But not so deep as to introduce
a new source of FRICTION."
The Big Bang Theory Season 2, Episode 5
I know that money can
be a source of FRICTION
The Big Bang Theory Season 12, Episode 3
I'm detecting a little FRICTION
between you two,
The Big Bang Theory Season 7, Episode 15
...to static FRICTION between the tire
and the road to zero.
The Big Bang Theory Season 2, Episode 5