Rolling without Slipping

## Question:

Anyway, here goes the exercise translated from Portuguese to English:

" A solid ball with 240g of mass rolls without slipping on a horizontal surface, with a constant speed of translation of the center-of-mass. The coefficients of static and kinetic friction between the ball and the surface are, respectively, 0.32 and 0.21. Taking as positive the direction of the speed of translation of the ball, what is the friction force between the ball and the surface, in Newtons? "

My teacher is still saying the correct answer is zero. I argued with him by e-mail, anonymous, and he just ignored my arguments and said to go talk with him personally. I know he just want to humiliate me, so I want to be sure of what I am saying. He asked me jesting, in the last email, if I knew what is ABS in cars or if I knew that in snow one shall use second gear, never first. I don't know what is his point, what it has to do with this case, I think he just don't want do admit he is wrong.

In the meantime, rolling without slipping, is a very particular case that I am not sure I understand, so I am not completly sure about this. Anyway, either kinetic or static there must be a Friction! That's my point, what I have been arguing.

When doing the exercise I supposed there was another force not mentioned... and calculated the Friction with: Friction = Normal * Uk.

I just could not conceive the friction didn't exist, given the conditions.
Well, what do you say?