An hour with… the Double Gravity Well

Individual Science Centre exhibits usually get no more than a few minutes of attention, but spending a whole hour near one can be interesting…

The Gravity Well is a classic and very popular exhibit, probably found in every science centre. We have a few of them as well, some of them set up as donation collection points for visitors to dispose of their coins in an interesting and educational way.

A Gravity Well exhibit is essentially a round table with a hole in its centre and a table surface curved in such a way that it gets steeper and steeper towards the centre, until it forms an almost vertical wall around the hole. This funnel is meant to represent the gravitational field around an object in space. So the movement of a coin or ball rolling around the hole is similar to that of a planet orbiting around a star (except for the friction between ball and table, that is).

Our Mathematics exhibition has an interesting variation on the theme – the Double Gravity Well. This is an oval table with two holes, creating a beautifully curved double funnel.

On busy days, this exhibit becomes a hub of social interaction. Every kid wants to have a go at rolling a ball and observing its progress, but nobody has the patience to wait for somebody else’s ball to complete its run. The result is a constant launching and capturing of balls, mid-orbit collisions and negotiations of launch priorities. After some time, these often non-verbal negotiations lead to collaborations. For example, I observed an older kid helping a little one who couldn’t reach for the large balls (from the nearby Packing exhibit) that were blocking the holes at the centre.

The exhibit demonstrates as much about human behaviour as about physics. The group dynamics among school boys is very different to that between siblings. And the latter behave differently if their parents, their domestic helper, another adult or other kids are present.

When given a chance, most kids – and adults – will start experimenting. How many times will the ball go around before falling through a hole if it’s launched along the side? And if it’s launched at a different angle? Can we roll it between the two holes without it veering off towards one of them? How can we make the ball roll in a figure eight around the two holes? Can we make the ball roll straight from the edge into a hole?

While I was watching from afar, two boys came up with a fun game of chance for two people: position yourselves on either side of the exhibit, launch the ball, wait for it to drop, and see in whose collection tray the ball emerges (there are two such trays). What a great opportunity to study probability and chance, which happens to be the topic of the one-armed bandit exhibit right behind the Double Gravity Well…

There is a panel with some instructions on the side of the exhibit, and the text also mentions potential and kinetic energy, binary star systems and more. But nobody ever seems to have the need to read that…

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