Observe the motion of the two-stage rocket and the corresponding velocity-time graph below. The rocket has two consecutive fuel stages followed by a free-fall motion (no fuel). In the two fuel stages, the rocket experiences an upward acceleration of +10 m/s/s and +4.29 m/s/s respectively. This acceleration is depicted by the slope on the velocity-time graph. After ten seconds, the second fuel stage ends and the rocket is acted upon only by the force of gravity. It subsequently experiences a downward acceleration of -10 m/s/s. Note however, that from 10 to 16 seconds, the rocket continues moving upward (the velocity values are positive). During these six seconds, the rocket is moving upward but slowing down (the acceleration is downwards or negative as denoted by the negatively sloped line). It is not until after t =16 seconds that the rocket begins to move downwards.
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