The natural appearance of unnatural incline speed.

In three experiments, college students provided judgments about a marble's speed along a nonlinear incline. Each experiment revealed widespread support for the slope-speed belief, a mistaken belief holding that an object's speed at any point depends on the slope at that point. In truth, an object's incline speed varies with its elevation. In Experiment 1, participants relied solely on a diagram. In Experiments 2 and 3, participants observed computer animations depicting the descent of a marble at speeds conforming to either the slope-speed belief or Newtonian theory, and they rated the slope-speed version as more "natural" than the correct version. The task in Experiment 1 gauged participants' consciously available knowledge, but the perceptual realism of the slope-speed animations suggests that the slope-speed belief is also held outside awareness. By contrast, virtually all previously identified false beliefs about motion appear unnatural once animated.