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: Partial derivative problem... why is my answer wrong?

  1. Nov 29, 2017 #1
    1. The problem statement, all variables and given/known data
    The entire problem is in the attached picture. I have been checking and double checking for about an hour, found solutions online which agree with my solution, but I cannot find any answer beside -3.697 m/s which is marked wrong by the computer program.

    2. Relevant equations
    Is the homework program wrong or am I somehow missing something?

    df/dt = (2x-y)/(2(x^2+y^2-xy)^(1/2))(dx/dt) + (2y-x)/(2*(x^2+y^2 -xy)^(1/2))(dy/dt)

    3. The attempt at a solution
    All of this works out to -3.697. I have tried rounding, leaving the answer as postive, nothing seems to work. Very frustrated.
    upload_2017-11-28_22-25-24.png
     
  2. jcsd
  3. Nov 29, 2017 #2

    Delta²

    User Avatar
    Homework Helper
    Gold Member

    I also get 3.697... but where did u get the minus sign from, you input the velocities as negatives?
     
  4. Nov 29, 2017 #3
    The minus sign comes from the fact that the distances x and y at time t will be given by x = 21 - 5t and y = 25 - 3t hence dx/dt = -5 and dy/dt = -3. But the computer will not accept it either way. Unless any one else can see some mistake we've both made, I will now go complain to my teacher!

    Thanks
     
  5. Nov 29, 2017 #4

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I agree with your answer.
     
  6. Nov 29, 2017 #5

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    More simply: the two people get closer together as ##t## increases.
     
  7. Nov 29, 2017 #6

    DrClaude

    User Avatar

    Staff: Mentor

    Have you tried fewer decimals? Four significant digits is a bit much considering the numbers given.
     
  8. Nov 30, 2017 #7
    It turns out that whoever wrote my homework program has never in their life heard of significant figures. Given the values involved the answer should contain only 1 sigfig, but the solution was to input SIX MORE DECIMAL PLACES from my calculator.
     
  9. Dec 1, 2017 #8

    DrClaude

    User Avatar

    Staff: Mentor

    And I suggested you use fewer :rolleyes:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted