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All your formular is of "constant accelerated motion".
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This is the expression for the average acceleration:
a = F/m
m = 9000lbs/32.2 = 279.5 lb-s^2/ft
a= 5864lb/279.5lb-s^2/ft
a = 20.98 ft/s^2
It is all in the formula, blackberry.
Think about it.
Do you know what the acceleration would be if just gravity is acting alone on the airplane?
32.2ft/s^2.....
Do you think our acceleration is going to greater or smaller than the acceleration of gravity at the beginning of the dive when we have the most excess thrust?
GREATER THAN
Why is our average acceleration lower than that of gravity???
20.98 ft/s^2
< 32.2ft/s^2......
Because there is an aerodynamic braking force acting on the aircraft that lowers the average acceleration.
Yes the formulation requires constant acceleration but that does not mean it does not accurately describe the motion of the aircraft.