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Quick Mathematica Question

  1. Oct 3, 2014 #1
    I have equation of motion that our instructor wants to replace one or both of the masses with a negative mass in Mathematica. I tried m1=-m1, but I get an infinite recursion. What is the best way to do this? The equations of motion is a monster, so I don't really want to do this by hand.

  2. jcsd
  3. Oct 3, 2014 #2


    User Avatar
    Gold Member

    Just a thought: Replace m1 by mm1 or some such, then replace mm1 by - m1. This may allow you to fool the computer.
  4. Oct 3, 2014 #3
    Yes, I ended up doing something just like that ##m_1=-m_{11}##.

  5. Oct 4, 2014 #4


    Staff: Mentor

    The easiest way is:
    eq /.{m1 -> -m1}

    This goes through the expression eq and replaces every occurrence of m1 with -m1. It is very worthwhile learning this technique of using replacement rules.
  6. Oct 4, 2014 #5
    Awesome, thank you.

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