Constant-speed motion looks mechanical
Aliases: linear interpolation · constant-velocity easing · mechanical motion
What it is
Linear interpolation holds the derivative of position with respect to time constant. Real objects almost never travel that way: they spend energy to start and dump momentum to stop. On a screen the result is a velocity discontinuity at both ends, as if the object were being dragged on a wire. Linear easing is not a neutral default; it is a curve that manufactures acceleration jumps at the endpoints.
Why it happens
People are sensitive to biological versus mechanical motion. Constant speed plus an instantaneous start or halt leaves a spike in the acceleration profile (large jerk). The visual system reads that velocity profile as “pulled by an external force,” not as an object arriving on its own. Representational momentum adds to this: after watching something move, people expect it to coast to rest; a linear clip that drops to zero cuts that expectation, so the landing looks abrupt.
Linear can still be right for a looping spin or a progress bar, because those motions are already understood as a constant process with no “arrive at a resting pose” problem. As soon as the object must rest at the end, the endpoint shock of constant speed shows.
Studying it
Run the same displacement with a linear curve and a decelerating-arrival curve; collect naturalness ratings and a forced choice of “dragged / came to rest by itself.” Gaze after arrival is useful — an abrupt brake often triggers a corrective fixation. Independent variables: curve family, travel distance, whether the object is construed as having mass. Dependent variables: naturalness, forced choice, corrective fixations after arrival.
Do not use “was there animation” as the measure; the two curves can share a duration exactly.
Where it stops holding
- Progress indicators, infinite spins and scrubbing a timeline often want constant speed, because speed itself encodes steady progress or finger-binding.
- On very short durations the curve is undersampled, so linear and ease-out become indiscriminable.
- Deliberate mechanical aesthetics (industrial look, a conveyor in a game) can keep constant speed as a semantic choice, not as an un-tuned curve.
Applying it
- When an object must rest at the end, decelerate into arrival so cruise velocity does not drop to zero in one sample.
- Keep linear for processes that really are constant; do not use it as the global default interpolator.
- Check: plot position against time from a recording. If the slope does not fall near the endpoint and the object’s meaning is “come to rest,” constant speed is fighting the landing.