A train travels 9 meters in the first second of travel, 9 meters again during the second second of travel, and 9 meters again during the third second. What is its acceleration?

Zero

Explanation:

Acceleration is defined as the rate of change in speed. The speed during the first second is 9m/1s = 9m/s. During the second second is again 9m/1s = 9m/s and again 9m/s for the third second. Being the speed the same, the acceleration is 0

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