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

You’re driving along, minding your own business, and—TING! Something smacks your windshield. You didn't see it coming, but the glass is cracked. Where the heck did it come from, and why did it hit your windshield? As I always say, you don’t understand something unless you can model it. So let’s try a little thought experiment. We’ll start with a basic scenario, where someone ahead of you drops an object in the road, and then see what we need to make it work. Follow the Bouncing Ball OK, imagine a car is driving in front of you at 30 miles per hour. A child in the back seat takes a bouncy ball and drops it out of the window. Why would they do this? Who knows why children do things. This is what it would look like to someone on the side of the road: Is that what you expected? The ball bounces up, but it continues to move along with the car. This is because when the ball is released, it's horizontal velocity is the same as the car’s. The only change is in its vertical velocity: The gravitational force causes it to move down until it hits the ground, at which point, it rebounds upwards. As you can see, it won’t hit the car behind. Oh, but what about air drag? It’s true, as the ball moves through the air, it collides with air molecules. This produces a force in the opposite direction. The magnitude of this drag force depends on the speed of the moving ball and its size. Plugging in some reasonable values, its motion now looks like this.