# Homework Help: Acceleration and velocity question

1. Sep 20, 2007

### Spikyhed

1. The problem statement, all variables and given/known data
OK, here is the problem...

A plane starts at rest. It must be moving at 75 meters per second to get into the air. The runway the the plane starts on is only 950 meters long. What is the minimum acceleration the plane must have to get into the air by the time that the runway ends?

2. Relevant equations
Acceleration=(change in velocity)/time

and maybe:
Velocity=distance/time

3. The attempt at a solution
I got 12 and 1/3, but i am about 100 percent sure thats not right.

I divided 950 by 75... and thats all i could come up with. =(

2. Sep 20, 2007

### learningphysics

Here's a slightly different question... what is the acceleration required to get to 75m/s at the end of the 950m? Assume the plane starts at rest.

Use your equations for displacement during uniform accelerated.

3. Sep 20, 2007

### Spikyhed

OK, A=change in velocity/time so A=75/time, and to get time, its distance/velocity. so thats A=75/(950/75).

so.

if i am doing everything right, the answer sould be 5.92105 and so on. But the answer in the book says its approx. 3. I dont see where i am going wrong, i have been at this for close to 20 min.

4. Sep 20, 2007

### learningphysics

The equation d = v*t is only true for 0 acceleration... d = v*t is the wrong equation to use... you should have equation at your disposal for accelerated motion.

5. Sep 20, 2007

### Spikyhed

It might be on the site, but this is my first time ever posting. So i only have the 3 equations given by the teacher, which are V=D/T, A=(change in V)/T, and finally,
D=Vinitial*t+(1/2)a(t^2)

am i missing an equation or something?

6. Sep 20, 2007

### learningphysics

There's an equation you can use to directly get acceleration... but you can also use two of the above equations if you are supposed to only use these 3 equations...

Try to use the above 2 equations to find acceleration. V = D/T doesn't apply here.

7. Sep 20, 2007

### Spikyhed

I feel so stupid. OK. I apologize for being this dense.
I dont see a way to start with either, because they both have 2 unknown variables (time and acceleration).

8. Sep 20, 2007

### learningphysics

Don't feel bad... takes time to get used to this stuff...

You can solve two equations with two unknowns... solve for t in one equation (getting t in terms of a)... substitute t into the other... equation...

9. Sep 20, 2007

### Spikyhed

Oh my gosh, why didnt i think of that!

thanks so much