1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Hard arc length problem

  1. Apr 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A hawk flying at 2 m/s at an altitude of 80 m accidentally drops its prey. The parabolic trajectory of the falling prey is described by the equation below until it hits the ground, where y is its height above the ground and x is its horizontal distance traveled in meters. Calculate the distance traveled by the prey from the time it is dropped until the time it hits the ground. Express your answer correct to the nearest tenth of a meter.

    y = 80 - x^2/80

    ans : ____m

    3. The attempt at a solution

    arrggggg: here it is :

    y' = -1/40x

    1 + (f '(x)) = 1 + 1/1600 x^2

    [tex]\int\sqrt{1 + 1/1600*x^2}[/tex]


    1/40[tex]\int \sqrt{1600 + x^2}[/tex]

    using trig sub and letting x = 40tan( [tex]\Theta[/tex] )

    40 [tex]\int sec( \Theta )^3[/tex]


    [tex]\frac{40}{41}[/tex][tex] sec(\vartheta)tan(\vartheta) + ln| sec(\vartheta)tan(\vartheta) | [/tex]


    1/40[tex]\int \sqrt{1600 + x^2}[/tex] = [tex]\frac{40}{41}[/tex][tex] sec(\vartheta)tan(\vartheta) + ln| sec(\vartheta)tan(\vartheta) | [/tex]

    I am very lost if this is the integral. Any help please....
  2. jcsd
  3. Apr 11, 2009 #2
    Double check antiderivative: missing a plus sign.

    Not sure where 41 came from. Should be 2? Missing parentheses, too.

    Now convert back to x.
  4. Apr 11, 2009 #3
    the 41 is from the from the integral of 1/40 * sec^3 x
  5. Apr 11, 2009 #4
    Well, what is the integral of sec^3 x by itself?
  6. Apr 11, 2009 #5
    nope your right.

    This is what I got substituting it :

    20 ( sqrt(40+x^2) / x * x/40 + ln | sqrt(40+x^2) / x * x/40 | ) ( from 0 ->80 ?)

    not sure what the limits are.

    Last edited: Apr 11, 2009
  7. Apr 11, 2009 #6
    What is sec(theta)? What is tan(theta)? And where's that missing plus sign?

    Limits look good.
  8. Apr 11, 2009 #7
    sec(theta) = sqrt(40+x^2)
    tan (theta) = x/40

    since originally x = 40tan(theta)
  9. Apr 12, 2009 #8
    Hello tnutty,

    You are making things more difficult than necessary. The integral to solve is the following:
    [tex]\int \sqrt{a^2+x^2}dx[/tex]
    By doing the substitution you proposed, you end up with
    [tex]\int \frac{d \theta}{cos^3(\theta )}[/tex]
    This is not easier than the first one :-( What you can do is start with partial integration, giving:
    [tex]I=x\sqrt{a^2+x^2}-\int \sqrt{a^2+x^2}dx+a^2\int \frac{dx}{\sqrt{a^2+x^2}}[/tex]
    One of these is equal ti I itself and the other one can be solved almost directly by setting [itex]x=a sinh(t)[/itex]
    It looks difficult, but it is a lot easier than the trig way you used. The problem there is that you don't have the basic goniometric relation [itex]sin^2(x)+cos^2(x)=1[/itex] in it, but you do have [tex]cosh^2(x)-sinh^2(x)=1[/itex] for the last integral.

    Try it and let us know if this works.

  10. Apr 12, 2009 #9
    \int \frac{d \theta}{cos^3(x) dx}[/tex]

    ALL the integral below should have dx but it did not register.

    [tex]\int cos^2(x) * cos(x)dx[/tex]

    cos^2(x) = 1-sin^2(x)

    [tex]\int(1- sin^2(x) * cos(x)dx [/tex]

    u = sin(x)
    du = cos(x) dx

    [tex]\int( 1 - u^2) du[/tex]


    u - 1/3u^3


    sin(x) - sin^3(x) / 3 + C

    I think this might be a easier way to integrate than what was suggested, at least to me.

    I have one question. How did you find that the integral to solve is :

    \int \sqrt{a^2+x^2}dx[/tex]
  11. Apr 12, 2009 #10
    Hello tnutty,

    Let's reframe this step by step. First the setting up of the integral is correct, you got:

    Which is the same as the one I gave:

    [tex]\int \sqrt{a^2+x^2}dx[/tex]

    I left out the factor before the integral and set [itex]1600=a^2[/itex] to make the writing a bit easier. So no discussion on this part so far. The next step is to solve the integral, you used a trig substitution and obtained (beware of the naming, you're teacher will certainly not be pleased on an exam if you do this):

    Now, why are you solving this as:

    [tex]\int\ cos^3(\theta)d\theta[/tex]

    It is a different function. So the integral to solve has to be:

    [tex]\int\ \frac{1}{cos^3(\theta)}d\theta[/tex]

    This is where I would try to find something else and that was what I explained in my earlier post.

    Hope this helps,

  12. Apr 12, 2009 #11
    With your original approach, I think of [tex]\int\sec^3\theta\,d\theta[/tex] as a standard example. It will be in any table, and it will be almost certainly be worked out as a sample problem in your calc book. Look it up and you'll see you almost have it (but you're missing a plus sign).

    The problem is that substituting back to get in terms of x, your tan(theta) is correct, but your sec(theta) isn't. If you drew a triangle, you labeled it wrong or are using it wrong. If you used a trig identity, it's not right, or you used it wrong.

    Or use coomast's approach.
  13. Apr 12, 2009 #12


    User Avatar
    Homework Helper

    It's not an easy integral if you don't know the hyperbolic functions, but your substitution will work as well it will just take a bit more work.

    Here is a start.

    \int \sec^3 \theta\, d\theta &= \int \frac{1}{(1-\sin^2 \theta)^{\frac{3}{2}}}\, d\theta
    &= \int \frac{1}{(1-u^2)^2}\, du
    &= \int \left(\frac{1}{(1-u)(1+u)}\right)^2\, du
    &= \frac{1}{4} \int \left(\frac{1}{(1-u)^2}+\frac{1}{(1+u)^2}+\frac{1}{1-u}+\frac{1}{1+u}\right)\, du

    Try to check the steps and continue from here on. The substitution [itex]u=\sin \theta[/itex] was used.
    Last edited: Apr 12, 2009
  14. Apr 12, 2009 #13
    First IBP, then use trig identity:

    I&=\int\sec^3\theta\,d\theta \\
    &=\sec\theta\tan\theta-\int\sec\theta\tan^2\theta\,d\theta \\
    &=\sec\theta\tan\theta-\int\sec\theta(\sec^2\theta-1)\,d\theta \\
    &=\sec\theta\tan\theta-\int\sec^3\theta\,d\theta+\int\sec\theta\,d\theta \\

    Now add I to both sides and divide by 2:


    Or memorize it! :)
  15. Apr 13, 2009 #14
    Sweet, I figured out the answer = 118.3.

    I was using the wrong trig identity. Should have used x = 40tan(theta).

    If you want the solution let me know. Otherwise thanks for helping me.

    Such a long process for this problem. Feel like shooting that hawk for dropping its food.
  16. Apr 13, 2009 #15
    Very good, you found it. It can be written as: [itex]L=20\cdot [2\sqrt{5} + ln(2+\sqrt{5})][/itex]

    best regards,

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook