Elsevier

Acta Psychologica

Volume 194, March 2019, Pages 51-62
Acta Psychologica

Intuitive physics of gravitational motion as shown by perceptual judgment and prediction-motion tasks

https://doi.org/10.1016/j.actpsy.2019.02.001Get rights and content

Highlights

  • In Experiment 1, participants rated the naturalness of the falling motion of simulated polystyrene or wooden spheres.

  • In Experiment 2, participants were asked to predict the time-to-contact (TTC) of the sphere with the floor of the room.

  • In both experiments, the implied mass of the falling sphere affected participants’ responses.

  • An explicit ‘heavy-fast, light-slow’ heuristic or an implicit association between mass and falling speed can explain the results.

Abstract

In Experiment 1, we explored participants' perceptual knowledge of vertical fall by presenting them with virtually simulated polystyrene or wooden spheres falling to the ground from about two meters high. Participants rated the perceived naturalness of the motion. Besides the implied mass of the sphere, we manipulated the motion pattern (i.e., uniform acceleration vs. uniform velocity), and the magnitude of acceleration or velocity. Results show that relatively low values of acceleration or velocity were judged as natural for the polystyrene sphere, whereas relatively high values of acceleration or velocity were judged as natural for the wooden sphere. In Experiment 2, the same stimuli of Experiment 1 were used, but the sphere disappeared behind an invisible occluder at some point of its trajectory. Participants were asked to predict the time-to-contact (TTC) of the sphere with the ground by pressing a key at the exact time of impact of the lower edge of the sphere with the floor of the room. Results show that the estimated TTC for the simulated wooden sphere was slightly but consistently smaller than the estimated TTC for the simulated polystyrene sphere. The influence of the implied mass on participants' responses might be the manifestation of two processes, namely an explicit ‘heavy-fast, light-slow’ heuristic, and/or an implicit, automatic association between mass and falling speed.

Introduction

Since gravitational force is a constant presence in our everyday life experience, one might expect that people should have an accurate knowledge of how gravity affects the motion of objects. Yet, if it were so, it wouldn't have taken several centuries from Aristotle's ideas to Galilei's experimentation to shed light on the physical phenomenon (see, e.g., Darling, 2006). Research in the field of intuitive physics has shown that people do not have a good intuitive understanding of the physics of gravitational motion. Shanon (1976) showed that a significant minority of a group of students incorrectly believed that objects fall at a constant speed rather than with uniform acceleration as predicted by Newton's theory (see also Champagne, Klopfer, & Anderson, 1980). Additionally, most people believe that heavier objects fall faster than lighter ones (Champagne et al., 1980; Shanon, 1976). Due to the presence of air resistance, there is actually a small positive relationship between the mass of a falling object and its downward acceleration (see Baurès, Benguigui, Amorim, & Siegler, 2007; Oberle, McBeath, Madigan, & Sugar, 2005); However, this positive relationship is largely overestimated by people without formal instruction in Physics (Vicovaro, 2014). For instance, when two plastic bottles of identical shape and size – one empty and one filled with water – are dropped simultaneously from just one meter high, most people believe that the filled bottle will touch the ground well sooner than the empty bottle. Actually, the two bottles will arrive to the ground almost simultaneously, meaning that their downward accelerations are nearly identical, since one meter is not enough to appreciate the effect of air resistance.1 The belief in a strong positive relationship between mass and acceleration is persistent to change, and it is intuitively appealing not only to laypersons but also to fourth-year physics university students (Sequeira & Leite, 1991; for a comprehensive review on students' misconceptions about gravity see Kavanagh & Sneider, 2007).

In most of the studies in the field of the intuitive physics of gravitational motion, participants were asked to predict (i.e., to reason) about the unseen motion of a hypothetical object (e.g., Champagne et al., 1980; Sequeira & Leite, 1991; Vicovaro, 2014). By contrast, relatively little is known about people's ability to discriminate between physically plausible and physically implausible vertical falls if presented with realistic simulations of the event. Although people may fail in abstract reasoning tasks, it seems reasonable that they might have accurate knowledge of a phenomenon at a perceptual level, because the visual perception of an ongoing physical event would allow them to draw representations of physically-based, previously experienced events (Gravano, Zago, & Lacquaniti, 2017). In accordance with this hypothesis, it has been shown that judgments about physical events are more consistent with physical laws when participants are presented with virtually simulated events, as compared to when they are required to reason about abstract paper-and-pencil problems (e.g., Hecht & Bertamini, 2000; Kaiser, Proffitt, & Anderson, 1985; Kaiser, Proffitt, Whelan, & Hecht, 1992). Nevertheless, although realistic simulations of physical events may trigger stored representations of previously experienced events, this is not always the case. As studies have occasionally reported, marked discrepancies between physical laws and participants' judgments of realistic simulations of ongoing physical events can occur (e.g., Rohrer, 2003; Vicovaro, 2018; Vicovaro, Hoyet, Burigana, & O'Sullivan, 2014).

The study of subjective judgments of virtually simulated vertical fall has been limited so far by at least two technical issues. The first one is that an object that moves vertically downward on a computer screen, with approximately 1 g = 9.81 m/s2 of acceleration, remains visible for a very short time, making it difficult for the observer to evaluate the ‘naturalness’ of the motion. The second one concerns how the manipulation of the implied mass of the falling object actually occurs.

As to the first issue, researchers have attempted various approaches to increase the duration of the visible motion. For instance, Bozzi (1959) presented the participants with two-dimensional animations of objects descending along frictionless inclined planes, rather than presenting them with simulated vertical falls. From a physical viewpoint, a descent along a frictionless inclined plane is equivalent to a free fall, but lasts longer – on passing, this is exactly the reason why Galileo performed his experiments on an inclined plane. Bozzi (1959) found that the motion pattern that was judged as most ‘natural’ by participants was an accelerated one in the first third of the descent, followed by a constant velocity motion in the last two thirds of the descent. At a perceptual level, this would correspond to a constant velocity motion (see Zago & Lacquaniti, 2005). A nearly opposite pattern of results was reported by Shanon (1976), who employed edited slow-motion videos of balls falling vertically downward and found that uniformly accelerated motion was correctly judged as more natural than constant velocity motion. It is however unclear whether participants' judgements of such videos can be extended to more realistic scenarios. Twardy and Bingham (2002) presented the participants with virtual animations depicting a featureless ball falling with a parabolic trajectory from high above and that, at the end of the fall, bounced several times upon the ground. They found that animations that represented gradual increases in simulated gravity were on average judged as ‘natural’ as animations that represented Earth's gravity, whereas animations that represented gradual decreases in simulated gravity were judged less ‘natural’ than those that represented Earth's gravity. However, participants' naturalness judgments in Twardy and Bingham's (2002) study were based on the kinematic features of the bounces, rather than on the motion patterns from the beginning of the descent to the contact with the ground.

Unfortunately, the generalizability of the results of these previous studies is hindered by the fact that the involved stimuli did not represent realistic falls – as for Bozzi's (1959) and Shanon's (1976) works – or by the fact that participants' judgements were not based on the vertical fall of an object – as in the case of Twardy and Bingham's (2002). In Experiment 1 of the current study, we presented participants with wall projections of virtually simulated vertical falls, and we asked them to evaluate the ‘naturalness’ of each fall. The use of wall projections allowed us to maximize the duration of the simulated falls, while preserving their realism.

As to the second technical issue, i.e., the manipulation of the implied mass of the falling object, Bozzi (1959) achieved it by means of different sizes and, intriguingly, found that relatively high (low) velocities were perceived as most ‘natural’ for descents of large-sized (small-sized) objects.2 Since large-sized objects are typically associated with a larger mass than smaller-sized ones, this finding might indicate a positive relationship between an object's implied mass and its perceived ‘natural’ velocity along an inclined plane. However, the well-known negative relationship between size and perceived velocity (Brown, 1931) implies a confounding between size and perceived velocity in Bozzi's (1959) study. Specifically, it cannot be excluded that the relatively high velocities that were judged as most ‘natural’ for large-sized objects were actually the same – at a perceptual level – as the relatively low velocities that were judged as most ‘natural’ for small-sized ones. One of the aims of Experiment 1 is to disentangle the contributions of the falling speed/acceleration and of the implied masses on the judged naturalness of the falls. We did so by varying the implied mass of the falling object through manipulations of their simulated materials, rather than through manipulations of their size.

In Experiment 1 of the current study, we presented participants with virtually simulated material objects that fell vertically to the ground from about two meters high. Participants were asked to rate the perceived naturalness of each fall. Three factors were orthogonally manipulated, namely the implied mass of the falling object (i.e., light or heavy), the motion pattern (i.e., uniform acceleration vs. uniform velocity), and the magnitude of acceleration/velocity. Hypothetically, if participants could retrieve some memory-stored representation of previously experienced phenomena, then simulated falls which are characterized by ≈1 g acceleration should be rated as more natural than those characterized by physically implausible motions. Moreover, naturalness ratings should be largely independent of the implied mass of the simulated objects, because mass has but a small influence on the acceleration of objects falling from about two meters high.

Section snippets

Participants

Thirty graduate or undergraduate students at the University of Padova participated in the experiment on a voluntary basis, and received €5 for their participation. They were aged from 20 to 34 years (M = 23.67 years, 95% CI [22.4, 25]), 18 were females and 12 were males. None of them had studied or were studying physics at the University. They had studied physics at the high school for at most three years. All participants were naive to the purpose of the experiment, and gave written informed

Experiment 2

Results of Experiment 1 suggest that judgments of the perceived naturalness of vertical falls were likely driven by a ‘heavy-fast, light-slow’ heuristic, rather than by representations of physically-based, previously experienced vertical falls. A possible interpretation of this finding is that instructions to judge the ‘naturalness’ of the motion may have led the participants to reason about the meaning of ‘natural’ or ‘physically plausible’ motion in the case of vertical fall. This may have

General discussion

People without formal instruction in Physics tend to explicitly believe that heavier objects fall faster than lighter ones (Champagne et al., 1980; Sequeira & Leite, 1991; Shanon, 1976; Vicovaro, 2014). Contrary to what could be expected on the base of previous studies that compared explicit and perceptual judgements of physical events (Hecht & Bertamini, 2000; Kaiser et al., 1985; Kaiser et al., 1992), results of Experiment 1 showed that participants responded according to a ‘heavy-fast,

Author's note

Original materials used to conduct the research will be made available upon request. Raw data can be downloaded from here: https://osf.io/z45t2/?view_only=95743205eb5d47619855e836f96d7c9a

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