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I'm confused with the nature of the force produced by tires that pushes the car forward. This is what I understand:

Let us have a driving torque applied on the wheel axle clockwise. This will produce a force on the bottom of the wheel trying to push the ground backwards. According to Newton's 3rd Law, the ground will exert a force of equal magnitude towards the right. According to some textbooks, this is the driving force accelerating the car forwards.

However, other text books say that this 'driving force' is the static friction. I guess both of them are right, but I don't see the equivalence. For me, if the force exerted on the tire by the ground is in the direction of motion, static friction will point backwards, opposing the eventual relative motion (slipping).

It is probably a very basic misunderstanding, but I would be grateful if anyone could clarify this.

miku tess
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I've read on some textbooks that it is the reaction to the force that tires exert on the ground as a result of the drive torque on the axle.

Yes, that's correct. Forces occur in pairs. If you push on a wall, then the wall feels a force from you and you feel a force from the wall. If you have no other (horizontal) forces acting on you (perhaps you're standing on ice), then that force will cause you to accelerate.

The same happens with the tire and the car. The torque from the axle creates a turning tendency. This creates a force on the ground. The tire pushes the ground in one direction and the ground pushes back in the other direction. If this force is strong enough, the car moves. (Just as if your push on the wall is strong enough, you will move away from the wall).

Many others say it comes from static friction preventing relative motion.

Friction is the method that couples the tire and the ground together. If friction is removed (car on ice), then the torque from the axle only spins the wheel. There's no overall motion of the vehicle. With friction present, the force pair between the tire and the ground appears.

For me, if the force exerted on the tire by the ground is in the direction of motion, static friction will point backwards, opposing the eventual relative motion

If you gun the engine and the tire spins (or if you're on ice or something with minimal friction), it spins in a way where the bottom of the wheel moves backward (to the rear of the car) and the top of the wheel moves forward.

Friction creates forces that oppose relative motion. Since the wheel "wants" to move backward, that means that the friction force (on the tire) points forward. The relative motion we are talking about isn't between the car and the ground, it's between the bottom of the tire and the ground.

BowlOfRed
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  • Okay, but then, wouldn't the static friction point backwards? – miku tess Jan 08 '20 at 20:45
  • Can you expand on your comment? Static friction opposes relative motion. The axle is trying to push the bottom of the wheel in one direction (which direction depends on if the engine or the brake is providing the torque), and friction from the road creates a counter-torque. If you make your comment more explicit, we can talk about it. But when the engine is under power, it creates a situation where friction accelerates the car forward. – BowlOfRed Jan 08 '20 at 21:26
  • Hi, I have edited the question to try to explain my doubt better. Hope it is clearer now. Infinite thanks for your help and dedication. – miku tess Jan 08 '20 at 22:03
  • Added a bit to expand on the edit. – BowlOfRed Jan 08 '20 at 22:51
  • Nice, that has totally clarified my issue with the direction of static friction. But I still don't see this: in the abscense of friction (wheel on ice), you will have a force trying to push the ground backwards and the subsequent reaction pushing the tire forward. What do you mean by 'this force is not strong enough', isn't this force just dependent on the torque applied? I intuitively see the wheel will spin and don't move forwards, but then where does that force go? Thank you. – miku tess Jan 08 '20 at 23:47
  • There is no force. There's no "conservation of force" law. Forces can appear and disappear as necessary. The torque from the engine is still there, but it accelerates the wheel in this case instead of creating a force on the ground. – BowlOfRed Jan 08 '20 at 23:50