Marianne Dyson’s Science Snacks Newsletter
November 7, 2015
Hello, and a special welcome to those of you who joined my monthly mailing list/newsletter at my book signing and talk to the NASA Alumni League at JSC on November 5. I call it Science Snacks because I discuss one cool science fact or news item (the “snack”) in each issue that I hope writers and readers of science fact & fiction will enjoy. This month’s topic is freefall.
“When you see astronauts floating in space on TV, it is easy to think there is no gravity there. But really there is. In fact, if you built a tower more than 200 miles (322 km) tall, as high as the space station’s orbit, gravity would be about 90 percent as strong at the top of the tower as on the ground. If you stepped off the top of the tower, you would drop to Earth. So why doesn’t the space station fall to Earth?”
Space Station Science, page 20
Say NO to zero G!
Since Space Station Science: Life in Freefall, was originally published in 1999, I have spoken to tens of thousands of students. When I toss a ball up in the air and ask why it falls down, the students have no problem answering that gravity pulls it down. Then I ask why the space station stays in orbit, and always someone shouts that it is because there is no gravity in space. I then ask them to raise their hands if they think there is no gravity in space. When I first started speaking at schools, about half the hands would go up. The rest admitted they didn’t know. Now almost all the hands shoot up, even those of the teachers.
There is in fact no such thing as “zero g” despite the insidious use of this unfortunate term. The force of gravity is LESS the farther you get from the center of the large mass that is pulling on you by the square of the distance you are from it. But it never goes to zero! (At the center of the Earth, the mass would pull you equally in all directions, so it may be “as if” there is no gravity, but you are still subject to Earth’s gravity, and the sun’s gravity, etc.)
The force of gravity at the Earth’s surface is 1g=GMm/D² where M is Earth’s mass, m is your mass, and D is 4000 miles, the radius of Earth. If you divide gravity at the surface by gravity 200 miles above the surface, all the terms cancel out except for D². So 4000 x 4000 divided by 4200 x 4200= 16,000,000/17,640,000 or 0.90. Thus, a person on a platform 200 miles up weighs 90 percent of what they weigh on the surface.
Why then, do astronauts float in space? The answer: because they are falling!
To demonstrate this at schools, I hold my space duck out beside me and ask the kids to imagine that we are skydiving. We step out of the plane together. Gravity immediately pulls us down. We appear to be floating next to each other as we fall at the same rate. Things float in space because they are falling.
But unless we deploy our parachutes, we will hit the ground. (I address the misconception that heavy things fall faster by dropping two plastic containers, one empty and one full of coins and letting them see they hit at the same time. I explain that falling in air depends on shape, not mass. I reference the Apollo 15 demo of the feather and hammer and note that their shapes didn’t matter because there is no air on the Moon. So if the space station is falling, whether in air or not, why doesn’t it hit the ground?
To demonstrate the answer, I have a little toy sponge shuttle on a string. I toss it up and it falls back down, gravity is pulling it down. Then I spin it around in a circle and say that gravity is like the string. As long as the space ship is high enough that it won’t run into anything (including air that slows it down) and continues going fast enough (about 5 miles a second), it can balance gravity. But if it slows down, it falls to the ground. That’s how we return from space: we slow down by firing engines and using friction with the air (and our shape to increase that friction).
It is far harder to undo a misconception than to explain that things float because they are falling. So please do your part to promote science literacy and ban the use of the term “zero g” from your lexicon! Practice saying, “When in FREEFALL, everything floats!”
Speaking of Science
- Thursday, November 12, I’m speaking at Bethany Elementary in Plano, Texas.
- Saturday, December 5, 10-4, Freeman Library, 16616 Diana Ln, Houston, 77062, Local Authors book signing benefiting Friends of the Library
If there is a particular science topic you’d like me to address in a future Science Snacks, please send me an email. Note, you can order copies of my books via my website.