**"Gravity"**reminds many people of this:

#Gravity according to Neil Tyson #GravityMovie http://t.co/a0VAIaEx10

— Living On Earth (@livingonearth) October 19, 2013

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,A video explainer of the Cavendish experiment

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.

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 Cavendish balance to measure G universal gravitational constant - Prof. Hunt #AmericanRiverCollege http://t.co/r9hJfNksAV

— Ken Kubo (@kmkubo) October 16, 2013

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.

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.**Back for another Physics/Astronomy lunch lecture! Today Prof. Victor Zarate #AmericanRiverCollege pic.twitter.com/uvLMsbqlYm

— Ken Kubo (@kmkubo) October 9, 2013

Prof. Zarate #AmericanRiverCollege talking about gravitationally bound systems | Physics lunch lecture pic.twitter.com/eUmFP7Uf8c

— Ken Kubo (@kmkubo) October 9, 2013

**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.

*Source: Wikipedia (Orbit)*

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

*Source: NASA*

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

Here's another example of some gravitationally bound systems...

The waxing crescent Moon in conjunction with Venus tonight pic.twitter.com/DWv47FGoaZBut there are less desirable systems. For example, you probably don't want to be one of these gravitationally bound objects.

— Jason Major (@JPMajor) September 8, 2013

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