How to Spot Fake Video Stunts—With Science

I hope you already know that you shouldn’t trust crazy-awesome videos on the internet; there’s a lot of fake stuff out there. But don’t worry, it’s possible to use physics and video analysis to see if they are indeed fake or actually real.

In this case, it’s this very cool-looking soccer trick. A guy kicks a soccer ball towards a wall with two holes in it (in the upper corners of a painted soccer goal). At the same time, another dude tosses a ball parallel to the wall. When the balls collide, each one ricochets into one of these holes. It looks magical. But alas, this is fake. If you look closely, you can see a cloud make a weird move, indicating a video edit (as spotted in an observant tweet).

But it’s more than just fake clouds. This soccer trick also breaks some physics rules. Really, this is the fun part—using some fundamental ideas to show that the video is fake.

The Motion of the Tossed Ball

I’m going to start with the ball that’s tossed from the guy on the side. I can easily measure the motion of this one because it’s moving across the camera’s field of vision. Using the Tracker video analysis tool, I can thus mark the horizontal and vertical location of the ball in each frame of the video. Also, by looking at the frame rate, I can put a time stamp on those coordinates.

With that, I get the following plot of horizontal position vs. time for the tossed ball:

The key thing to see here is that the data is linear. This means the ball moves in the horizontal direction with a constant speed (which is the slope of the line). I get –6.844 m/s (about 15.3 mph). Is that OK? Well, if you throw a ball, there is only one force acting on it after it leaves your hand (assuming it’s going slow enough to ignore air resistance), and that is gravity. Since the gravitational force pulls only in the downward direction, it doesn’t affect horizontal velocity. With no horizontal forces, there’s no change in horizontal motion. (That’s what forces do.) So this checks out.

What about the vertical motion? The downward-pulling gravitational force depends on the mass of the object as well as the local gravitational field (g = 9.8 newtons per kilogram). Since the vertical acceleration also depends on the mass, free-falling objects will all move with the same acceleration—no matter what the mass. This vertical acceleration has a value of –9.8 m/s2. Now, how do you measure the acceleration of a soccer ball from the video? If an object has a constant acceleration, then its position should agree with the following kinematic equation.





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