Calculating Takeoff Time for an Aircraft

[Miri]

Homework Statement

An aircraft has a lift-off speed of 40m/s. How long does it take the aircraft to be airborn after a takeoff run of 320m?

Homework Equations

I know that I have to divide 320m by the speed (40m/s). But if I do that, I get a wrong result. The result should be 16s and this is only true if I use half of the speed (20m/s). Can somebody explain me why I have to take half of the speed? Thanks

 

[Borek]

At the beginning plane was not moving, then it started to accelerate. Its speed was not constant.

 

[Doc Al]

Can somebody explain me why I have to take half of the speed?

As Borek said, the speed is not constant. Hint: What's the average speed during takeoff?

 

[Miri]

Oh yes..average speed =40/2...thx