Question

# Approximately when did the peak speed occur? (a) 25 s (b) 20 s (c) 10 s (d) 5 s

Solution

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The peak speed occurs when the acceleration becomes zero.

It is like a skydiver while falling down. The initial speed of him is zero and it increases until the drag force due to the air will be equal to the weight of the skydiver. At this moment, (when the two forces became equal), the speed of the skydiver is reached its maximum value.

But because we do not know if the acceleration of the airplane becomes zero at the end of the test, or not, then we will choose the last moment of acceleration in the given table.

That's because the acceleration is still in the positive direction, the speed of the airplane is increasing.

Note that, if the angle changed to the right side of the vertical dashed line (becomes a negative), then the acceleration of the airplane is negative and it is slowing down.

That is not the case which means that the airplane is speeding up along its test process.

Therefore, the peak speed occurs at the time of 25 s which is choice a.

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