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We know that the frictional force always opposes the relative motion. Recently, I happened to read somewhere that "It is the frictional force which causes a car to accelerate on road". My question is

a) How can the frictional force cause "the car to move forward"? According to the law, the friction always opposes the relative motion, and suppose the relative motion of the car is in the forward direction with respect to the road, so the friction must act in an opposite direction (that is backwards) and not help "the car to move forward". Or am I getting wrong somewhere?

b) The car pushes the ground backward which in turn pushes the car forward. This particular reaction force should have helped in the motion of the car. Shouldn't it be the this reaction force responsible for the car's motion and not the frictional force(since the friction will in an opposite direction to the relative motion)?

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  • $\begingroup$ "The car pushes the ground backward which in turn pushes the car forward" What do you think this force is? Also, think about how the tire is going to move relative to the ground when you try to rotate it $\endgroup$ – Aaron Stevens Aug 10 '18 at 16:34
  • $\begingroup$ Isn't this the normal reaction force? $\endgroup$ – Devanshu Pandey Aug 10 '18 at 16:35
  • $\begingroup$ It can't be friction since friction is opposite to relative motion? $\endgroup$ – Devanshu Pandey Aug 10 '18 at 16:36
  • $\begingroup$ Remove friction. In what direction will the wheels rotate? This is the direction friction opposes. You are thinking about the relative motion between the car and the ground rather than the wheel and the ground. Also, what do you mean by "normal reaction force"? $\endgroup$ – Aaron Stevens Aug 10 '18 at 16:41
  • $\begingroup$ OK I got it I was wrong indeed...but isn't it more appropriate to say that "friction helps the car to accelerate" than to say "friction accelerates the car"? $\endgroup$ – Devanshu Pandey Aug 10 '18 at 16:47
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here is an easy way to visualize this.

we see the car in profile, accelerating to the right. it is rear-wheel drive. now zoom in on the rear wheel. the engine, through the transmission, is urging it to rotate clockwise. if there were no friction between it and the pavement, it would spin freely in place and the car would not accelerate.

Now zoom in on the contact patch of the tire on the pavement, and assume there is friction. the tire is being urged to turn to the right but the friction force, acting right at the contact patch, forces the tire to engage the pavement. the tire pushes against the pavement towards the left, through the friction force. the pavement resists and through the friction force, applies a force towards the right to the tire. this force is opposite in sense to the direction the tire wishes to rotate but is in the same sense as the intended movement of the car. it is transmitted through the tire, wheel, and suspension to the frame and furnishes the force which accelerates the to to the right.

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    $\begingroup$ Standing behind a car trying to accelerate on a gravel road will make it painfully clear that the contact patch is exerting a frictional force backwards on the loose gravel... $\endgroup$ – DJohnM Aug 10 '18 at 17:46
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I do not fully understand the context of your quote, but it is technically true that friction causes/allows a car to accelerate.

Static friction is actually what is responsible for wheels and rolling to work.

Think of the point of contact for a tire as actually stuck to the ground from static friction. When you apply a torque to the axle, it rotates the wheel and tries to pull that point backwards. But since the point is "stuck" to the ground, instead the entire car falls forwards and lands conveniently on the next point of contact of the tire, which happens to be the patch of tire right in front of the last point of contact. Repeat this many many times as the tire rotates and the car will roll forward.

Without friction the car would just spin out continually.

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