I hope you already know you shouldn’t believe every crazy-awesome thing you see 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 what’s real and what’s not.
So, some guys tweeted out this cool-looking soccer trick: One dude kicks a ball toward a wall that has an outline of a soccer goal on it, with small two holes in the upper corners. At the same time, another guy tosses a ball parallel to the wall. When the balls collide, they ricochet into the holes like billiard balls. It looks magical. But alas, it’s 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 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 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. 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: