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I fully understand why the friction force points forwards on the back wheel. In my head, if i were to think about a rouge, unpowered, solo wheel, rolling along the ground for example, the friction force on the wheel is still pointing backwards. Furthermore, in my mind, I dont see a difference between this rouge unpowered wheel and the front two wheels of the car.

Simply stated: If both the front and back wheels of a car are spinning in the same direction, shouldn't the friction forces be pointing in the same direction? How would the ground even know whether the wheels are powered or not?

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  • $\begingroup$ Okay do you think when the friction on the front wheel will be in the same direction as the back wheel then the car will be able to move? Simply $NO$, if the friction on the front wheel will act in the same direction as the back wheel then it will produce the torque opposite to the direction as torque provided by the engine and the car won't move at all it's very simple to understand with this example just draw a diagram and try to resolve the direction of torque when friction is in the same.... $\endgroup$ Jan 21, 2022 at 7:50
  • $\begingroup$ .....direction as that of back wheel and for the case in which the friction is in the opposite direction you will automatically get your answer! $\endgroup$ Jan 21, 2022 at 7:50
  • $\begingroup$ Just to be clear, when you ask in which direction the force "points," are you asking about the force that the road exerts on the passive wheel? or are you asking about the equal and opposite force that the wheel exerts on the road? $\endgroup$ May 17 at 18:17

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Imagine for a moment that the car is moving from left to right and there is no friction under the front wheel. That wheel will just slide and not rotate. Now if you want to make that front wheel rotate clockwise by applying a force to the bottom of the wheel, which direction must you apply that force?
The ground "knows" which wheel is powered because that wheel is applying a horizontal force to the ground which friction opposes. For the freely rotating front wheel the ground is applying a horizontal frictional force that makes it rotate, i.e. in the same direction as the rotation.

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With the back wheels rotating clockwise and accelerating a car forwards, the force on the back wheels due to the ground is in the forward direction.

The front wheels need to rotate in a clockwise direction faster and this is done by a torque applied to the front wheels by the ground in a clockwise direction.
So the forces on the wheels due to the ground which produce that torque are in the backward direction.

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Think about it... friction force always oppose the intended direction of relative movement. On the rear wheels, the contact path is pushing the ground backward and there is a negative relative motion between rear Tyre( tyre surface moving backward as compared to ground) and ground, causing ground to oppose the motion via a friction force against the intended direction of relative motion.Since the front wheels are constrained by the vehicle body, it needs to have same translation motion. However, if you look at the contact path, there is a positive relative movement, thus ground applier friction in opposite direction, which is backward.

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