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I have heard a lot of people saying that when the tires of a car rotates backward, a friction acts in forward direction on the tire and the car moves forward.

Now according to my physics book,

" When a wheel rolls without slipping over a horizontal plane, the surface at contact do not rub against each other. The relative velocity of the point of contact of the wheel with respect to the plane is zero, if there is no such slipping. There is no sliding or static friction in such an ideal situation."

Now since the relative velocity is zero the friction should be zero and the car should not move forward ??

If all of the above is true, i am greatly confused about how does a car moves ??

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  • $\begingroup$ you're confusing "kinetic" friction, which is zero when the tire is not slipping, and "static" friction, which is non-zero and what propels the car forward. $\endgroup$ Nov 30, 2019 at 3:20
  • $\begingroup$ The book is not correct. There is no sliding friction, but there can be static friction. $\endgroup$
    – Dale
    Nov 30, 2019 at 3:33
  • $\begingroup$ Static friction comes into play when any body tends to move ,then how can static friction make any body move ? $\endgroup$
    – user223406
    Nov 30, 2019 at 3:35

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There are 2 types of friction: static and dynamic. Static friction is the friction that prevents an objects from sliding, until sufficient force is applied. Once the object is moving, dynamic friction takes effect.

You can think of static friction as the effect of the little imperfections on the 2 surfaces meshing with each other, hence preventing movement. Under dynamic friction, the peaks of the imperfections slide over each other, hence generating far less friction.

Dynamic friction is smaller than static friction, which explains why modern cars use ABS (anti-lock braking systems). This prevents the brakes from completely stopping the wheels from turning, and hence keeps the friction in the static range, which is higher and hence has more stopping power.

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