Word Problems with Parabolas

1. Feb 22, 2012

darshanpatel

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

Roads are often designed with parabolic surfaces to allow rain to drain off. A particular road is that is 32 feet wide is .4 feet higher in in the center then on the sides.

a) Find an equation if the parabola that models the road surface. (Assume that the orgin is at the center of the road.)

b) How far from the center of the road is the road surface .1 feet lower then in the middle?

2. Relevant equations

-None-

3. The attempt at a solution

x^2=4py
x^2= 4(.4)y
x^2=1.6y

^part (a) Correct?

x^2=1.6(.3)
x^2=.48
x = .693 feet away from the center

^Part (b) -Correct?

I kind of have a basis but I am a little wary about the answers.

2. Feb 22, 2012

Dick

The form x^2=4py is fine. If the origin is the center of the road then a point at the center of the road is x=0, y=0 and x is the distance from the center of the road and y is the elevation of the road. What should y be if x is 16?

3. Feb 22, 2012

darshanpatel

y=.4

but what next?

4. Feb 22, 2012

Dick

Use that to solve for p. p isn't equal to 0.4.

Last edited: Feb 22, 2012
5. Feb 22, 2012

darshanpatel

so like x^2=4py

x^2=4p(.4)
(1/.4)x^2=4p

4p=1/.4
p=1/1.6

x^2=4(1/1.6)y
x^2=6.4y

is that right?
becuase the first time i thought that (0, .4) was the focus, and p=.4 from the focus...

6. Feb 22, 2012

Dick

(1/.4)x^2=4p. x=16. What's p? It's not 1.6 either.

7. Feb 22, 2012

darshanpatel

p=160

so the equation for the graph would be x^2=640y?

and plugging in .3 for y
would get me answer to part (b)
13.856 feet from center?

8. Feb 22, 2012

Dick

Yes for the first part. .4 is how far the road is below the center at x=16. The problem is asking "How far from the center of the road is the road surface .1 feet lower then in the middle?". I wouldn't plug in .3 for y.

9. Feb 22, 2012

darshanpatel

oh, i was thinking of like where is the road only .3 feet or whatever,

do i plug .1 in?

Solved it for .1 and got 8

10. Feb 22, 2012

Dick

I really don't like responses like "do i plug .1 in?". Do you or don't you? Sketch a picture of the road surface and tell me.

11. Feb 22, 2012

darshanpatel

What do you mean you don't like them? I wasn't trying to be rude or anything.
You do plug in .1

12. Feb 22, 2012

Dick

I didn't think you were trying to be rude or anything. I just like hearing "You do plug in .1" better than the "?" thing. Yes, you are right. And for the equation of the road surface they might like x^2=(-640)y better than x^2=640y. x^2=640y is sort of 'upside down'.

13. Feb 23, 2012

darshanpatel

x^2=640y is upside down?

I don't understand.

14. Feb 23, 2012

Staff: Mentor

It's upside down relative to the road surface. The parabola above opens up - you want a parabola that opens down, so that water will drain off the road. That's what Dick meant by "upside down."

15. Feb 23, 2012

darshanpatel

ohhhhhhhhhhhhhhhhhhhhh, thank you, that makes a lot more sense, how would you show the work so that the number comes out to x^2=-640y?

Would you use -.4 when making the equation becuase it is 'upside down' now?

But would it matter because the answer comes out to the same thing no matter what?

Last edited: Feb 23, 2012
16. Feb 23, 2012

Staff: Mentor

Start the work you did earlier with x2 = -4py (with p > 0) to get a parabola that opens down.

17. Feb 23, 2012

darshanpatel

Can you help me with how the graph would look?

It's not for my homework, its just so I can understand it.

How would the parabola open and where?

Also to get part (b)

would you plug in -.1 for y and solve?

Becuase when I just plugged in .1 it would give me a negative answer

18. Feb 23, 2012

Staff: Mentor

How do you think the graph should look? You want the water to drain off the road, so it should have the high point at the center of the road's cross section.

This means you want a parabola that opens down. If the cross section of the road looked like a parabola that opened up, water would collect in the middle of the road, which isn't good.

Do as Dick suggested many posts ago by putting the vertex of the parabola at the origin. The left and right edges of the road will be at (-16, -.4) and (16, -.4).

If you substitute the x and y values of either point into x2 = -4py, you should be able to solve for p, which will be a positive number.
If the high point of the road is at (0, 0), every other point will have y values that are negative. You should get two x-values when y = -.1.