Has a rock hit your windshield, or did your windshield hit a rock?

Now the ball behaves more as you would expect. The horizontal speed decreases so that it falls behind the original car. But it is still not going to touch your windshield. With air resistance and energy loss with the ground impact, every bounce is slightly lower than that earlier. It will be fine.

Roll out

Now say that child drops a rock out of the window. Children! Or maybe there is a truck with gravel, and part of it slides through a crack. When the rock hits the road, the movement can change in a few ways. First, there is a frictional force between the rock and the road, which will reduce the horizontal speed of the rock. As we saw above, it is slower.

Secondly – and this may seem strange – because a rock is irregularly formed, it is possible that it turns and the sidewalk touches in a way that will be skipped higher than it started.

Wait a minute! Isn’t that a violation of the law? Do you know, the law of preserving energy? No, it’s an energy transfer. A rotating and moving rock has both rotation and translational (linear) kinetic energy. Some collisions can convert rotation energy into translational energy, so that the rock bounces higher. Higher is bad.

This is what that could look like:

So you sail along the highway at 70 miles per hour, for example – and there is a rock up in the air at the top of his path. It is temporarily at rest, but that is not. Let the court note: the rock did not hit you. You hit the rock. But the effect is the same. Anyway, you need a new windshield.

Rock and Roll

That is a plausible scenario, but what if there is no truck for you? In fact, a more usual cause of windshield damage is a rock that lies on the road that is made by another vehicle. You may think it will be shot back on the car behind it, but that is not entirely true. The rock is still not moving backwards.

Imagine this: a wheel rolls along the road and a rock is clamped between the tire tracks. When the rock comes into contact with the ground, it is at rest. This is how that would look like: