An airplane is sitting on a runway waiting to take off. the airplane begins to move down the runway and it takes 10 s for the airplane to leave the ground. the speed of the airplane at takeoff is 40 m/s. what is the acceleration of the airplane while it is moving down the runway?

Acceleration = (change in speed) / (time for the change)

-- You said that the airplane has to speed up from zero ("sitting") to 40 m/s, so the change in speed is 40 m/s.

-- You said that it has to roll for 10 seconds to build up enough speed to take off, so the time for the change is 10 s .

Acceleration = (40 m/s) / (10 s)

Acceleration = (40/10) (m/s)/s

Acceleration = 4 m/s²

That seems like no problem.  It's only like about 41% of 1 G .  That would not even spill the drinks in First Class, or wake up the passengers who are already asleep (like me).

the answer is 3.1 his answer is wrong mines is right

probably not if net force is zero