A Matter of Some Gravity

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A Matter of Some Gravity
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By Robert Bernstein

The latest Humanist Society talk was by UCSB Physics Professor Roger Freedman. His title: A Matter of Some Gravity – What Newton Never Knew About Gravitation

Here are my photos, along with his slides, which he has generously shared.

As a physics person myself, I was curious how Roger Freedman would present to a general audience. And I was curious what he meant about "What Newton Never Knew". I was not sure if he planned to go into relativity or even into quantum gravity.

Instead, he mostly told us all of the amazing things that Newton did know. Due to his own discoveries as well as those of his predecessors and contemporaries such as Galileo and Kepler.

Freedman talked of Millennium lists of top people. Newton was in the top five on all the lists!

Newton appeared on a postage stamp and on the One Pound Note in England when there was such a thing.

Freedman handed out an iClicker device to allow audience members to register their answers to various physics questions. It was a valuable window into whether people were following along and whether they were learning from his talk.

Freedman actually wanted people to discuss their answers with their neighbors before responding!

The first question: A force is a push or a pull. If an object moves in a circle at a constant speed, which of these forces must act on it?

A. a force that pulls the object inward toward the center of the circle

B. a force that pushes the object outward away from the center of the circle

C. a force that pushes the object forward in the direction that the object is moving

D. both A and B

E. all of A, B and C

Most of our group answered E that you need all of the forces. The correct answer is A.

HSSB President Roger Schleuter insisted that a force was needed to keep the object moving forward (as in C) because of friction through the air or water.

Freedman invited us to think about planets and other bodies in space where friction would be minimal. In space there are no strings attached. Just gravitation. And Newton's great insight was that the gravity that made an apple fall is the same gravity that keeps the moon falling toward the Earth. "Universal gravitation."

You are even attracted gravitationally to the person sitting next to you. Note to astrologers: That force is actually larger than the force of any other planet on you.

Next question:

Our Moon has 1/81 as much mass as does Earth. Compared to the gravitational force that Earth exerts on the Moon, the gravitational force that the Moon exerts on the Earth is

A. 1/81 as great

B. smaller, but by a factor other than 1/81

C. equally strong

D. 81 times greater

E. greater, but by a factor other than 81

Our group answered A, B and C in equal numbers.

The correct answer is C. This is not specific to gravity. If you are sitting in a seat, the chair pushes up on you exactly as hard as you are pushing down on the seat. Otherwise you would be rising or falling. This is Newton's law that for every action there is an equal and opposite reaction.

The Moon doesn't exactly orbit the Earth. The Earth and the Moon actually orbit each other around a common center of mass. That common center is about 1700km below the Earth's surface. It makes the Earth wobble.

Note that the larger the orbit size of an orbiting body, the slower the speed. Mercury is close to the sun and orbits at 48km/sec. Neptune is the furthest planet and orbits at 5km/sec.

Freedman did a demonstration of a ball swinging on a string to show the same principle.


If a planet moves in a circular orbit around the Sun at a constant speed, which of these forces must act on it?

A. a gravitational force that pulls the planet inward toward the Sun

B. a force that pushes the planet outward away from the Sun

C. a force that pushes the planet forward in the direction that the planet is moving

D. both A and B

E. all of A, B and C

Note that this is really the same question as before, but applied to planets. Most who voted got it right! They learned!

That very morning was a launch from Vandenberg that placed a satellite in orbit. Newton's laws work!

But Newton's laws can only be calculated from equations for two bodies. For three or more bodies it is necessary to resort to computer simulations.

Question: Which statement about the rings of Saturn is correct?

A. the rings are a solid disk, like a DVD or a vinyl record

B. the rings are made of solid fragments in individual orbits

C. the rings are made of liquid drops in individual orbits

D. the rings are a sheet of liquid

E. the rings are gaseous


Half our group said B; the other half said E. The correct answer is B. Saturn's rings are made of particles.

The F ring of Saturn was discovered in 1979. There are two moons orbiting on either side of that ring called Pandora and Prometheus. They are irregularly shaped rocks 104km and 136km across.

Freedman showed us a video from the Cassini spacecraft that displayed ripples in the F ring as the moons passed by! They are "sheperds".

Here is the video:

Note that the Cassini Division in the rings was discovered by the astronomer Cassini in1675; Newton did know about that.

Saturn's moon Mimas was discovered in 1789. It is 400km in diameter. It is what clears the Cassini Division. And it is in orbital resonance with Cassini division particles. The resonance is 2:1. As if you were pushing someone on a swing every other time they came toward you.

He noted that Mimas looks a lot like the Star Wars Death Star. But no one knew that when Star Wars was made!

Newton knew about volcanism, even though it was not going on in England!

Question: Where does the energy come from to cause Earth’s volcanic activity?

A. internal heat left over from Earth’s formation

B. the decay of radioactive elements in 

Earth’s interior

C. the energy of sunlight

D. both A and B in roughly equal amounts

E. all of A, B and C in roughly equal amounts


He claimed the answer is D (both internal heat from Earth's formation and from radioactivity)

Our Moon does not have volcanic activity. Its heat has dissipated away and its composition of materials is in different proportions than Earth.

Jupiter's larger moons were discovered in 1610. Newton could see them orbiting at different speeds. They are about the same size as our moon, but they are quite different. Io has no craters. It has sulfur eruptions. Jupiter's moons are in resonances with each other:

Ganymede 4:1

Europa 2:1

Io 1:1

Tidal forces on Io squeeze and release it. This heats it up. UCSB Physics professor Stan Peale had predicted this before the eruptions were observed.

Europa has ice geysers and a subsurface ocean. 100km deep. More water than in all of Earth's oceans. The Europe Clipper mission is planned to investigate. There has been speculation life could exist in such a place.

Freedman also noted that Europa endures a huge magnetic field from Jupiter. Enough to fry electronics.

Earth's tilt of 23.5 degrees causes our seasons. It varies only about +/- one degree over a 40,000 year cycle. Mars has a similar tilt of 25 degrees. But it varies wildly.

It turns out our large moon stabilizes Earth's axis. This makes for stable seasons and climate.

Question: How can you use gravity to send a signal into space?

A. jump up & down

B. spin around

C. make snow angels

D. any of A, B or C

E. crazy question – none of these!


Freedman claimed "make snow angels" is the correct answer. But the signal would be far too small to be detected by any known technology.

All objects curve space and all objects respond to curved space. This, according to Einstein's General Theory of Relativity. Objects orbiting each other should generate gravity waves according to a 1916 prediction by Einstein. These waves will stretch and compress objects as they pass. But he doubted we would ever be able to detect such a small signal.

Many attempts have been made to detect gravity waves since the 1970s. Then came a system of three huge detectors in Hanford (Washington), Pisa (Italy) and Livingston (Louisiana).

The idea was to look for a signal appearing in all three detectors to eliminate noise. And, the three detectors would allow the source of the signal to be determined by triangulation.

On August 17. 2017 they got their first signal. It was due to two orbiting neutron stars spiraling into each other. The power release was more than the power of all the stars of the Milky Way in the last fraction of a second. This happened 100 million years ago and the gravity waves had just arrived here.

The gravity waves were less than the diameter of a proton. Not an easy measurement! Scientists have converted the signal to an audio signal that we can listen to.

Here is a video of this historic event:

Astronomers were able to turn their telescopes toward the source of the gravity waves and observe the collision. One thing they learned: It produced gold. No one knew for sure before that where gold came from.

I would like to think Professor Freedman struck a difficult balance: Informing our listeners of some of the wonders of modern physics and astronomy without overwhelming us.

Here http://www.santabarbarahumanists.org/ is the Humanist Society of Santa Barbara web site for more information about upcoming events!

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sbrobert Oct 13, 2018 06:17 PM
A Matter of Some Gravity

Thank you for the correction! Yes I indeed meant that the tilt of the Earth's axis varies over a 40,000 year cycle, not a 40 year cycle! I am honored that you read my article carefully and caught that! I had it correct in my notes, but made a typo when transfering "40K" from my notes to the article. Thanks!

a-1539918552 Oct 12, 2018 08:08 AM
A Matter of Some Gravity

Slight correction: The tilt of the Earth's axis varies over a 40,000 year cycle.

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