A mechanic is examining a tire. The moment of inertia about its spindle is given by 1/2 mR². He hoists the tire up, sets it with an angular velocity of ω_i, and brings it back to the ground. Right at the moment when the tire makes contact with the ground, the center of mass of the tire has a velocity of zero. After touching the ground, the tire initially skids on it, and then travels forward and in the end starts to roll without skidding. Determine the direction in which the friction force acted on the skidding tire.