Did a Rock Hit Your Windshield, or Did Your Windshield Hit a Rock?

Now the ball behaves more like you would expect. Its horizontal velocity decreases so that it falls behind the original car. But it’s still not going to hit your windshield. With air drag and loss of energy on impact with the ground, each bounce is a little lower than the one before. You should be fine.

Rock Out

Now say that kid drops a rock out the window. Kids! Or maybe there’s a truck carrying gravel, and some of it slips through a crack. When the rock hits the road, its motion can change in a few ways. First, there’s a frictional force between the rock and the road, which will reduce the rock’s horizontal velocity. As we saw above, slower is bad.

Second—and this can seem weird—because a rock is irregularly shaped, it’s possible that it spins and hits the pavement in a way that makes it skip up higher than it started.

Hold on! Isn’t that a violation of the law? You know, the law of conservation of energy? Nope, it’s an energy transfer. A rotating and moving rock has both rotational and translational (linear) kinetic energy. Some collisions can convert rotational energy to translational energy, which makes the rock bounce higher. Higher is bad.

Here’s what that might look like:

So you’re cruising along the highway at, say, 70 miles an hour—and there’s a rock up in the air at the top of its path. It’s momentarily at rest, but you aren’t. Let the court take note: The rock didn’t hit you. You hit the rock. But the effect is the same. Either way, you need a new windshield.

Rock and Roll

That’s a plausible scenario, but what if there is no truck in front of you? Actually, a more common cause of windshield damage is a rock lying in the road that gets kicked up by another vehicle. You might think it gets shot back at the car behind, but that’s not quite right. The rock still doesn’t move backward.

Picture this: a wheel is rolling along the road and a rock gets wedged between the tire treads. When the rock comes in contact with the ground, it’s at rest. Here’s what that would look like: