Tuesday, October 22, 2013

Attracted by Gravity: From the ARC Physics/Astronomy Lunch Lectures

Nowadays the mention of "Gravity" reminds many people of this:


 However, for the ARC Physics/Astronomy lecture series, Gravity has a more attractive meaning...

 I. The Cavendish Balance

 Sir Isaac Newton reasoned that any two objects have a gravitational attraction on each other, and he expressed the gravitational force as:

But what is the value of G, the gravitational constant?

 During one Physics/Astronomy lunch lecture,  Prof. Chuck Hunt (Physics, ARC) described Henry Cavendish's ingenious device to measure the gravitational constant G.

 Some background information from PASCO
The gravitational attraction of all objects toward the Earth is
obvious. The gravitational attraction of every object to every
other object, however, is anything but obvious. Despite the lack
of direct evidence for any such attraction between everyday
objects, Isaac Newton was able to deduce his law of universal
gravitation.
However, in Newton's time, every measurable example of this
gravitational force included the Earth as one of the masses. It was
therefore impossible to measure the constant, G, without first
knowing the mass of the Earth (or vice versa).
The answer to this problem came from Henry Cavendish in 1798,
when he performed experiments with a torsion balance,
measuring the gravitational attraction between relatively small
objects in the laboratory. The value he determined for G allowed
the mass and density of the Earth to be determined. Cavendish's
experiment was so well constructed that it was a hundred years
before more accurate measurements were made.
A video explainer of the Cavendish experiment

 Prof. Hunt presented a classroom version of Cavendish's balance from the science education company PASCO that can be used to accurately determine G

PASCO's Gravitational Torsion Balance (based on Cavendish's classic design)


The Cavendish balance measures the gravitational attraction between the large and small masses in the device, and the attraction can be monitored by the movement of a light beam from the balance.



Cavendish Experiment from MIT Video

Prof. Hunt showed an example of the light beam measurements he used with the PASCO balance.


And the graph from the data plot.


And the math to determine G. You can find more details in the PASCO instruction manual.


Here's the gravitational constant.



II. Gravitationally Bound Systems

In a previous lunch lecture, Prof. Victor Zarate (Physics, ARC) talked about another aspect of gravity: Gravitationally Bound Systems.


What is a gravitationally bound system?



Here's a primer on circular motion and orbits from the Physics Classroom. NASA also has an education article on orbits.



A good example of a gravitationally bound system is the International Space Station (ISS) in orbit around Earth.

Source: NASA

Google: International Space Station

Khan Academy has an explainer on calculating the speed of the International Space Station (ISS) to stay in orbit.



Prof. Zarate kindly provided a copy of his presentation slides so that you can learn more about the math underlying gravitationally bound systems, as well as other examples.

 Here's another example of some gravitationally bound systems...
But there are less desirable systems. For example, you probably don't want to be one of these gravitationally bound objects.
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