Acceleration and velocity question

[Spikyhed]

Homework Statement

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?

Homework Equations

I can't be too sure about this but i believe we use the formula:
Acceleration=(change in velocity)/time

and maybe:
Velocity=distance/time

The Attempt at a Solution

I got 12 and 1/3, but i am about 100 percent sure that's not right.

I divided 950 by 75... and that's all i could come up with. =(

 

[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.

 

[Spikyhed]

OK, A=change in velocity/time so A=75/time, and to get time, its distance/velocity. so that's 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 don't see where i am going wrong, i have been at this for close to 20 min.

 

[learningphysics]

OK, A=change in velocity/time so A=75/time, and to get time, its distance/velocity. so that's 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 don't see where i am going wrong, i have been at this for close to 20 min.

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.

 

[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?

 

[learningphysics]

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?

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...

A=(change in V)/T, and finally,
D=Vinitial*t+(1/2)a(t^2)

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

 

[Spikyhed]

I feel so stupid. OK. I apologize for being this dense.
I don't see a way to start with either, because they both have 2 unknown variables (time and acceleration).

 

[learningphysics]

I feel so stupid. OK. I apologize for being this dense.
I don't see a way to start with either, because they both have 2 unknown variables (time and acceleration).

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...

 

[Spikyhed]

Oh my gosh, why didnt i think of that!

thanks so much