What would I do without internet videos? It’s not just viral cat memes but also amazing humans like this guy, Xiao Qiang. In this short clip, the kung fu fighter appears to leap onto a bucket of water several feet off the ground—then bounce off the surface of the water like a trampoline. Whaaaat?
It’s an illusion, of course, but one that is made possible only by awesome strength and athleticism. Even when you know how it’s done, it still looks cool. So I’ll give you a clue right up front: As he flies through the air, watch a spot around the middle of his body.
Yep, once he pushes off the ground, his center of mass actually follows a normal parabolic trajectory. Just like when you toss a ball into the air, the only force acting on him at that point is the gravitational interaction with Earth. That means he has a constant downward acceleration, producing that familiar path. At this level, it’s normal projectile motion.
But he’s not just a rigid ball; his body is still working as he moves through the air, and that’s where the magic happens. To sort it all out, I ran this clip through my Tracker video-analysis app.
Plotting the Motion
Usually when I do video analysis to track the motion of something through space, I graph vertical position in each frame as a function of time. But there’s a problem in this case. Someone clearly messed with the frame rate in this clip to highlight the “water jump” portion. That means we can’t get a stable time scale.
Instead, how about we plot vertical position against horizontal position in each frame? If an object is moving through the air with only gravity acting on it, the horizontal velocity will be constant. This means we’ll still get a parabolic graph—it’s just a little harder to analyze.
What I’ve done here is trace the movement of three different parts of his body: his head, his feet, and his center of mass. Normally the center of mass would be around a person’s belly, but it changes as you move your arms and legs up or down, so this is a rough estimate.
So check it out: The center of mass (COM) follows the parabolic path we expect. But look at his feet. They reach the top and start moving down—then they bounce back up again, as if he really is jumping off the water. What’s really happening, as this graph shows, is that he’s pushing his feet hard away from his body near the top, then pulling them back up.
