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Shot Speed: measure a football kick from one video
Day 12 of the series: an app built with Claude in 13 minutes that turns a side-on clip into a shot speed, and why it needs your frame rate.
Shot Speed measures how hard you kicked a football from a single side-on video. Tap the ball, tap it again a few frames later, and the app gives you a speed. It's Day 12 of the series, and I built it with Claude in 13 minutes, inside a one hour limit.
The reason for building it is simple. Turf football always turns into fights, and the biggest one is whose shot is harder. Even the pros can't agree.
How it works
A video is many still pictures every second, called frames. In the app you tap the ball in one frame, then tap it again a few frames later.
That gives you two positions. To turn them into a distance, the app needs a ruler. A football is 22 centimetres wide, so the ball itself becomes the measuring tape. Compare how far it moved with how wide it looks, and you know how far it flew.
Then it's basic maths. Distance, divided by the time between those two frames, is the speed.
Testing it
Before trusting it on a real kick, I made a computer video of a ball moving at exactly 72 km/h. The app said 72. That told me the measuring logic was sound.
Then I ran a clip that looked like a 100 km/h shot. The app said 29.
Why it read 29
The clip was slow motion. The camera captured many more frames each second than the video plays back, so the shot looks slower than it really was. The maths uses the time between frames, and if you assume the wrong frame rate, you get the wrong time.
Here's the example. This video plays at 25 frames a second, and nothing in it says how fast it was filmed. If it was filmed at 120, the real shot was 140 km/h. Same clip, different truth.
So the app asks how you filmed it. Your phone's slow motion setting tells you. A random video online doesn't.
What to take from it
If you build anything that measures speed from video, treat the frame rate as an input, not something you can read off the file. A computer-made test at a known speed is a cheap way to check the maths, and a real clip is a cheap way to find what the test missed.
If you want to try the app, comment SHOT on the reel and I'll send it. Then send the reel to the friend who calls every shot a rocket.
