Lift speed sets the inertial initial velocity
Aliases: fling velocity · coast initial speed · flick
What it is
A fling hands the contact’s velocity to the content at the instant the finger leaves, so the content continues from that speed on its own. Tracking ends; simulated physics begins. Initial velocity is not the whole-path average. It is speed in a short window before lift. Pick the window wrong and a hard throw feels lazy, or a light send flies away.
Why it happens
While tracking, content is locked to the contact; lift opens the lock. If speed dropped to zero, a long list could only travel as far as one finger stroke, and browsing would be unaffordable. Writing terminal speed as v0 and integrating position under friction lets one arm motion cover many screens. Sampling jitter spikes instantaneous speed, so the last 30–80 ms are smoothed; too wide a window eats a whip-crack acceleration, too narrow treats last-frame noise as a throw. Convert pixels/frame into density-independent units or the same throw travels farther on a high-DPI panel. A fling is not a swipe: a swipe fires a discrete command and snaps to a detent at lift; a fling fires a continuous v0, and where it stops is the deceleration’s job.
Studying it
With a robot arm or a calibrated throw, record content v0 and first-frame displacement at a known terminal finger speed. Independent variables include the estimation window, whether the last frame is dropped, and refresh rate. Dependent measures include gain of v0 relative to finger speed, variance across repeated throws, and reports of “tracking suddenly let go.” Also test a slow lift (meant to stop where tracking ended) for a falsely injected v0.
Where it stops holding
A pager that spends fling v0 on skipping several pages fights the “one swipe, one page” swipe meaning; above-threshold flings should usually collapse to at most one detent, not free flight. A short, non-scrollable page should write no lift speed as v0, or you get an empty bounce. Trackpad inertia is gained; a touchscreen v0 calibration does not move there unchanged.
Applying it
- Take smoothed speed from a few tens of milliseconds before lift as
v0, calibrate in millimetres, and clamp so noise cannot fly the screen and a light lift cannot either. - On a slow lift (terminal speed inside tracking noise) set
v0to zero so content stays where the finger left. - Have one person throw ten times at the same effort and check first-screen distance variance; then lift slowly and confirm it is not a fling. Align high-speed video of fingertip leaving glass with the first frame of free coast.