Video motion tracker: click a moving object frame by frame and get position, velocity and acceleration (video analysis for physics)

Open a phone video, set the scale with a ruler in the shot, click the object in each frame and get x(t), y(t), velocity and acceleration, with straight-line and parabola fits (g from a thrown ball). The video is played in your browser and never uploaded.

Keep it one tap away for everyday use.

Video
Scale
Click it
x, v, a

Nothing is uploadedWorks offline once open

Your video

MP4 or WebM from a phone or camera. It is played here, in your browser; nothing is uploaded.

Frames per second of the video Move on by this many frames after each click

Most phones film at 30 or 60 frames per second (slow motion at 120 or 240: type the rate it was filmed at, and times come out in real seconds). Film side-on with the phone still, and put a ruler or object of known length in the plane of the motion for the scale.

Points and scale

Time (s), x and y (pixels from the top-left of the frame), one point per line

Scale point 1, x and y (px) Scale point 2, x and y (px) Real distance between them (m) Origin, x and y (px; empty = first point)

You can also paste points from another tracker. Without a scale, distances stay in pixels. y counts upwards from the origin.

What it does

Video analysis is one of the best ways to see the physics in everyday motion: a ball thrown across a room, a toy car rolling down a ramp, a swinging pendulum, a jumping friend. This tool lets you do it with any phone video, entirely in your browser: the video file is opened on your device and never uploaded anywhere.

You step through the video frame by frame and click the object in each frame. The tool records the time and position of every click, converts pixels to metres with a scale you set from something of known length in the shot, and works out:

position x(t) and y(t), with y measured upwards,
velocity and acceleration at every point, from differences between neighbouring points,
a straight-line fit to the horizontal motion (constant speed) and a parabola fit to the vertical motion (constant acceleration), with uncertainties. For a thrown ball the vertical acceleration should come out close to −9.81 m/s².

It draws position, velocity and acceleration against time, shows a table, and downloads everything as CSV or as a report.

How to use it

Open a video (MP4 or WebM). Film side-on with the phone held still, and include a ruler, a metre stick or an object of known length in the same plane as the motion.
Type the video’s frames per second (usually 30 or 60; for slow motion, the rate it was filmed at).
Press Set the scale and click both ends of the ruler, then type its real length in metres.
Optionally press Set the origin and click the point that should be (0, 0).
Go to the first frame you want and click the centre of the object. The video moves on by the chosen number of frames; keep clicking. Undo last point and the frame buttons and slider help you fix mistakes; clicking a frame again replaces its point.
Read the results, or download them.

The points list is ordinary text (time, x and y in pixels), so you can also edit it, paste points from another tracker, or open a points file.

Worked example

Press Try a made-up example to load eight clicks of a ball thrown sideways, 1/30 s apart with one frame skipped, and a scale of 200 pixels for 1 m. The page shows a horizontal speed of 1.4989 ± 0.0062 m/s from the straight-line fit and a vertical acceleration of −9.829 ± 0.050 m/s² from the parabola fit, within half a per cent of free fall. The horizontal acceleration is −0.02 ± 0.20 m/s², zero within its uncertainty, as it should be with little air drag. Over the 0.2667 s the ball moves 0.401 m across and 0.347 m down. The point-by-point accelerations scatter much more (from about −10 down to −5 at the ends), which is why the fits are the numbers to quote.

Tips

Higher frame rates and a fast shutter (bright light) give sharper positions. Motion blur is the main source of click error.
Keep the camera square on to the plane of motion and far enough away that perspective does not change the scale across the frame.
Clicking every frame gives the most points, but every second or third frame spreads the same click error over a longer time, which steadies the velocities.

FAQ

Is my video uploaded? No. It is opened with a local link inside your browser and played there. Nothing about it, or the points, leaves your device.

Why is acceleration so noisy point by point? Taking a difference of a difference doubles the effect of every small click error. The parabola fit uses all points together and is far more reliable.

My video will not open. Browsers play MP4 (H.264) and WebM; some phone formats (HEVC) only play in some browsers. Convert the clip, or record in a “most compatible” setting. The tool is for study and planning, not a substitute for professional judgement.

Where the numbers come from

Finite differences on uneven time steps (second-order central differences, as in numpy.gradient) and least-squares polynomial fits with standard errors: standard numerical methods.
Checked against numpy.gradient and numpy.polyfit(cov=True), and in the browser against a generated test video of a ball moving at 1.2 m/s sideways and falling at 9.81 m/s².
The example points are made up.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.