 1 km = 1000 m and 1 h = 3600 s. Use an inverse check: convert the answer back. If the starting and final values do not match, inspect the factor and unit direction.
Uniformity: straight motion is uniform only when equal distances are covered in equal time intervals. From a table of positions, subtract consecutive positions. Equal changes for equal time steps support uniform motion; unequal changes show non-uniform motion. Speed and uniformity answer different questions: speed tells how much distance per unit time, while uniformity tells whether that rate stays constant.
Instrument check: a stopwatch measures intervals, a clock shows time, a speedometer displays vehicle speed and an odometer records distance travelled. Select an instrument whose range and smallest division suit the task. A one-second wall clock cannot reliably settle a race decided by hundredths of a second.
One-minute numerical routine: circle the given values and units; write the formula; convert before substitution; calculate; attach the unit; reverse the operation; and ask whether the size is sensible. For an investigation, add variables, repeated trials and one method improvement. This routine earns accuracy because every step can be inspected.
Teach it to a partner: Create three cards. Card A gives distance and time and asks for speed. Card B gives speed and time and asks for distance. Card C gives distance and speed and asks for time. Swap cards with a partner, but require the solver to state the target unit before calculating. After solving, the writer checks the answer by reversing the operation. Add a fourth card with equal-time positions so the partner must classify motion as uniform or non-uniform. This quick peer check reveals whether a student understands the relationship or is only guessing which numbers to multiply.
Reasonableness anchors: Normal walking is much slower than a road vehicle, a classroom pendulum often has a period of seconds rather than hours, and converting a fixed speed from m/s to km/h makes the number larger. Anchors are not substitute data, but they help spot answers such as 900 km/h for a walking trial or 0.02 s for a hand-timed swing. When an answer looks surprising, recheck units first.
Final notebook audit: Before you finish, underline every measured quantity once and every calculated quantity twice. Check that each table heading carries a unit, each pendulum trial uses the same oscillation count, and each speed answer names the unit requested. Then read your conclusion aloud: it should describe only the tested interval or conditions, not make an unlimited claim. If a result differs from the others, keep the original entry, note the likely reason, and repeat the trial using the written method. This short audit protects both the arithmetic and the scientific honesty of your work.
Textbook Questions ki Taiyari
Prepare four question types. Explain: describe a timekeeper or periodic motion with its repeating process. Calculate: write formula, convert units, substitute and check. Investigate: name the variable changed, variables controlled, number of trials and observation. Interpret: compare equal-time distances in a table and justify uniform or non-uniform motion.
For pendulum questions, draw a support, thread, bob, mean position and two extreme positions. State your rule for one oscillation. For speed questions, write the required unit before working. For an investigation answer, explain why timing many oscillations reduces the importance of a small stopwatch reaction error.
When the question gives a real journey with changing speed, say that total distance divided by total time is the average speed for the whole trip. Do not introduce advanced velocity or acceleration formulae; they are outside this chapter’s required scope. When a table has equal intervals, show consecutive differences so the conclusion is visible.
Extra Practice with Answers
Question 1. A timer starts at 9:46 AM and stops at 10:11 AM. Find the interval.
Answer: 14 min to 10:00 plus 11 min gives 25 min.
Question 2. A pendulum takes 32.4 s for 18 oscillations. Find its time period.
Answer: T = 32.4 s ÷ 18 = 1.8 s.
Question 3. A scooter covers 150 m in 12 s. Find its average speed.
Answer: Speed = 150 m ÷ 12 s = 12.5 m/s.
Question 4. A bus moves at 45 km/h for 2.4 h. Find the distance.
Answer: Distance = 45 km/h × 2.4 h = 108 km.
Question 5. A cyclist covers 30 km at 12 km/h. Find the time.
Answer: Time = 30 km ÷ 12 km/h = 2.5 h, or 2 h 30 min.
Question 6. Convert 10 m/s to km/h.
Answer: 10 × 3.6 = 36 km/h.
Question 7. Convert 72 km/h to m/s.
Answer: 72 ÷ 3.6 = 20 m/s.
Question 8. Positions at 0, 8, 16 and 24 s are 0, 20, 40 and 60 m. Classify the motion.
Answer: Each equal 8 s interval adds 20 m, so the motion is uniform. Speed = 60 ÷ 24 = 2.5 m/s.
Question 9. Positions at 0, 5, 10 and 15 s are 0, 7, 18 and 30 m. Classify the motion.
Answer: Interval distances are 7 m, 11 m and 12 m, so the motion is non-uniform.
Question 10. Why is timing 20 oscillations usually better than timing one?
Answer: The same small reaction-time error forms a smaller fraction of the longer total time; dividing gives a more stable estimate.
Self-Assessment
Self-check 1. What is the SI unit and symbol of time?
Answer: The SI unit is the second, symbol s.
Self-check 2. Define one oscillation of a pendulum beginning at extreme A.
Answer: The bob moves from A to the other extreme B and returns to A.
Self-check 3. Write all three forms of the speed relationship.
Answer: Speed = distance ÷ time; distance = speed × time; time = distance ÷ speed.
Self-check 4. What does an odometer measure?
Answer: It records the distance travelled by a vehicle.
Self-check 5. How do you identify uniform motion from an equal-time table?
Answer: Consecutive distances covered in each equal interval are equal.
Small improvement for today: Pick one journey and record distance, total time and units before calculating. Then reverse your operation to check the answer. A correct method is more valuable than a fast guess. If your table and calculation still disagree, share the exact values on the Ask a Doubt page so the unit and interval can be checked step by step.