A car is moving without sliding means that the friction between wheels and the ground is static friction. This is the force causing an object to move forward, therefore, its direction is the same as the moving direction of the car.

My question is: For the horizontal forces acting on any moving(without sliding) car, $\ F_{fs}$ always oriented forward, what's the backward force to balance $\ F_{fs}$ so as to keep the car moving uniformly? Is that the Force produced by engines?

Just to clarify, I am referring to auto cars (with engines).

Any help or thoughts are appreciated!!!

  • $\begingroup$ Maybe air resistance and friction between wheels and shafts. $\endgroup$ – lucas Apr 30 '16 at 19:48
  • $\begingroup$ An ideal ball would roll indefinitely on a flat non-slippery surface; so there is no force there that would push it forward or backward. Similarly a car rolling on wheels without slippage - in case there is no air resistance, friction in the axles etc - would not have any horizontal force in either direction. $\endgroup$ – Maxim Umansky Apr 30 '16 at 21:22
  • $\begingroup$ For my question, we probably don't want to take the force of friction out of consideration because static friction is the reason why the wheel is moving. I am wondering if the car has a close to 100% inner efficiency and it is designed to have close to zero air resistance, does it means that this car cannot move uniformally? @MaximUmansky $\endgroup$ – JTing May 1 '16 at 18:20
  • $\begingroup$ @Jingting Liu - With 100% inner efficiency and with no air resistance there is no friction force between the road and the wheel, this friction force sets the kinematic constraint $V=\omega R$ where V is the speed of the car, R is the radius of the wheel, and $\omega$ is the angular velocity of the wheel; once this constraint has been set the friction between the wheel and the road is not there any more. It is useful to think about a ball rolling on a non-slippery flat surface - do you think for the rolling ball the friction against the floor is the reason why it continues rolling? $\endgroup$ – Maxim Umansky May 1 '16 at 21:40
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    $\begingroup$ @Jingting Liu - If the car is accelerating/decelerating then yes, this is the force that pushes it forward/backward. If the car is not accelerating/decelerating then this horizontal force between the wheels and the road is just absent. Consider an object on inclined plane not sliding down because of the friction force that keeps it there. If the plane is horizontal then the friction force is not there, it is not needed, the object is not "trying" to slide along the plane. Same with the car without friction and acceleration - the lower point of the wheel is not trying to slide along the road. $\endgroup$ – Maxim Umansky May 1 '16 at 22:01

I agree that friction in the drive mechanism reduces thrust, rather than opposing the motion of the car. However, this is not the case for wheels which are not in the drivetrain - ie where there is front/rear wheel drive instead of 4-wheel drive. Friction in non-drivetrain wheel mechanisms are then sources of resistance to motion. If the car has rear-wheel drive, then the static friction on the rear wheels is forwards and the friction on the front wheels is backwards.

In both types of wheel there is also "rolling resistance" which is the net horizontal component of mostly vertical forces caused by deformation of the tyre.


  • $\begingroup$ Thanks for the answer, I like the idea of non-drivetrain wheel mechanisms that you've brought up. I am confused about if the mechanism efficiency of the car itself is close to 100% and it is designed with really low air resistance, does it mean that it can hardly move uniformly? because the static friction which is the thrust is large and it's possible to artificially decrease its resistances. $\endgroup$ – JTing May 1 '16 at 18:10
  • $\begingroup$ What do you mean about the car not being able to move uniformly? If resistance to motion is small, the car will simply continue to accelerate until these forces increase to balance the thrust. $\endgroup$ – sammy gerbil May 1 '16 at 18:24

what's the backward force to balance $F_{fs}$ so as to keep the car moving uniformly? Is that the Force produced by engines?

The forwards force comes from the torque produced by the engine of the car and is transferred into the ground via static friction. The retarding force that keeps the car moving at the same speed is mostly air resistance (as well as a large assortment of inefficiencies in the actual mechanics of the car).

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    $\begingroup$ The inefficiencies in the mechanics can only reduce the force produced by the engine and indirectly reduce the ground static friction. These cannot be described as "balancing $\ F_f$". I agree with that air resistance is one of the factors. However, if that's the only factor, it means that a car can hardly move uniformly when air resistance is close to zero. This does not fit in the reality in my opinion. I think I we missing some important factors... $\endgroup$ – JTing Apr 30 '16 at 20:37
  • $\begingroup$ I didn't say they were balancing $F_f$ - I said they were retarding forces that keep the car from experiencing a net acceleration. However, in the context of the question I can understand how that could have been ambiguous. $\endgroup$ – Ben Bartlett May 1 '16 at 21:12
  • $\begingroup$ @JingtingLiu The major internal force which you (and the other answers here) are missing is engine braking, which is basically vacuum in the cylinders slowing down the the engine, thus slowing down the wheels, thus slowing the car via friction between the wheels and the road. Engine braking and wind resistance are the two major forces at work. Engine braking is far greater than wind resistance in most situations, especially at low speeds (put your car in 1st gear then roll down a hill and see what happens), I'm surprised it isn't mentioned. $\endgroup$ – Jason C Oct 13 '16 at 4:41

A free body diagram will show a car in motion has air drag force, gravity force and friction force on it. The net force keeps the car accelerated, decelerated, or moving at constant speed.

Friction force is due to relative motion between wheel and ground. Engine output spins the wheels (torque from power train system balances the torque produced from the frictional force).

Air drag force is proportional to car velocity to the power 2. It works opposite to the car moving direction. Zero air drag occurs when car is in stall. On horizontal surface, this is the only force on the car external opposite to the frictional force.

If the car is on slope, it can either move itself (down slope) or it need extra force from the frictional force.

There are many areas that consumes engine output such as frictions inside power delivery system and wheel sliding friction.


protected by Qmechanic May 1 '16 at 18:01

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