An object starts at x = 0 m on a straight track. It travels 15 m in the positive direction in 5 s, remains still for 4 s, then travels 21 m in the negative direction in 7 s. Each moving stage has constant speed and direction changes take negligible time. Find its final position, total distance, average speed and average velocity for the whole journey. Explain why a moving-stage speed cannot replace the whole-journey average.

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Final position -6 m; distance 36 m; average speed 2.25 m/s; average velocity -0.375 m/s. Choose the stated positive track direction and preserve the signs. The final coordinate is 15 − 21 = -6 m, which is also the displacement because the starting coordinate is zero. Distance counts every travelled section without cancellation: 15 + 21 = 36 m. The stationary stage adds no distance but still occupies time. The complete elapsed time is 5 + 4 + 7 = 16 s. Therefore average speed is total distance divided by this time: 36/16 = 2.25 m/s. Average velocity uses signed displacement instead: -6/16 = -0.375 m/s. Its sign identifies the overall displacement direction, not every direction used along the route. The individual moving speeds are 3 and 3 m/s. Neither alone includes both travel stages and the pause, so neither is automatically the correct whole-journey average.

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