WHAT IS GRAVITY?

When asked what holds you to the Earth or what holds the planets in orbit around the Sun, the easy answer is: "gravity".

Q. But what is gravity?

A. professor will tell you about Kepler's Laws. If he get's technical, he'll tell you that gravity is a "force" related to the mass of the two objects and the inverse square of the distance between them.

It all sounds so scientific and so precise...but just what is this "force" anyway?
What is this "power" that works at a distance...and has no substance you can get your hands on...yet we all know the effects of it?


Again, just what is gravity? What is it like anyway?



We'll, to get a handle on this, let's consider what you, like me, have probably been told in the past.
"Gravity? It's like a ball at the end of a string that a person is spinning around. "

Great! So your hand is like the Sun, the ball like the Earth, and the string like gravity.
Or your hand is like the Earth, the ball like the Moon and the string, the gravity. OK?

Great...so the string is like gravity. Ok, let's ask a few questions to try to get a handle on what gravity is like, so we might better understand what this term is that we use so casually.

Q. How big a diameter would the string have to be to hold the Moon in it's orbit around the Earth? I.e., how big would the string have to be for it to replace gravity?

"Well the string might break," you retort.

OK, let's try something stronger...let's use wire in place of the string...in fact, let's use the strongest wire you can find...piano wire...and ask the question again.

But before we continue...you may want to determine how big (in terms of the diameter) the Moon and the Earth are.


Moon diameter: 2160 miles (3476 km) across. (This distance is about one fourth the diameter of the Earth, and 400 times smaller than that of the sun.)

Earth diameter: At the Equator: 7,926.28 miles (12,756.1 km)


Q. How big a diameter would the wire have to be to hold the Moon in it's orbit around the Earth? I.e., how big would it have to be to replace gravity?



A. A mile?

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Bigger....
A. 10 miles?
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Nope...
A. 100 miles?
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Nope again....try
A. 1,000 miles?
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No! Bigger
A. 2,000 miles (about the diameter of the Moon)
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No! Bigger....
A. 5,000 miles?
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No....Bigger yet
A. It turns out that using the highest tensile strength wire (piano wire), and neglecting the weight of the wire itself, the wire would have to be...

~7,000 MILES in DIAMETER!!!

THAT'S ALMOST

THE DIAMETER OF THE EARTH

and

~4 TIMES THE DIAMETER OF THE MOON!!

Now...let that sink in a little...then maybe we'll continue by asking some more questions...if you're up to it?

WHAT'S AT THE OTHER FOCI OF THE ELLIPSE?

Q. What is the nature of an ellipse?



A. It's like a flattened or stretched circle where, among other things, the center is stretched into two foci. Technically any point on the surface is equal to the sum of the distances from one foci to that point on the circumference and the other foci to that same point on the circumference.



FACT: Our planet's orbits can be described by elipses. At one focus is the Sun whose gravitational force bends the planet's orbit around to follow an elliptical orbit.



QUESTION: What is at the other focus that prevents the planet's orbit from sailing off out into infinity in the universe on a parabolic trajectory, and instead pulls it back around at the apogee and starts another trip around the Sun.
HINT: Mass of whatever is "holding" the earth in it's orbit would have to be of the same order as the Sun...but there is nothing obvious out there with that amount of mass.
Q. So what is it that must have a mass equal to that of the Sun and yet is invisible and resides at the other focus ... of EACH planet?