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: Drawing direction fields

  1. Aug 5, 2008 #1


    User Avatar

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

    (i'm not sure if this is the correct forum please move it if incorrect)
    my problem is with generally drawing direction fields - i dont really know what to do. i have notes but i can't make head nor tail of them. this is an example of a question concerning them:


    2. Relevant equations

    i know that if you stick the requirments into newton's 2nd law adding constant k you get that equation, and i know to find the terminal velocity you integrate and find v, but i dont know how to sketch the direction field.

    3. The attempt at a solution

    i see that

    dv/dt = -g + k/m

    but this seems to be independant of both v and t so how do i plot it as a graph?

    even if there were a v or t in that equation i still wouldn't be comfortable to draw the direction field because i don't know what to do >_>;

    Last edited: Aug 5, 2008
  2. jcsd
  3. Aug 5, 2008 #2
    Think of direction fields as a [tex]{dy \over dx} = f(x,y)[/tex]. At each point [tex](x,y)[/tex] on the graph, you draw a tic-mark with slope equal to f(x,y). So here, instead of x and y, we have v and t, and f(v,t)=-g+k/m. What's the slope of each tick mark going to be?
  4. Aug 8, 2008 #3


    User Avatar

    i'm not sure....some kind of straight line?
  5. Aug 8, 2008 #4


    User Avatar
    Science Advisor

    Check the meaning of the word "slope". It is a number not "some kind of line". What does the slope of a line mean?
  6. Aug 12, 2008 #5


    User Avatar

    ok i looked up slope and now know it is a number but the answer to this question still eludes me!

    i dont know how you'd put the equation on a graph

    the notes i have for this involve looking at the independence of say, x of the equation so im rather stumped. also i just suck at graphs...
    Last edited: Aug 12, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook