Review of Newton's Aquarium - Educational Physics Software

Review of Newton's Aquarium™
by David Pooley, Physics Graduate Student, University of Waterloo

While in high school, two questions directed me into the world of science: why? and how? Why is the sky blue? How do planes fly? Answers to these questions increased my knowledge and whet my appetite for more. Nevertheless, it rapidly became apparent to me that to gain understanding, another question is equally important: what if? Newton's Aquarium™ excels at giving students the chance to explore this question regarding Newton's Law of Universal Gravitation and basic celestial mechanics.

Strengths
Newton's Aquarium™ calls itself a "solar system construction set". It allows the student to place stars, planets, moons and asteroids together in a fictional 3-dimensional universe with user-defined masses, positions and velocities. More significantly, the student can modify the universal law of gravitation by altering the distance exponent or by changing the gravitational constant, G. This allows, for example, experiments with a distance-cubed law or a significantly weaker gravitational force. A click of the mouse starts the universe in motion, visually displaying the answer to "what if".

One nice feature of the program is its ability to graphically display force, acceleration and velocity vectors. The convention of representing vector magnitude by line length is followed. These vectors can be shown or hidden for each body individually or globally toggled on and off for all bodies. While the vector nature of force and velocity is perhaps obvious, I personally found it harder to grasp for acceleration when learning this material. The ability to "see" acceleration vectors, especially for circular type motion, makes understanding easier.

Similarly, the importance of center of mass took a while to sink into my brain. Watching the center of mass location on the screen, and seeing its relation to object motion, helps to solidify what it represents. Although it is unlikely that Newton's Aquarium™ would help a student do calculations in a center of mass coordinate system, the student may be in a better position to understand why this is often important.

To help you get started right away, Newton's Aquarium™ comes with a number of sample universe documents. The user is free to run them as is, modify them or create new universes altogether. Creating a new universe is relatively simple (the point-and-click interface is well laid out and simple to use), but the process can be somewhat tedious. Most of the initial configurations I created from scratch turned out to be very unstable. Either the planets would fly off to nowhere or quickly collide with one another. This was especially true for any scenarios I felt were "interesting". Thus, I increasingly found myself modifying the sample universes provided ("orbit layers" and "asteroids" were my two favourite). The ability to add textual descriptions to each universe was also quite helpful. It allows the universe designer to clearly indicate what is being demonstrated, or what the user should be looking for.

Shortcoming
One minor annoyance I had with Newton's Aquarium™ concerned plot rates. Each universe stores its own plot rate and timestep. On the surface this may sound reasonable. However, the apparent speed default of "fastest" (5 levels in total) combined with the given timesteps produced simulations that were far too fast to follow on my PowerMac 7300/180. This forced me to decrease the plot rate every time I opened a new universe. Admittedly, I could save the change once made, but an overriding global setting for plot rate or timestep would have been more convenient.

Another feature I would like to see is access to velocity, force and acceleration values. Clearly, the program is calculating these numbers, but to my knowledge there is no way to view them. It is felt that seeing these values would help give an intuitive understanding of reasonable celestial mechanics numbers. For example, most people would be surprised to learn that the earth is moving around the sun at the heady rate of 30000m/s!

Conclusion
Over all, the program was enjoyable to use. The interface is clean and unobtrusive, focusing on the graphical simulation. While just playing around with Newton's Aquarium™ may become tiresome after a while, the ability to load new universes with new features helps to maintain interest. By changing and experimenting with these features, exploring "what if" scenarios becomes an open-ended pursuit.