There should be a button that removes the "lock" from changing values to -ve for mass, friction and many other values. So you should be able to type "-"/"letters" in the values.
Pros:
You get to do many stuffs easily (IF, Invalid Rotation, -ve Mass and much more)
Cons:
You wont know when you do something wrong like double -ve sign (Mass=--40)

If you tick Dynamic Ground, you should be able to change the values.

Sumyungho a dit :

If you tick Dynamic Ground, you should be able to change the values.

Only so much. He wants to be able to set negative mass and all other sorts of glitches within the map editor.

I would be fine with it but I'm confident that they won't implement it.