# Our study of gravity ranges from slinkies to black holes to sandwich throwing.

The Physics class has been busy this block investigating gravitation, oscillations, and waves. While we can't bring black holes into the lab, we have reveled with fascination at the recent detection of the gravitational signature of colliding black holes in the LIGO experiment. The students have been doing the calculation of orbiting binary systems similar to these and have had their first introduction to the idea of a field. They have found the far-reaching conclusions from Newton’s universal law of gravitation and how it explains the astronomical observations made by Kepler. Celestial motion has become a natural extension to applying their ideas about energy and angular momentum conservation.

Among the really interesting results of gravitation is a terrestrial puzzle which the students have discovered in a problem set. If a sandwich is dropped through a hole drilled through the center of earth, it oscillates and arrives back at its starting point well before you’re hungry again, in a little more than an hour. It turns out that this is true no matter which two points you drill a hole through. There have been some rumors about trying to reduce commute time times by digging straight tunnels between Post Street and student residences. If you notice any unusual activity in your backyard, don't be alarmed: it's for the benefit of science!

As we were studying oscillations as projections of circular motion, it was wonderful to have observations that linked our study of the sandwich to the circular motion of a sandwich thrown with such speed that it begins to orbit the earth. Again, it was a voyage of discovery when students found the thrown sandwich takes the same time to go around the earth, except now the meeting was not quite as gentle. Though we have understood the mathematics, a physical connection drawn between the projections of circular motion to this problem is still an interesting work in progress.

We have studied how oscillations are ubiquitous, and why small angle approximations enable physical systems to be mathematically tractable. This has naturally led to our study of waves, and this is the first major mathematical abstraction that the students are working with, as we deal with wave functions which vary both in space and time. We have been exploring this with motions of a really long slinky, and building demos of mechanical wave machines, and we will wind up the block with an analysis of musical instruments and sound.

*-- Kaushik Basu*