Doing work against the friction force, temperature, and heat In Feynman's his introduction to the second law of thermodynamics, he said,

We know that if we
  do work against friction, say, the work lost to us is equal to the heat produced.
  If we do work in a room at temperature $T$, and we do the work slowly enough,
  the room temperature does not change much, and we have converted work into
  heat at a given temperature.

I suppose that if the temperature doesn't change, then the work done must go into the potential energy part of the internal energy of the gas in the room,  but then why must we do the work slowly?
The last statement is also quite confusing. As I understand, work and heat are essentially the same thing: a mechanism of transferring energy from one system to another. So is it just an abuse of language when he says, "we have converted work into heat"?
 A: Feynman wants to talk about constant temperature, because that simplifies the physics: "we have converted work into heat at a given temperature"
If friction transfers a lot of energy quickly, the material where friction is acting rises in temperature:  Those parts get hot, and it's no longer a constant-T situation.
So you have to have the friction act slowly, giving time for the energy lost to friction to spread out into the room and eventually into the environment of the room.  Since those are quite large, their temperature doesn't change significantly when the energy is added across all of that material.
As to work and heat being the same:  No, they're not.  That's the point.  Later, in the book, you'll see more about that. 
His "converted work into heat at a given temperature" is saying that work, the mechanical transfer of energy, has been changed into heat, the thermal transfer of energy. (Later you'll learn that, while it can go this way, going the other way is more complicated)
If you find yourself thinking that Feynman is committing an "abuse of language", that's a good clue that you're acting on an incorrect assumption or a misunderstanding.  His knowledge of physics and how to explain it were both above average.
