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: Problem with easy diffrential equation

  1. Sep 26, 2006 #1
    Problem with an easy diffrential equation, the problem is explained in the picture ....

    http://img209.imageshack.us/img209/7341/diffproblemsf9.jpg [Broken]

    Kindly TNS
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 26, 2006 #2


    User Avatar
    Science Advisor

    You have
    [tex]\frac{dm_p}{dt}= \dot{m_p_i}[/tex]
    In that case
    [tex]m_p= \dot{m_p_i}t[/tex]
    would be correct only if [itex]\dot{m_p_i}[/itex] was a constant.
  4. Sep 26, 2006 #3
    First, check out https://www.physicsforums.com/showthread.php?t=8997

    So you can type the math notation in [itex] \LaTeX [/itex].
    To see how I typed things in the "math" click on the images and you will see the code. It's very easy, and the preferable way to communicate.


    So your equation is:
    [tex] \frac{d m_p}{dt}= m_{pi} [/tex]

    Now your teacher says that [itex] m_p = m_{pi}t [/itex].

    You are saying this:
    Let [itex] m_{pi} = m t^4 [/itex].
    [tex] m_p = \frac{m_{pi} t}{5} [/tex]

    You are not consistent with your notation. You should be careful here. For example, you introduce the variable [itex] m [/itex] and then it just disappears. However, with your argument you have:

    [tex] \frac{d m_p}{dt}= m_{pi} [/tex]

    [tex] m_{pi} = m t^4 [/tex]

    If we sub this in:
    [tex] \frac{d m_p}{dt} = (m t^4) [/tex]

    Now you say:
    [tex] m_p = \frac{m_{pi} t}{5} [/tex]

    What happens if you differentiate this?
    [tex] \frac{d m_p}{dt} = \frac{d \left( \frac{m_{pi} t}{5} \right)}{dt} = ? [/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook