You might think the coolest part of the SpaceX Falcon Heavy test was the Tesla with a spaceman riding inside, flying out into space. Yeah, sure, that part was cool. But for me, the best part was this footage of the Heavy's two side boosters returning to the launch pad. There are a bunch of cool physics things you can do with a video like this. For me, I'm going to answer two questions. First, how far away was this camera from the boosters when they turned on the engines? Second, what kind of acceleration did the boosters have while slowing down? Watch the video above and be sure to have the sound on. Notice that you see the rockets turn on before you hear them? Actually, there are several sounds and I'm not exactly sure what each one is. I know there are multiple sonic booms—but I'm not sure if they happen before or around the same time as the rocket engine. Actually, Destin from Smarter Every Day has a very nice video about the sounds of the Falcon Heavy launch—listen to it with earphones. So, let's assume that the really loud sound is from the engines turning on. We see them before we hear them because light travels way faster than sound. In fact, it wouldn't be crazy to assume that the light from the engines travels to the camera in zero time—at least that's what I'm going to do. This means that the time between seeing the rockets and hearing them is due to sound traveling over some distance. By knowing the speed of sound and the time difference, I can calculate the distance. In normal conditions, the speed of sound is approximately 343 m/s. From the video, the time between the flash and the sound is about 9.8 seconds. Here is the calculation of the distance.