Speed vs Average Speed — Difference (with Table & FAQs)
Speed vs Average Speed compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 9 Physics, free and no sign-up.
TL;DR: Speed vs Average Speed compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 9 Physics, free and no sign-up.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
The main difference between Speed and Average Speed: Speed is the distance traveled per unit time at a specific instant, while average speed is the total distance traveled divided by the total time taken. Ave
Speed vs Average Speed — Comparison Table
| Basis | Speed | Average Speed |
|---|---|---|
| Definition | Distance per unit time at an instant | Total distance divided by total time |
| Calculation | Speed = distance/time for small intervals | Average speed = total distance/total time |
| Time Interval | Instantaneous value at a moment | Over the entire journey |
| Variation | Can change at each instant | Single value for entire trip |
| Speedometer | Shows speed at each moment | Cannot show average speed directly |
| Example | Car speedometer reads 60 km/h | Car traveled 120 km in 2 hours at average 60 km/h |
Key Points
- Speed is an instantaneous quantity while average speed is a derived quantity over a time interval
- A vehicle can have varying speed but constant average speed for a trip
- Average speed does not provide information about the actual path taken
- For uniform motion, speed and average speed are equal
Speed at an instant
Speed is the distance travelled by an object in unit time: speed = distance ÷ time. It is a scalar quantity, meaning it has magnitude but no direction, and its SI unit is metre per second (m/s). Kilometre per hour is also common, and the conversion is 1 km/h = 5/18 m/s.
When people say 'the car was doing 60', they mean instantaneous speed — the speed at that particular moment, which is what a speedometer shows. It can change from second to second.
Average speed over a journey
Average speed is the total distance travelled divided by the total time taken. It is a single number that summarises a whole journey, however irregular that journey was.
The definition is the important bit: total distance over total time. It is NOT the average of the individual speeds. A car that covers half a journey at 40 km/h and the other half at 60 km/h does not average 50 km/h — it averages 48 km/h, because it spends more time at the slower speed. This trap appears in exams constantly.
Uniform and non-uniform motion
An object is in uniform motion when it covers equal distances in equal intervals of time, however small those intervals. In that special case, the instantaneous speed and the average speed are the same number.
In non-uniform motion the object covers unequal distances in equal intervals — which describes almost all real motion, such as a bus in traffic. Here the instantaneous speed keeps changing, and the average speed is the only single figure that describes the whole journey.
Speed versus velocity, and average velocity
Speed is a scalar and uses distance, which is the total path length covered. Velocity is a vector and uses displacement, which is the shortest straight-line distance from start to finish, with direction.
So average speed = total distance ÷ total time, but average velocity = total displacement ÷ total time. For an object that returns to its starting point, the displacement is zero, so the average velocity is zero — even though the average speed is clearly not. A runner completing one lap of a track is the standard example.
Average velocity can also be found as (initial velocity + final velocity) ÷ 2, but only when the acceleration is uniform. Applying that shortcut to non-uniform motion is a common error.
The two-part journey formula
For a journey covered in two equal distances at speeds a and b, the average speed is 2ab ÷ (a + b) — the harmonic mean, not the arithmetic mean. For 40 and 60 that gives (2 × 40 × 60) ÷ 100 = 48 km/h.
But if the two speeds are maintained for equal times rather than equal distances, the average genuinely is (a + b) ÷ 2 = 50 km/h. Read the question carefully to see whether it is equal distances or equal times — the two give different answers, and examiners know it.
Speed vs Average Speed — Worked Examples
- A car covering 120 km in 3 hours — Average speed = 120 ÷ 3 = 40 km/h, regardless of how much it sped up or slowed down along the way.
- A runner completing one full lap of a 400 m track in 50 s — Average speed = 400 ÷ 50 = 8 m/s. Average velocity = 0, because the displacement is zero — she finished where she started.
- Half a trip at 40 km/h, half at 60 km/h — Average speed = 2(40)(60) ÷ 100 = 48 km/h, not 50, because more time is spent on the slower half.
- A speedometer reading 55 km/h — Instantaneous speed — the speed at that exact moment, not the average for the journey.
Common Mistakes Students Make
- Averaging the two speeds directly when the distances are equal. Use 2ab ÷ (a + b) instead.
- Using displacement in an average-speed calculation. Average speed uses total distance travelled.
- Saying average velocity and average speed are always equal in magnitude. They are equal only when the motion is in a straight line without reversing.
- Applying average velocity = (u + v) ÷ 2 to non-uniform acceleration. It holds only for uniform acceleration.
- Forgetting the unit conversion: 1 km/h = 5/18 m/s, not 18/5.
Diagrams
Where This Shows Up in Real Life
What your speedometer does and does not tell you
A speedometer shows instantaneous speed — how fast you are moving at this exact moment. It says nothing about the journey as a whole. A trip computer showing "average 42 km/h" is doing something different: dividing total distance by total time, including every traffic light and jam. This is why a drive that touched 80 km/h can still average 35 — the stops count in the time but add nothing to the distance.
Average speed cameras on highways
Older cameras measured instantaneous speed at one point, so drivers braked for the camera and accelerated after. Average-speed enforcement reads a number plate at two points a known distance apart and divides distance by time. Because it measures the whole stretch, braking for one camera achieves nothing. It is a direct, practical use of the definition — and it is why the two quantities must never be confused.
Why athletics uses average speed
A 100 m sprinter finishing in 10 s has an average speed of 10 m/s, but she did not run at 10 m/s throughout — she started from rest, accelerated for roughly the first 40 m, and peaked well above 11 m/s. The average conceals the variation, which is exactly what an average is for and exactly why coaches use split times to see the detail underneath it.
Estimating travel time realistically
Route planners quote an average speed far below the road’s speed limit because they build in junctions, traffic and stops. Multiplying distance by the limit gives a time no journey achieves. Using a realistic average speed instead is the everyday application of total distance over total time, and it is why 200 km on an Indian highway rarely takes the two hours a 100 km/h limit suggests.
Solved Numericals — Step by Step
Problem 1. A car covers the first 100 km at 50 km/h and the next 100 km at 100 km/h. Find its average speed for the whole journey.
- Time for the first stretch = 100 ÷ 50 = 2 h.
- Time for the second stretch = 100 ÷ 100 = 1 h.
- Total distance = 100 + 100 = 200 km. Total time = 2 + 1 = 3 h.
- Average speed = total distance ÷ total time = 200 ÷ 3.
Answer: 66.67 km/h — NOT 75 km/h. Averaging the two speeds is wrong because more time is spent on the slower stretch. The shortcut 2ab/(a+b) = 2(50)(100)/150 gives the same 66.67.
Problem 2. A boy runs 400 m around a circular track in 100 s and returns to his starting point. Find his average speed and average velocity.
- Average speed = total DISTANCE ÷ total time = 400 ÷ 100 = 4 m/s.
- He finishes exactly where he began, so his DISPLACEMENT is zero.
- Average velocity = displacement ÷ time = 0 ÷ 100.
Answer: Average speed 4 m/s; average velocity 0 m/s. This is the standard illustration that speed uses distance while velocity uses displacement.
Problem 3. A train travels at 60 km/h for 30 minutes, then at 40 km/h for the next 45 minutes. Find its average speed.
- Convert times to hours: 30 min = 0.5 h, 45 min = 0.75 h.
- Distance 1 = 60 × 0.5 = 30 km. Distance 2 = 40 × 0.75 = 30 km.
- Total distance = 60 km. Total time = 0.5 + 0.75 = 1.25 h.
- Average speed = 60 ÷ 1.25.
Answer: 48 km/h.
Board-Style Questions with Model Answers
Define speed and average speed. How do they differ? (3 marks)
Speed is the distance travelled by an object per unit time, and in ordinary use it means instantaneous speed — the speed at a particular moment, which is what a speedometer displays. Average speed is the total distance travelled divided by the total time taken for the whole journey. The difference is one of scope: instantaneous speed describes a single instant and can change continuously, while average speed summarises an entire journey in one number and conceals all the variation within it. They are equal only when the motion is uniform, that is when the object covers equal distances in equal intervals of time.
Distinguish between distance and displacement, and hence between speed and velocity. (5 marks)
Distance is the total length of the path actually covered by a moving object, regardless of direction. It is a scalar quantity, is always positive, and can never decrease during a journey. Displacement is the shortest straight-line distance between the initial and final positions, together with the direction. It is a vector quantity, can be positive, negative or zero, and its magnitude is always less than or equal to the distance. Speed is distance divided by time, so it is a scalar and is always positive. Velocity is displacement divided by time, so it is a vector with both magnitude and direction. The clearest illustration is an athlete completing one full lap of a circular track: the distance is the full circumference so her average speed is non-zero, but her displacement is zero because she ends where she started, so her average velocity is zero.
A car travels the first half of a distance at 40 km/h and the second half at 60 km/h. Find its average speed and explain why it is not 50 km/h. (3 marks)
Let the total distance be 2d. Time for the first half is d divided by 40, and for the second half d divided by 60. The total time is therefore d/40 + d/60, which is (3d + 2d)/120 = 5d/120 = d/24. Average speed is total distance over total time, that is 2d divided by d/24, which gives 48 km/h. It is not 50 km/h because average speed weights by TIME, not by distance, and the car spends longer on the slower half of the journey. The general formula for two equal distances is 2ab/(a+b), which here gives 2 × 40 × 60 ÷ 100 = 48 km/h. Note that if the two speeds were maintained for equal TIMES rather than equal distances, the answer genuinely would be 50 km/h.
When are the average speed and the magnitude of average velocity equal? (2 marks)
They are equal only when the distance travelled equals the magnitude of the displacement, which happens when the object moves along a straight line in one fixed direction without reversing. In that case the path length and the straight-line separation between start and finish are the same. If the object changes direction, doubles back, or moves along a curved path, the distance exceeds the magnitude of the displacement, so the average speed becomes greater than the magnitude of the average velocity.
Define uniform and non-uniform motion with an example of each. (2 marks)
An object is in uniform motion when it covers equal distances in equal intervals of time, however small those intervals are taken to be; its speed does not change, so instantaneous and average speed are identical. An example is a car moving steadily at 60 km/h on a straight empty highway. An object is in non-uniform motion when it covers unequal distances in equal intervals of time, so its speed keeps changing. An example is a bus travelling through city traffic, repeatedly slowing at junctions and accelerating away. Almost all real motion is non-uniform, which is why average speed is such a useful summary.
Assertion–Reason Questions (CBSE Format)
For each question choose: (a) both A and R are true and R is the correct explanation of A; (b) both are true but R is not the correct explanation; (c) A is true but R is false; (d) A is false but R is true.
1. Assertion (A): The average speed of a body can be zero only if it never moves.
Reason (R): Average speed is total distance divided by total time, and distance is never negative.
Answer: (a) — Both are true and correctly linked. Distance can only accumulate, so the only way the total is zero is if no motion occurred. Average velocity, by contrast, is zero whenever the body returns to its start.
2. Assertion (A): A car travelling half a journey at 40 km/h and half at 60 km/h averages 50 km/h.
Reason (R): The average of two numbers is their sum divided by two.
Answer: (d) — The assertion is false — over equal DISTANCES the answer is 48 km/h, because more time is spent at the lower speed. The reason is a true statement of arithmetic that simply does not apply here.
3. Assertion (A): An object moving in a circle at a steady rate has zero acceleration.
Reason (R): Its speed does not change.
Answer: (d) — The assertion is false: the direction of the velocity changes continuously, so the object is accelerating. The reason is true but describes speed, not velocity.
4. Assertion (A): The magnitude of displacement can never exceed the distance travelled.
Reason (R): Displacement is the shortest path between the initial and final positions.
Answer: (a) — Both are true and the reason explains the assertion. The shortest route between two points cannot be longer than any actual path taken between them.
Quick Self-Check
Q1. A body covers 100 m in 20 s and the next 100 m in 30 s. Its average speed is:
- A. 5 m/s
- B. 4 m/s
- C. 4.5 m/s
- D. 3.33 m/s
Answer: B. 4 m/s — Total distance 200 m, total time 50 s, so 200 ÷ 50 = 4 m/s. Averaging 5 and 3.33 would wrongly give 4.17.
Q2. A runner completes exactly one lap of a circular track. Her average velocity is:
- A. Equal to her average speed
- B. Zero
- C. Double her average speed
- D. Half her average speed
Answer: B. Zero — She ends where she began, so her displacement is zero and average velocity = displacement ÷ time = 0.
Q3. Which quantity is a vector?
- A. Speed
- B. Distance
- C. Velocity
- D. Time
Answer: C. Velocity — Velocity has both magnitude and direction. Speed, distance and time are scalars.
Q4. 72 km/h expressed in m/s is:
- A. 20 m/s
- B. 25 m/s
- C. 36 m/s
- D. 12 m/s
Answer: A. 20 m/s — Multiply by 5/18: 72 × 5 ÷ 18 = 20 m/s.
Key Terms Glossary
- Speed
- Distance travelled per unit time. A scalar, always positive. SI unit m/s.
- Instantaneous speed
- The speed at one particular moment — what a speedometer shows.
- Average speed
- Total distance divided by total time for a whole journey.
- Velocity
- Displacement per unit time. A vector, so it carries a direction and can be negative or zero.
- Average velocity
- Total displacement divided by total time. Zero if the object returns to its start.
- Distance
- The total length of the path actually covered. Scalar, never decreases.
- Displacement
- The shortest straight-line distance from start to finish, with direction. Vector.
- Uniform motion
- Equal distances covered in equal intervals of time. Instantaneous and average speed are equal.
- Non-uniform motion
- Unequal distances in equal intervals — the speed keeps changing.
- Scalar
- A quantity with magnitude only: speed, distance, time, mass.
- Vector
- A quantity with magnitude and direction: velocity, displacement, acceleration, force.
- Harmonic mean
- The 2ab/(a+b) formula used for average speed over two EQUAL DISTANCES.
One-Minute Revision
- Average speed = TOTAL DISTANCE ÷ TOTAL TIME. Never the average of the individual speeds.
- Equal DISTANCES at a and b → 2ab/(a+b). Equal TIMES at a and b → (a+b)/2. Read which one is given.
- 40 and 60 over equal distances = 48 km/h, not 50. This is the most-set trap in the chapter.
- Speed uses DISTANCE (scalar). Velocity uses DISPLACEMENT (vector).
- One full lap: average speed is non-zero, average velocity is ZERO.
- Average speed is zero ONLY if the object never moved. Average velocity is zero whenever it returns home.
- Uniform motion → instantaneous speed = average speed. Non-uniform → they differ.
- 1 km/h = 5/18 m/s. So 72 km/h = 20 m/s.
Syllabus reference: NCERT Class 9 Science, Chapter 8 — Motion.
Frequently Asked Questions
What is the difference between speed and average speed?
Speed is how fast an object is moving at a particular moment or over a stretch. Average speed is the total distance travelled divided by the total time taken for the whole journey.
How do you calculate average speed?
Average speed = total distance ÷ total time. It takes into account stops and changes of speed during the journey.
Can average speed differ from the actual speed?
Yes. If the speed changes during a trip, the average speed can be different from the speed at any single instant.
What is the difference between speed and average speed?
Speed usually means the instantaneous speed — how fast an object is moving at one particular moment, which is what a speedometer shows. Average speed is the total distance travelled divided by the total time taken for the whole journey, and it smooths out all the variations along the way.
Can average speed be zero?
Only if the object never moves at all. Average speed uses total distance, which can never be negative and is zero only if no distance was covered. Average velocity, by contrast, is zero whenever an object returns to its starting point.
Why is the average of 40 km/h and 60 km/h not 50 km/h?
Because over equal distances the object spends more time travelling at the slower speed, so the slower speed carries more weight. The correct formula for equal distances is 2ab ÷ (a + b), which gives 48 km/h. If the two speeds were held for equal times instead, the answer would genuinely be 50 km/h.
Can an object have constant speed but changing velocity?
Yes. An object moving in a circle at a steady rate has constant speed, but its direction changes continuously, so its velocity is changing. That is why circular motion at constant speed still involves acceleration.