Monday, 17 February 2014

How children's understanding of gravity changes as they grow older

What happens if you drop a ball in a falling elevator and why? Your answer will of course depend on the sophistication of your understanding of the laws of physics. Psychologists in France and the Netherlands have used similar questions to test the understanding of 144 children and teenagers aged 5 to 18 years. The results show how children's naive understanding of gravity matures through different stages as a result of their first-hand experience and exposure to formal teaching and cultural explanations.

Soren Frappart and her colleagues tested the children's understanding in six contexts using cartoons, pictures and models. The young participants were asked to say what would happen if Billy dropped a stone on earth; in a lift in free fall; in a spaceship orbiting the earth; on the moon; on a planet with no air; and on a planet with air. Specifically, in each case, the children and teens had to indicate whether the stone would go down, float, or go up, and then explain why*.

The children's answers fell into three distinguishable levels of understanding, with each tending to be reached at different ages. A significant proportion (23 per cent) of the 5-year-olds said that the stone falls in all six contexts, and their explanations were mostly intuitive (e.g. "a stone can only fall"), or they gave no explanation at all. Presumably their answers were based largely on their first-hand experience that things fall.

Starting from age 7 and up to age 15, the children's and teens' answers were more sophisticated, but still reflected a far from complete understanding of gravitational laws. Around half of them were consistent in that they stated that the stone falls on earth and other planets with air; floats in a spaceship, on the moon and on a planet with no air; and goes up in a free-falling lift (although they were less consistent here). The fact that this response style kicked in at age 7 probably reflects the children's exposure to cultural concepts, such as the image of astronauts floating in space.

Scientific justifications for the stone's movement on earth began to emerge at age 12 (likely reflecting the introduction of relevant lessons in school), but scientific justifications for the stone's movement in other contexts didn't emerge until age 18.

Also at age 18, a new consistent pattern of responding emerged, one even more scientifically accurate than the younger age groups - that is, 27 per cent of 18-year-olds said the stone would go down on earth, on a planet with or without air, and on the moon. They said it would float in the spaceship orbiting the earth. They were inconsistent in their answers about the free-falling lift, although most said the stone would go up.

As well as providing the most comprehensive study to date of how children's understanding of gravity develops with age, this new research also informs a debate in psychology about whether children's naive, changing understanding about the world is incoherent and inconsistent, or if instead it is based on a succession of discrete mental models - each one akin to a flawed but coherent scientific theory. At least in the narrow context of gravitational laws examined here, these new results are consistent with the idea that children gradually acquire a series of largely coherent mental models of how the world works.


Frappart S, Raijmakers M, & Frède V (2014). What do children know and understand about universal gravitation? Structural and developmental aspects. Journal of experimental child psychology, 120, 17-38 PMID: 24361806

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

*Unfortunately, for those adult readers with a poor understanding of physics (including me), this journal paper did not provide the correct answers to the questions! I believe that the correct answers are as follows, but please correct me if I'm wrong: on earth, on the moon, and on a planet with or without air, Billy is stable with his feet on the ground, but when he drops the stone it is pulled downwards by gravity. In an orbiting spaceship and in a free-falling lift, the stone appears to float when dropped because Billy, the spaceship/lift are also falling and subject to the same gravitational forces as the dropped stone. 


Rachel said...

I had to think really hard about the lift, and ended up at the same conclusion as you: The stone appears to float because gravity is acting on the lift, Billy, and the stone, all exactly the same. We have to assume that Billy is falling with the lift and didn't get into it mid-fall. Of course the stone, and Billy, are falling relative to the building.

I wonder whether an alternative to a theory based explanation might be one based on similarity. As you say, experience teaches that stones fall when dropped on Earth, and culture might expose the idea that things float in space. The question then becomes, "Is this situation more similar to Earth or to space?" Uncertainty about which features are relevant would lead a planet without air to be judged more similar to space than to Earth. A free-falling lift is notably dissimilar to either situation, hence the inconsistent answers, including, "Up," (and surely there must be task demand effects here - why would "Up" be offered as an option if it isn't the right answer to any of the questions?)

I suppose this account might just displace the "Theory" into "Relevance," but it feels like a different sort of explanation than mental models.

I also wondered what educational paths the 18 year-olds were taking, and whether it could be diverse in France and the Netherlands as it is in the UK by that age.

Anonymous said...

Your answers are correct Christian.

Gravity is a force that acts bringing together all entities with mass. A rock has mass to it, so the rest of the mass in the universe is attracting it every time.

Thus, in the question "what happens to the rock when ... X?", can't ever shed darkness on whether gravity operates on the rock or not. It is regardless of X the absolute fact that gravity is always present pulling at the rock. Even when poor Billy is holding it.

Then, the tricky question is "how will the rock move when Billy drops it when... X?".

And to that, the answer is another absolute fact according to Newton's laws. The rock will always move in the direction of the resultant force after taking all forces into account. Since the planet/moon is so massive and so close to the rock, the gravity pulling towards the planet/moon is the prevalent force and the rock will always move towards the planet/moon.

And that is what always happens to the rock. And that is what always happens to Billy too.

The problem is that when Billy moves at the same speed as the rock, then Billy's senses can be deceived. For example, just by looking at the rock, Billy won't be able to know if he is in free fall or in the orbiting ship.

Within these situations, only that of the orbiting ship needs a conceptual warning. The rock does fall, and an orbiting object is in constant free fall. Indeed, anything that is in free fall but not orbiting, will eventually hit the ground.

Great blog and a very interesting post. I'll be dropping by (hehehe) from now on.

Christian Jarrett said...

Hi Rachel - interesting points, thanks. If a child's theory about gravity is based on similarity it can still be a theory though, right? Good point about the "up" answer and demand effects, I hadn't thought of that.

Christian Jarrett said...

thanks for clarifying the physics!

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