Physics · Motion In A Straight Line · NEET
No. That is the single biggest mistake NEET students make. Average speed = total distance / total time. Only when the object spends EQUAL TIME at each speed does the answer become (v1 + v2)/2. When it covers EQUAL DISTANCE at each speed, the answer is the harmonic mean 2v1v2/(v1 + v2), which is always smaller. So never blindly average the speeds; first ask whether the times or the distances are equal.
Average speed can never be negative because distance is always positive. It can be zero only if the object does not move at all (total distance = 0). This is different from average velocity, which CAN be negative or zero. If an object returns to its start, its average velocity is zero, but its average speed is still a positive number because it did travel a real path.
If a body covers the FIRST HALF of the distance at speed v1 and the SECOND HALF at v2, the average speed is the harmonic mean: v_avg = 2v1v2 / (v1 + v2). Derivation: time1 = (d/2)/v1, time2 = (d/2)/v2, total distance = d, so v_avg = d / (time1 + time2). This exact case appeared in NEET 2023.
Average SPEED uses distance (total path length), while average VELOCITY uses displacement (straight-line change in position). Speed is a scalar and ignores direction, so it counts every metre travelled including back-and-forth motion. That is why average speed is always greater than or equal to the magnitude of average velocity.
The SI unit of average speed is metre per second (m/s), same as any speed. In common problems you may also see km/h. Its dimensional formula is [M^0 L^1 T^-1], the same as velocity, because speed is just the magnitude of velocity.
A vehicle travels half the distance with speed v and the remaining half distance with speed 2v. Its average speed is:
A toy car with charge q moves under a uniform electric field E. Its velocity increases from 0 to 6 m/s in 1 second. The field is then reversed and the car moves 2 more seconds. The average velocity and average speed between 0 and 3 s are respectively:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Average speed = total distance travelled / total time taken. In symbols, v_avg = (total path length) / (total time). It is a scalar quantity with SI unit m/s.
Average speed is a scalar. It has only magnitude and no direction, because it is built from distance (a scalar), not displacement (a vector).
Only when the object travels for EQUAL TIME intervals at speeds v1 and v2. If it covers equal DISTANCES instead, the average speed is the harmonic mean 2v1v2/(v1 + v2).
Average speed is always greater than OR EQUAL TO the magnitude of average velocity. They are equal only when the motion is along a straight line without any change of direction (no back-tracking).
Yes, but only if the object does not move at all, so total distance = 0. If it moves and returns to start, average velocity is zero but average speed is still positive.
