1. The problem statement, all variables and given/known data I'm trying to understand how static friction opposes the motion of a moving car. From what I have read, the car tyre doesn't move relative to the ground. The car tyre pushes on the ground and the ground pushes back - this is how the car moves. The force of friction is in the direction of movement. However, there are also frictional forces (which must also be static) trying to slow the car down otherwise the engine would not need to provide a force. So are there two static friction forces on a car; one trying to slow if down and opposing the motion of the car and the other providing the equal and opposite force to propel the car forwards? That doesn't really make any sense to me. Also we are doing this experiment where you change the material of a ramp that a car goes down and measure the acceleration using light gates. Would a higher frictional surface (e.g. sandpaper) decrease the acceleration? How do you explain this in terms of static friction? 2. Relevant equations None 3. The attempt at a solution Had a look on previous threads. I understand that it has to all do with static friction but I can't make sense of fricitonal forces which oppose and help motion.