# Very Simple Airplane problem

1. Nov 20, 2011

### hotshot1kille

1. The problem statement, all variables and given/known data

A plane must reach a velocity of 2.4 km/h to lift off. If it has 45s of runway time, what must its acceleration be?

vf= 2.4km/h or 0.666666666666 m/s

vi=0.0 m/s

Δt=45s

a= ?
2. Relevant equations

vf= vi+ aΔt

3. The attempt at a solution

vf- vi
------- = a
Δt

0.666666666-0.0
---------------- = a
45 s

a= 0.014814814 m/s^2
a= 1.5 * 10^2 m/s^2

i did all of this but i am getting the wrong answer. Can someone tell me what i am doing wrong?

2. Nov 20, 2011

### arpeggio

a = 1.5 * 10^ (-2) m/s^2

negative 2 not positive 2
otherwise it seems fine

3. Nov 20, 2011

### hotshot1kille

the answer is 1.5 * 10 ^ 2 m/s^2 [E]. but they don't state which direction is negative or positive. so i am still right?

4. Nov 20, 2011

### Andre

Seems like two orders of magnitude error here. And that lift off speed is a very leisure walk speed. Could it be that the question reads:
A plane must reach a velocity of 240 km/h to lift off.

5. Nov 20, 2011

### arpeggio

as for direction, if it didn't state which way the airplane was flying, then it doesn't matter
I would say it is a bad question

and for 10^-2

the answer should be 10^-2 not 10^2

think about it, 10^-2 =1/(10^2) = 1/100 =0.01
so the answer should be 1.5 x 0.01 =0.015

6. Nov 20, 2011

### hotshot1kille

I got it directly from the worksheet i have and it says it must reach a velocity of 2.4 km/h. I know it is not realistic in a real world, but considering that the teacher just made up numbers so we can practice.

7. Nov 20, 2011

### arpeggio

yes maybe that is the problem
the airplane cannot fly when its speed is slower than a hamster on treadmill

8. Nov 20, 2011

### hotshot1kille

ye i agree with you