How to track a video
- Film the motion face-on with a fixed phone (on a stand or resting), fairly far from the object, with a ruler or object of known length in the plane of the motion.
- Open the video (button or drag and drop). It is played by your browser and sent nowhere.
- Calibrate: 📏, then click both ends of the known length and enter its value.
- Set the axes: ✛ for the origin, ↗ to orient the x axis. By default the y axis points up, as in physics.
- Track: go to the first frame, click “This frame: t = 0”, then click on the object; the video moves on by itself. With a finger, a magnifier helps you aim. With 🎯 Auto-track, one click is enough: the object is found automatically on the following frames.
- Analyse: data table, vectors, graphs, model (line, parabola, sine), energy, then export or print.
How are velocity and acceleration computed?
Each point Mi is timed by its frame number: $t_i = (n_i - n_0)/f$, where $f$ is the capture frame rate. The velocity at Mi uses the two neighbouring points (central difference): $v_{x,i} = \dfrac{x_{i+1} - x_{i-1}}{t_{i+1} - t_{i-1}}$, and likewise for $v_y$. The acceleration is obtained the same way from the velocities. So there is no velocity at the first and last points, and no acceleration at the first two and last two.
Common mistakes
- Parallax: if the ruler is not in the plane of the motion, or the object moves towards the camera, distances are wrong. Film face-on and from a distance (zooming in a little).
- Variable frame rate: many phones record at a variable rate. The tool measures the video's frame rate; check it in the settings.
- Slow motion: a slow-motion video (120 or 240 frames/s) is often saved to play at 30 frames/s. Enter the real rate under “Capture frames/s”.
- Flipped y axis: image pixels are counted downwards. The tool points y upwards by default; free fall then gives $a_y \approx -9.8$ m/s².
- Motion blur: film in good light with a contrasting object; always click the same point of the object (its centre).
Going further
- Full guide: filming and tracking a video (equipment, phone settings, running the lab).
- Method sheet: working with a stroboscopic picture (speed, velocity vector, change in velocity, acceleration).
- Stroboscopic pictures: 5 printable worked exercises, which also open in the tool.
- Comparison: Tracker, FizziQ, NewMeca and others.
Frequently asked questions
Is my video uploaded to a server?
No. The video is opened and played by your browser, on your device. Only the tracking data (positions and settings) is kept in this browser for next time; you can clear it at any time.
Which video formats work?
Any format your browser can play: MP4 (H.264), WebM and often MOV. iPhone HEVC videos do not play everywhere: choose “Most Compatible” in the camera formats, or convert the video to MP4.
Can I track two objects?
Yes: “Add an object” creates a new series of points with its own colour (collision of two carts, two points on the same body…). The table shows the selected object; the graphs show all of them.
How can students work on the same video?
Set up calibration, axes and the starting frame, track the video if you want to provide an answer key, then open “Student mode” (section 6). The student link (or its QR code) fixes your settings, can hide velocities and accelerations so that they are computed by hand, and lets students compare their tracking with yours.
Can I compare measurements with theory?
Yes: choose a theoretical model above the graphs (uniform motion, free fall or throw, simple pendulum). Its initial conditions are fitted to your points while the law (g, pendulum period) is fixed; the curve appears on the graphs and the picture, with the mean gap between data and model.
My accelerations jump around from point to point: what can I do?
That is normal with a real video: a small tracking error is amplified twice by the derivatives. Choose “5” or “7 points, smoothed” for the velocity calculation, and measure your tracking uncertainty (click the same point several times): the tool shows it on every velocity and acceleration. For more precision, zoom into the picture (Ctrl + wheel or two fingers).
How do I share tracking with my students?
“Copy share link” or “Save (.json)”: the link and the file contain the points and settings, not the video. Send the video too; when it is opened, the points are put back frame by frame.
Can I make a paper worksheet?
Yes: “Print a student sheet” gives a stroboscopic picture on a white background, with the scale and the time between positions, followed by standard questions (velocities, vectors, change in velocity). “Print the full document” serves as the answer key.
Is it an alternative to Tracker?
For a kinematics lab, yes: manual or automatic tracking, calibration, axes, vectors, graphs, models and exports, with nothing to install, including on phones. Tracker (free, also available online for computers) goes further: more adjustable auto-tracking, dynamic models, multiple reference frames, advanced data analysis. See the detailed comparison.
How do I export to a spreadsheet?
“Copy” above the table, then paste into your spreadsheet. The CSV file uses commas and decimal points.
Five tips for a good video
- Keep the phone still, resting or on a stand: if the camera moves, the measured positions are wrong.
- Face-on and from a distance: stand perpendicular to the plane of the motion, 2 or 3 metres away, and zoom in a little rather than moving closer. Lens distortion and parallax are reduced.
- A known length in the plane of the motion: a metre ruler, an A4 sheet (29.7 cm), floor tiles… placed where the object moves, not in the foreground.
- Plenty of light and a contrasting object: the shutter speed is shorter and the object less blurred. A coloured ball in front of a light wall is ideal.
- High frame rate: 60 frames/s is enough for a 1 m drop (about thirty positions); 120 or 240 frames/s slow motion suits fast motion, as long as you enter that rate in the tool.
Working with a stroboscopic picture
On a stroboscopic picture, two successive positions are separated by the same time τ. If the positions are evenly spaced and in a line, the motion is uniform and straight: the velocity vector is constant and, by Newton's first law, the forces balance. If the spacing grows, the motion is speeding up.
To draw the velocity vector at Mi, measure the distance Mi−1Mi+1, divide it by 2τ, and draw a vector parallel to (Mi−1Mi+1), in the direction of motion, at the chosen scale. The tool does this for every point (“Velocity vectors” box); the printable student sheet leaves students to do it by hand.
The change-in-velocity vector Δv = vi+1 − vi−1 points the same way as the net force: for free fall it is vertical and downwards. Divided by the matching time, it gives the acceleration vector (“Acceleration vectors” box).
Three ready-made labs
- Free fall: drop a ball in front of a vertical ruler. Fit y(t) with a parabola or vy(t) with a line: the slope is the acceleration, to compare with g = 9.81 m/s².
- Projectile: throw a ball at an angle. Check that vx stays constant, that ay ≈ −g, and that the trajectory y(x) is a parabola.
- Pendulum: film a few swings, fit x(t) with a sine and compare the period with $T = 2\pi\sqrt{\ell/g}$. With the mass entered, the tool plots Ek, Ep and the total: mechanical energy slowly decreases because of friction.
The example videos on this page (free fall, projectile, pendulum, circular motion) are animations with a known scale and frame rate: use them to discover the tool and check the method before filming. To simulate these motions with or without friction, see the mechanics simulator.
Guide written and checked by Erwan Hautot · updated 7 November 2026.