Mathematica Replacing Mass in Equations of Motion: Quick Mathematica Question

[kq6up]

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.

Thanks,
Chris

 

[OldEngr63]

Just a thought: Replace m1 by mm1 or some such, then replace mm1 by - m1. This may allow you to fool the computer.

 

[kq6up]

Yes, I ended up doing something just like that $m_1=-m_{11}$.

Thanks,
Chris

 

[Dale]

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.

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.

 

[kq6up]

Awesome, thank you.

Chris