i have this burning question which i have thought about for a long time and yet i could not seem to figure it out. here it goes. when a wheel rotates to move a car or any vehicle, the friction would act on the opposite direction and so the friction (red arrow) would act in the same direction as the motion (black arrow), therefore propelling the car. However, given that this explanation is correct, how would the car travel at constant speed since theres no backward friction and there is always a resultant force (friction acting to the front) acting on the car? If you might say that the air resistance would oppose this force thereby allowing the car to travel at constant speed, i do not think that the air resistance is so huge to match the force, even if so, how then would the car be able to accelerate since the forces are always cancelled?