Monday, October 29, 2007

Zero Gravity (2)

This post is a continuation of an earlier post.

Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards.

Fred Hoyle
So somehow, people always use the term "zero gravity" to describe space as though it somehow escaped Earth's gravitational pull and is effectively receiving no attraction from the gigantic rock at all. You see terms like "there's no gravity in space" all over the place, spoken even by the astronauts themselves.

For example, let's look at a snippet from a supposedly educational article in The Star:
Half of all astronauts suffer space motion sickness when they encounter microgravity and feel as if they are falling. This affects our neurovestibular system, which is what helps our body keep proper orientation while on Earth. This feeling is not dissimilar to the sensation felt after you have been on a terrifying roller-coaster ride. It simply means that your body doesn’t know which way is up, down, left or right.
This paragraph is misleading.

When the astronauts encounter microgravity (a term used erroneously in the context of "zero gravity"), they don't only feel "as if they are falling" - they ARE actually falling. The astronauts feel weightless because the International Space Station is in an orbit around the Earth, and an orbit is a free-fall. And you feel weightless when you are free-falling.

I am not a textbook author, so I might not be good at explaining it. But here is a great explanation I found through a fabulous web page (which by the way, clarifies a lot of scientific misconceptions I blogged about some time in the past):
Everyone knows that the gravity in outer space is zero. Everyone is wrong.

Gravity in space is not zero, it can actually be fairly strong. Suppose you climbed to the top of a ladder that's about 300 miles tall. You would be up in the vacuum of space, but you would not be weightless at all. You'd only weigh about fifteen percent less than you do on the ground. While 300 miles out in space, a 115lb person would weigh about 100lb. Yet a spacecraft can orbit 'weightlessly' at the height of your ladder! While you're up there, you might see the Space Shuttle zip right by you. The people inside it would seem as weightless as always. Yet on your tall ladder, you'd feel nearly normal weight. What's going on?

The reason that the shuttle astronauts act weightless is that they're inside a container which is FALLING! If the shuttle were to sit unmoving on top of your ladder (it's a strong ladder,) the shuttle would no longer be falling, and its occupants would feel nearly normal weight. And if you were to leap from your ladder, you would feel just as weightless as an astronaut (at least you'd feel weightless until you hit the ground!)

So, if the orbiting shuttle is really falling, why doesn't it hit the earth? It's because the shuttle is not only falling down, it is moving very fast sideways as it falls, so it falls in a curve. It moves so fast that the curved path of its fall is the same as the curve of the earth, so the Shuttle falls and falls and never comes down. Gravity strongly affects the astronauts in a spacecraft: the Earth is strongly pulling on them so they fall towards it. But they are moving sideways so fast that they continually miss the Earth. This process is called "orbiting," and the proper word for the seeming lack of gravity is called "Free Fall." You shouldn't say that astronauts are "weightless," because if you do, then anyone and anything that is falling would also be "weightless." When you jump out of an airplane, do you become weightless? And if you drop a book, does gravity stop affecting it; should you say it becomes weightless? If so, then why does it fall? If "weight" is the force which pulls objects towards the Earth, then this force is still there even when objects fall.

So, to experience GENUINE free fall just like the astronauts, simply jump into the air! Better yet, jump off a diving board at the pool, or bounce on a trampoline, or go skydiving. Bungee-jumpers know what the astronauts experience.
So there we have it: Astronauts are not floating in zero gravity, they are constantly falling under the gravitational force. They feel just like a bungee jumper, a skydiver, or a person in a roller coaster going down a gigantic drop.

The concept of "zero-gravity" and freefall should be explained with the diagram on the right.

Say you are on top of a high mountain, and you throw an unfortunate guy forward with great force. If you throw him just lightly, he might land on, say, point D. If you throw him with even more velocity, he might end up landing on point E, F, or even B. But throw the poor boy with a sufficient velocity, and he will never land, as he will just keep on curving around the earth and missing the ground constantly. That is exactly what happens to a satellite in an orbit. An orbit is just a kind of freefall, on a special type of projectile motion which takes a form of an ellipse or a sphere.

[Read more about this diagram and its explanation in this Wikipedia article - Newton's Cannonball]

It might be just my personal peeve, but I have always been appalled by how little clarification is given about the term "zero gravity" in the media. When I was young, I had always thought that there is zero gravity pull in the space, and it was until quite late that I realized all the astronauts were just free-falling. And more people should learn about this misconception if we really want to educate our people about the space.

4 comments:

ShouFarn said...

In spirit of the great Douglas Noel Adams, as explained by Ford Prefect,

"The secret of flight is to throw yourself at the ground and miss."

sophisticatedsoul said...

Great enlightenment! ;)

Eric Fu said...

Thank you Mr. Yew, for posting such a informative and enlightening blog post!

It has been a while since I last look at physics from the window of intuition - what I have been doing in upper level physics courses are calculations that can be too cumbersome at times.

This post certainly reminds me of the notion of "orbiting" and "escape velocity." I was very intrigued by the idea of escape velocity when I learned it in high school - as long as you throw an object at a speed higher that the escape velocity (I think it is about 11km/s, if my memory serves me right). Attaining the speed of 11 kilometers per second is just like getting from Taman Aman (my neighborhood) to Keat Hwa (my high school) in one second. Sounds impossible...

When an object orbits around the earth, it experiences a force called the centripetal force that maintains the circular motion. For the mathematically inclined, the centripetal force is provided by the gravitational force, i.e.

\frac{mv^2}{r}=G\frac{Mm}{r^2}.

Solving for v,

v=\sqrt{\frac{GM}{r}}.

This gives us the velocity of the object that orbits around the earth at a distance r from the center of the earth.

youngyew said...

Shou Farn: Hah you quoted that line too when I was writing about the same topic in ReCom! :D It's indeed a great quote, by the way, as it captured the whole essence of an orbit.

Sophisticatedsoul: Glad that you liked it.

Eric Fu: Thanks for your mathematical comment Professor Fu! :P You and your LaTeX notations, hah. :P