Intuition might tell us that rough surfaces on clothing would inhibit the speed of an olympic athlete. why is this line of thinking not true in some cases? how can controlled roughness actually promote speed?

We associate roughness with a greater coefficient of friction, then we think that rough surfaces on clothing would inhibit the speed of an Olympic athlete.  This is not always true because the amount of roughness might improve the way air pass through the fabric.  

One example of controlled roughness that actually promote speed are the dimples on golf balls. Dimples create turbulence around the ball, which sucks air to the ball, making its displacement easier and promoting its speed.

f=ma newton

f=100x2.5=250n