Speed is a scalar: it tells you only how fast something moves (magnitude). Velocity is a vector: it tells you how fast AND in which direction. So speed uses total distance, but velocity uses displacement. Memory hook: "Velocity has a Vector arrow; Speed just has a Speedometer number."
Speed uses the full curved path length (distance), while velocity uses the straight arrow from start to end (displacement) plus its direction. Speed is always greater than or equal to the magnitude of velocity.
Your doubts, answered
Is speed the same as velocity? Why do teachers say they are different?
They are not the same. Speed only answers 'how fast?' (a number with a unit, like 40 m/s). Velocity answers 'how fast AND in which direction?' (like 40 m/s toward east). In physics, direction matters, so we call speed a scalar and velocity a vector. For NEET, this one idea decides whether you use total distance (speed) or displacement (velocity).
Can speed be greater than velocity? Or can they be equal?
Compare their magnitudes. Average speed = total distance / time. Magnitude of average velocity = displacement / time. Because distance is always greater than or equal to displacement, average speed is always greater than or equal to the magnitude of average velocity. They are equal only when motion is in a straight line without turning back (distance = displacement). If the object turns back, speed becomes larger.
Can velocity be negative but speed cannot?
Yes. Velocity can be positive, negative, or zero, because the sign shows direction (for example, +5 m/s means moving right, -5 m/s means moving left). Speed is only the magnitude, so speed is never negative. Speed of -5 m/s has no meaning; it is just 5 m/s.
A car goes around a circular track at a fixed 40 km/h. Is its velocity constant?
No. Its speed is constant (always 40 km/h), but its velocity keeps changing because the direction of motion changes at every point on the circle. Constant speed does not mean constant velocity. This is a very common NEET trap: uniform speed can still mean the velocity (and therefore acceleration) is changing.
If speed is constant, is the object still accelerating?
It can be. Acceleration means a change in velocity, not just a change in speed. If direction changes (like in circular motion) while the speed stays the same, the velocity is changing, so there IS acceleration. Only if BOTH the size and the direction of velocity stay fixed is the object truly not accelerating.
⚠️ The NEET trap ✗ Using the same total path length for both average speed and average velocity, so you report them as equal. ✓ Average velocity uses net displacement (which can be small or zero after turning back); average speed uses the full distance travelled. After a round trip, average velocity can be 0 while average speed is not. 🧠 When an object turns back, examiners check if you used displacement for velocity and distance for speed.
Real NEET questions
2018
A toy car with charge q moves on a frictionless horizontal plane under a uniform electric field E. Due to the force qE, its velocity increases from 0 to 6 m/s in one second. At that instant the direction of the field is reversed. The car moves for two more seconds under this field. The average velocity and the average speed of the car between 0 and 3 s are respectively:
A · 2 m/s, 4 m/s
B · 1 m/s, 3 m/s ✓
C · 1.5 m/s, 3 m/s
D · 1 m/s, 3.5 m/s
Solution: Phase 1 (0 to 1 s): acceleration a = +6 m/s^2 (since 0 to 6 m/s in 1 s). Distance = (1/2)(6)(1)^2 = 3 m, so position x(1) = 3 m, velocity = 6 m/s. Phase 2 (field reversed): a = -6 m/s^2. Position at t: x = 3 + 6(t-1) - (1/2)(6)(t-1)^2. At t = 2 s: x = 3 + 6 - 3 = 6 m (car still moving forward, reaches farthest point where v = 0). At t = 3 s: x = 3 + 12 - 24/... compute directly: x(3) = 3 + 6(2) - 3(2)^2 = 3 + 12 - 12 = 3 m. Net displacement (0 to 3 s) = x(3) - x(0) = 3 - 0 = 3 m. Average velocity = displacement / time = 3/3 = 1 m/s. For distance: car goes 0 to 6 m (forward 6 m) then 6 m back to 3 m (backward 3 m), total distance = 6 + 3 = 9 m. Average speed = 9/3 = 3 m/s. Answer: 1 m/s, 3 m/s.
2026
A particle moves along a straight line with position s(t) = a t^2 - b t + c, where a = 1 m/s^2, b = 6 m/s, c = 5 m. The average speed of the particle from t = 0 to t = 6 s is:
A · 12 m/s
B · 6 m/s
C · 3 m/s ✓
D · 0 m/s
Solution: s = t^2 - 6t + 5, so velocity v = ds/dt = 2t - 6. This is zero at t = 3 s, meaning the particle stops and reverses direction. So find distance separately for each part. From t = 0 to 3 s the particle moves backward, from t = 3 to 6 s it moves forward. Using the v-t triangle areas: on [0,3], |v| goes from 6 to 0, distance = (1/2)(3)(6) = 9 m; on [3,6], |v| goes 0 to 6, distance = (1/2)(3)(6) = 9 m. Total distance = 18 m. Average speed = distance / time = 18/6 = 3 m/s. Trap: average velocity = displacement/time. s(0) = 5, s(6) = 36 - 36 + 5 = 5, so displacement = 0 and average velocity = 0 (option D). Speed is NOT zero because the path length is 18 m. Answer: 3 m/s.
Solved Motion In A Straight Line NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the main difference between speed and velocity in one line?
Speed is a scalar (only magnitude, uses distance); velocity is a vector (magnitude and direction, uses displacement).
Do speed and velocity have the same unit?
Yes. Both are measured in metres per second (m/s) in SI units, or km/h. The unit is the same; the difference is that velocity also carries a direction.
Can average velocity ever be zero when speed is not?
Yes. In a round trip the object returns to its start, so displacement is 0 and average velocity is 0, but it did travel a real distance, so average speed is not zero.
Which is used in NEET kinematics equations, speed or velocity?
The equations of motion (v = u + at, etc.) use velocity because direction and sign matter. Speed is just the magnitude of velocity and is used when only 'how fast' is asked.
Is instantaneous speed always equal to the magnitude of instantaneous velocity?
Yes, at a single instant speed equals the magnitude of velocity, because there is no path length versus displacement gap over zero time. The difference only shows up for averages over an interval.
