How to track a video in physics

From filming to the data table: everything you need for a successful video analysis lab, with just a phone.

Equipment

A phone or tablet that records video (any recent model), on a stand, a holder or propped against books.
A metre ruler (or any object of known length: an A4 sheet is 29.7 cm, a 30 cm tile…).
A contrasting object: a brightly coloured ball, a marble, a small cart with a sticker.
A plain, light background and plenty of light (near a window or under bright lighting).

Frame rate: 60 frames/s if possible (“1080p at 60 fps”). Slow motion (120 or 240 fps) suits fast motion; note the rate you chose.
Format: on iPhone, Settings › Camera › Formats › “Most Compatible” (H.264 video, which every browser can play).
Focus and exposure: press and hold on the object before filming to lock them; avoid automatic zoom.
Stabilisation: if the phone is fixed, stabilisation is not needed and can even shift the picture slightly.

Put the ruler in the plane of the motion: where the object moves, not in front of it or behind it.
Place the phone facing the plane of the motion, with the lens axis perpendicular to it, 2 or 3 m away; zoom in a little (×2) rather than moving closer: parallax and distortion are reduced.
Check that the whole motion fits in the frame, with the ruler clearly visible.
Start recording, perform the motion, stop. A few seconds of video is enough.

Open the video in the tracking tool (button or drag and drop). It is sent nowhere.
Check the frame rate in the settings: the tool measures it; for slow motion, enter the real rate under “Capture frames/s”.
Calibrate (📏): click both ends of the ruler and enter its length.
Set the axes: origin (✛) on the first position, x axis (↗) along a horizontal line. Keep y upwards.
Go to the first frame of the motion and click “This frame: t = 0”.
Track (●) the centre of the object on each frame, or let auto-tracking (🎯) do it: click on the object, the tool finds it on the following frames and stops if it loses it.
To aim precisely, zoom in (Ctrl + wheel, two fingers or the + and − buttons): the view follows the object while tracking. A video filmed sideways is straightened with ⟳.
Check the points, fix them with ✋ if needed, then analyse: vectors, graphs, models, comparison with a theoretical model, energy. On a real video, choose “smoothed” derivatives if accelerations are too scattered.

Groups of 3: a thrower, a camera operator, someone in charge of the ruler. Each group films on its phone and tracks on the same device (by finger, with the magnifier) or on a computer.
Students export the table (CSV or copy to a spreadsheet) and save the session (.json) to hand in with their report.
The tool's student mode (section 6) gives a link or QR code: calibration and axes are fixed, velocities can be hidden so they are computed by hand, and each student then compares their tracking with the teacher's answer key.
Teachers can prepare a lesson without equipment: the worked exercises open directly in the tool, and the example videos on the tool page are ready to track.

The video will not open: it is probably HEVC; change the phone's format (see above) or convert it to MP4 (H.264).
Values are twice too big or too small: the frame rate is wrong (slow motion) or calibration used the wrong length.
Free fall gives a_y ≈ +9.8 m/s²: the y axis points down; untick “y downwards”.
Auto-tracking stops: the object is blurred or passes in front of a background of the same colour; track a few frames by hand and start again, or enlarge the tracked patch.