Many of us have gotten rock chips, but how many of us understand how those pesky rocks hit our windshield? A common misconception is that the car in front of us throws rocks “backward” and hits the following car’s windshield. A rolling tire cannot throw a rock backwards. A tire is a rolling object, thus every point along the tire is moving forwards. There is no force going in a backwards direction. Only direction part of a rolling object can go is a combination of up or down, and forward.
The velocity of the rock at any given point can be determined by adding it’s translational velocity at the center of mass (the orange arrow) with it’s rotational velocity.
Vrock= Vcenter of mass + Wrock Where V is the translational velocity, and W is the angular velocity
This can be simplified to Vrock=WDR Where D is the distance from the road at the point of contact in terms of R, the Radius. That is to say, that the velocity at the top of the tire would be Vrock=W(2R) =2Vcenter
That is to say, that the rock at the top of the tire may be going twice as fast as car itself. Similarly, at the point of contact of with the road, the velocity of the rock is 0.
So this leaves one to ask, how do those pesky rocks get thrown at a windshield? If it is hit by the following car, then it is because the rock was thrown somewhat vertically, slowed down by air resistance and the car behind it ran into the rock.
It can also be hit by a car going in the opposite direction. The magnitude of this collision will be much greater because it involves objects going in opposing directions. This is why the worse rock chips are often from cars going in the opposite direction, and why it is possible to throw rocks at yourself, which often do not do any damage.
Tires are thrown from tires because the centrifugal force expels snow, rocks, and other foreign objects.