⚛️ Physics · Topic 1 of 10
Start with the study notes on Motion, Measurement & Time (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
A unit is a fixed amount that we compare a quantity with. A standard unit means exactly the same amount to everyone, everywhere.
Long ago, people measured with parts of the body, such as the handspan, the footspan and the cubit (elbow to fingertip). These vary from person to person, so measurements did not agree.
Examples:
Remember: The SI unit of length is the metre (m), of mass the kilogram (kg) and of time the second (s).
Bigger and smaller units of length are made from the metre:
mm < cm < m < km
Examples:
Careful measuring avoids small mistakes:
Examples:
An object is in motion when its position changes with time. If its position does not change, it is at rest.
Rest and motion are relative: they depend on what you compare the object with.
Example: A passenger sitting in a moving bus is at rest compared with the bus, but in motion compared with a tree by the road.
Motion along a straight line.
Example: A car on a straight road, a soldier in a march-past, or a stone dropped from a height.
Motion along a circular path.
Example: A stone tied to a string and whirled round, or the tip of a clock's hand.
Spinning about a fixed axis.
Example: A spinning top, or the Earth turning on its axis.
Motion that repeats itself after equal intervals of time.
Example: A swing, a pendulum, the needle of a sewing machine, or your heartbeat.
Motion with no fixed path or pattern.
Example: A flying insect, or a footballer changing direction.
Remember: One object can show more than one type of motion. A ceiling fan rotates, while the tip of each blade moves in a circle.
Speed is the distance an object covers in one unit of time.
Speed = Distance ÷ Time
Its SI unit is metres per second (m/s). Road speeds are usually given in kilometres per hour (km/h).
Examples:
In uniform motion, an object covers equal distances in equal intervals of time, so its speed stays the same. In non-uniform motion, the speed keeps changing.
Examples:
For a whole journey, average speed = total distance ÷ total time.
Example: A train goes 120 km in 2 hours and then 180 km in 3 hours. Average speed = 300 km ÷ 5 h = 60 km/h.
Remember: In a vehicle, the speedometer shows the speed and the odometer shows the total distance travelled.
Time is measured using events that repeat at regular intervals.
A simple pendulum is a small metal ball (the bob) hung from a fixed point by a thread. One complete to-and-fro swing is one oscillation, and the time it takes is the time period.
Time period = Total time ÷ Number of oscillations
Examples:
Older timekeepers include the sundial (which uses the Sun's shadow), the water clock and the sand clock. Most modern watches use a vibrating quartz crystal.
Remember: For small swings, the time period depends on the length of the pendulum, not on the weight of the bob.
A distance–time graph shows how far an object has travelled at each moment.
| Misconception | Correct understanding |
|---|---|
| A heavier bob makes a pendulum swing faster. | For small swings, only the length of the pendulum changes its time period. |
| Average speed is the average of the two speeds. | Average speed = total distance ÷ total time. |
| You can mix units in a speed problem. | Convert first: 600 m in 10 minutes is 600 m in 600 s, which is 1 m/s. |
| An object is simply moving or not moving. | Rest and motion depend on what you compare the object with. |
| The speedometer shows how far a car has gone. | The speedometer shows speed; the odometer shows distance. |
Why are body-based units like the handspan unreliable?
Hands, feet and arms differ in size from person to person, so two people get different results.
Convert 350 cm into metres.
350 ÷ 100 = 3.5 m.
Where should your eye be when you read a ruler?
Directly above the mark being read, to avoid parallax error.
What type of motion does a swing show?
Periodic (oscillatory) motion.
A boy walks 600 m in 10 minutes. What is his speed in m/s?
10 minutes = 600 s, so speed = 600 ÷ 600 = 1 m/s.
What does a horizontal line on a distance–time graph show?
The object is at rest: its distance is not changing.
A pendulum makes 20 oscillations in 40 s. What is its time period?
40 ÷ 20 = 2 seconds.
Key takeaway: Measure with standard units and care, describe motion by its path, and find speed as distance ÷ time, always in matching units.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The SI unit of length is the:
Answer: A — Metre
The metre (m) is the SI (International System) unit of length. The kilogram is for mass, the second for time, and the litre is a unit of volume.
Q3. easy
How many centimetres are there in 1 metre?
Answer: A — 100
1 metre = 100 centimetres (cm). Also, 1 cm = 10 millimetres (mm), so 1 m = 1000 mm.
Q4. easy
How many millimetres are there in 1 centimetre?
Answer: A — 10
1 centimetre = 10 millimetres. The millimetre is the smallest common unit marked on an ordinary school scale.
Q10. easy
What is 'motion'?
Answer: A — A change in the position of an object with time
An object is said to be in motion if its position changes with time relative to its surroundings. If the position does not change, the object is at rest.
Q19. easy
Speed is defined as:
Answer: A — The distance travelled in unit time
Speed = distance travelled / time taken. It tells us how fast an object moves. A higher speed means more distance is covered in the same time.
Q20. easy
The SI unit of speed is:
Answer: A — Metre per second (m/s)
Since speed = distance/time, its SI unit is metre per second (m/s). Speed is also commonly measured in kilometres per hour (km/h).
Q25. easy
The SI unit of time is the:
Answer: A — Second
The second (s) is the SI unit of time. Larger units are the minute (1 min = 60 s) and the hour (1 h = 60 min = 3600 s).
Q2. medium
Why were ancient units like the 'cubit', 'handspan' and 'footspan' unreliable for measuring length?
Answer: A — They vary from person to person
Body-based units differ because people have different sizes of hands, feet and arms. This made trade and communication difficult, so standard units like the metre were adopted.
Q5. medium
How many metres are there in 1 kilometre?
Answer: A — 1000
1 kilometre (km) = 1000 metres. The kilometre is used to measure large distances such as those between cities.
Q6. medium
While measuring the length of an object with a scale, your eye should be positioned:
Answer: A — Directly above the point being measured
The eye must be exactly in line with (directly above) the point on the scale. Looking from an angle causes a 'parallax error' and gives a wrong reading.
Q8. medium
If the ends of your measuring scale are broken or worn, you should start measuring from:
Answer: A — A full mark like the 1 cm mark (and subtract it at the end)
If the zero end is broken, start from a clear full mark (say 1.0 cm) and subtract that starting value from the final reading to get the correct length.
Q9. medium
To measure the length of a curved line, the best method is to use:
Answer: A — A thread laid along the curve, then measured on a scale
A straight scale cannot follow a curve. A thread is placed along the curved line, marked, then straightened and measured with a scale (a divider can also be used).
Q11. medium
The motion of a car moving along a straight road is an example of:
Answer: A — Rectilinear (straight-line) motion
Motion along a straight line is called rectilinear motion. Examples include a car on a straight road, a marching soldier, or an object falling straight down.
Q12. medium
The motion of the moon around the Earth is an example of:
Answer: A — Circular motion
When an object moves along a circular path, it is in circular motion. Examples: the moon around the Earth, a stone tied to a string and whirled, the tip of a fan's blade.
Q13. medium
The spinning of a top or the Earth about its axis is an example of:
Answer: A — Rotational motion
Rotational motion is the spinning of an object about a fixed axis. The Earth's rotation on its axis (causing day and night) and a spinning top are examples.
Q14. medium
The to-and-fro (back and forth) motion of a swing or a pendulum is called:
Answer: A — Periodic / oscillatory motion
Motion that repeats itself after a fixed interval of time (like a swing, pendulum, or a vibrating string) is called periodic or oscillatory motion.
Q18. medium
'Rest' and 'motion' are relative. This means an object may be:
Answer: A — At rest with respect to one thing and in motion with respect to another
A passenger sitting in a moving bus is at rest relative to the bus but in motion relative to a tree on the road. Whether something is 'moving' depends on the reference point.
Q21. medium
A car travels 100 metres in 20 seconds. Its speed is:
Answer: A — 5 m/s
Speed = distance / time = 100 m / 20 s = 5 m/s.
Q22. medium
A cyclist covers 30 km in 2 hours. The speed is:
Answer: A — 15 km/h
Speed = distance / time = 30 km / 2 h = 15 km/h.
Q23. medium
When an object covers EQUAL distances in EQUAL intervals of time, its motion is called:
Answer: A — Uniform motion
In uniform motion the speed stays constant (equal distances in equal times). If the distances covered in equal times are unequal, the motion is non-uniform.
Q24. medium
The motion of a bus in busy city traffic, speeding up and slowing down, is an example of:
Answer: A — Non-uniform motion
In non-uniform motion the speed keeps changing, so the object covers unequal distances in equal time intervals - as with a bus in traffic.
Q26. medium
How many seconds are there in one hour?
Answer: A — 3600
1 hour = 60 minutes, and 1 minute = 60 seconds. So 1 hour = 60 x 60 = 3600 seconds.
Q27. medium
Early clocks were based on which idea?
Answer: A — Events that repeat after regular (periodic) intervals
The measurement of time relies on periodic (regularly repeating) events - like the swing of a pendulum, the rising sun, or dripping water - each occurring at fixed intervals.
Q31. medium
Which of these was an ancient device used to measure time using shadows?
Answer: A — Sundial
A sundial measures time by the position of the shadow cast by the sun. Other ancient time-measuring devices included water clocks and sand clocks (hourglasses).
Q32. medium
Modern watches and clocks that keep very accurate time usually use:
Answer: A — A vibrating quartz crystal
Most modern clocks and watches use the regular vibrations of a quartz crystal to keep time. They are far more accurate than older pendulum or water clocks.
Q33. medium
In a vehicle, the instrument that measures the SPEED is the:
Answer: A — Speedometer
The speedometer shows the speed of the vehicle (usually in km/h). The odometer measures the total distance travelled by the vehicle.
Q34. medium
In a vehicle, the instrument that measures the DISTANCE travelled is the:
Answer: A — Odometer
The odometer records the distance a vehicle has travelled. The speedometer shows its current speed.
Q37. medium
A steeper (more slanted) line on a distance-time graph indicates:
Answer: A — A higher speed
The steeper the slope of the distance-time graph, the greater the distance covered per unit time - so the greater the speed.
Q39. medium
Which is the correct order of length units from SMALLEST to LARGEST?
Answer: A — millimetre, centimetre, metre, kilometre
From smallest to largest: mm (millimetre) < cm (centimetre) < m (metre) < km (kilometre).
Q40. medium
The distance between two cities is best measured in:
Answer: A — Kilometres
Large distances, such as those between cities, are measured in kilometres. Millimetres and centimetres are used for very small lengths.
Q41. medium
The thickness of a coin is best measured in:
Answer: A — Millimetres
Very small lengths, like the thickness of a coin or a wire, are measured in millimetres (mm).
Q44. medium
Which of the following is NOT a standard SI unit?
Answer: A — Handspan
A handspan is a non-standard, body-based unit that varies from person to person. The metre, kilogram and second are standard SI units.
Q45. medium
The invention of the wheel was important for transport because it helped:
Answer: A — Make movement of goods and people easier (rolling reduces effort)
Before the wheel, people carried goods on their heads or dragged them. The wheel made carts possible, greatly easing the transport of people and goods.
Q47. medium
A stone tied to a string and whirled in a circle shows which motion?
Answer: A — Circular motion
The whirled stone moves along a circular path, so it is in circular motion.
Q49. medium
A measuring tape is more suitable than a metal scale for measuring:
Answer: A — The girth (round measurement) of a tree trunk
A flexible measuring tape can bend around curved surfaces (like a tree trunk or a person's waist), which a rigid metal scale cannot.
Q51. medium
If a pendulum completes 20 oscillations in 40 seconds, its time period is:
Answer: A — 2 seconds
Time period = total time / number of oscillations = 40 s / 20 = 2 s per oscillation.
Q52. medium
Which is the correct relationship between the units of time?
Answer: A — 1 hour = 60 minutes = 3600 seconds
1 hour = 60 minutes, and each minute = 60 seconds, so 1 hour = 60 x 60 = 3600 seconds.
Q54. medium
Which of these is an example of periodic motion in nature?
Answer: A — The heartbeat (beating at regular intervals)
The heartbeat repeats at regular intervals, making it periodic. Other natural examples include the phases of the moon and the swinging of a pendulum.
Q56. medium
Why do we need standard units of measurement?
Answer: A — So that measurements mean the same to everyone everywhere
Standard units ensure that a measurement (like '1 metre') is exactly the same everywhere and for everyone, making trade, science and communication reliable.
Q58. medium
The odometer of a car reads 57,321 km at the start and 57,521 km at the end of a trip. The distance travelled is:
Answer: A — 200 km
Distance travelled = final reading - initial reading = 57,521 - 57,321 = 200 km.
Q59. medium
Which of these is the correct formula for speed?
Answer: A — Speed = Distance ÷ Time
Speed = Distance travelled ÷ Time taken. From this, Distance = Speed × Time and Time = Distance ÷ Speed.
Q7. hard
The error caused by placing the eye at an angle (not directly in front of the reading) is called:
Answer: A — Parallax error
Parallax error occurs when the line of sight is not perpendicular to the scale, making the reading appear shifted. It is avoided by keeping the eye directly above the mark.
Q15. hard
A bird flying in the sky, changing direction unpredictably, shows:
Answer: A — Random motion
Random motion is irregular motion with no fixed path or pattern - like a flying insect, a running football player changing directions, or dust particles in air.
Q16. hard
The blade of a moving ceiling fan shows which combination of motions?
Answer: A — Circular (and rotational) motion
The fan rotates about its central axis (rotational motion), while the tip of each blade moves along a circle (circular motion). An object can show more than one type of motion.
Q17. hard
The needle of a sewing machine moves up and down repeatedly. This is an example of:
Answer: A — Periodic (oscillatory) motion
The up-and-down repeating motion of a sewing-machine needle repeats at regular intervals, making it periodic motion. (The wheel driving it shows rotational/circular motion.)
Q28. hard
A simple pendulum consists of a small metal ball (bob) suspended from a fixed point by a thread. One complete to-and-fro swing is called:
Answer: A — One oscillation
One oscillation of a pendulum is one complete to-and-fro movement (from one extreme position, to the other, and back). The time for one oscillation is the 'time period'.
Q29. hard
The time taken by a pendulum to complete ONE oscillation is called its:
Answer: A — Time period
The time period is the time for one complete oscillation. A pendulum takes the same time for each oscillation, which is why pendulums were used in clocks.
Q30. hard
The time period of a simple pendulum depends mainly on:
Answer: A — The length of the pendulum
For small swings, the time period depends on the length of the pendulum (longer pendulum = longer period). It does not depend on the mass of the bob or how far it is pulled.
Q35. hard
On a distance-time graph, the motion of an object moving with UNIFORM speed is shown by:
Answer: A — A straight (inclined) line
For uniform motion, equal distances are covered in equal times, so the distance-time graph is a straight sloping line. A curved line indicates changing (non-uniform) speed.
Q36. hard
On a distance-time graph, a HORIZONTAL straight line (parallel to the time axis) means the object is:
Answer: A — At rest (not moving)
If the distance does not change as time passes (a flat, horizontal line), the object is stationary - it is at rest.
Q38. hard
A train travels 120 km in the first 2 hours and 180 km in the next 3 hours. Its AVERAGE speed for the whole journey is:
Answer: A — 60 km/h
Average speed = total distance / total time = (120 + 180) km / (2 + 3) h = 300 km / 5 h = 60 km/h.
Q42. hard
Convert 2.5 kilometres into metres.
Answer: A — 2500 m
1 km = 1000 m, so 2.5 km = 2.5 x 1000 = 2500 m.
Q43. hard
Convert 350 centimetres into metres.
Answer: A — 3.5 m
1 m = 100 cm, so 350 cm = 350 / 100 = 3.5 m.
Q46. hard
Which quantity must you know to calculate an object's speed?
Answer: A — The distance travelled and the time taken
Speed = distance / time. You need both the distance covered and the time taken to calculate speed.
Q48. hard
Which pair correctly matches the motion with an example?
Answer: A — Rectilinear - a march-past; Periodic - a pendulum; Rotational - a spinning top
A soldier marching straight = rectilinear; a pendulum swinging = periodic; a spinning top = rotational. Matching motions to examples is a common exam skill.
Q50. hard
An athlete runs 400 m around a circular track and returns to the starting point in 50 s. The distance travelled is:
Answer: A — 400 m
Distance is the total path length covered, which is 400 m (the full track). (Note: the displacement - straight-line distance from start to end - would be 0, since the athlete returns to the start.)
Q53. hard
A boy walks 600 m to school in 10 minutes. His speed in metres per second is:
Answer: A — 1 m/s
First convert time to seconds: 10 min = 600 s. Speed = distance / time = 600 m / 600 s = 1 m/s.
Q55. hard
Two objects cover the same distance. Object A takes less time than Object B. Which has the greater speed?
Answer: A — Object A
For the same distance, the object taking less time has the greater speed (speed = distance/time; smaller time means larger speed).
Q57. hard
A simple pendulum is pulled to one side and released. Its amplitude (how far it swings) does NOT affect its time period. This principle was used by scientists to build:
Answer: A — Accurate pendulum clocks
Because a pendulum's time period stays nearly the same regardless of the swing size, it keeps steady time. Galileo's observation of this led to the invention of accurate pendulum clocks.
Q60. hard
The motion of the hands of a clock is best described as:
Answer: A — Circular (and periodic) motion
The tip of a clock's hand moves along a circle (circular motion), and it repeats its position at regular intervals (periodic motion) - so it shows both circular and periodic motion.