A Weighty Subject

5/13/07 – 5/20/07
by C. Zaitz

My husband recently lost about 35 pounds due to having a very overactive thyroid gland. After an intense potion of radiation, it calmed down, so we started going back to the gym for workouts. He picked up a 35 pound weight and gave it to me to feel how much weight he’d lost. It seemed very heavy, but he carried it around for years. I started to think about weight and how we measure it, and how hard it is to lose it.

Weight is a combination of how much gravity pulls down on us, and how much “us” there is. Often in everyday life we confuse “mass” and “weight.” Mass is an intrinsic property of something, measured in pounds or kilograms. It’s how much “stuff” there is. If our mass was 150 kg on earth, it would still be 150 kg on the moon. But we wouldn’t weigh as much there, because the moon has much less gravity than earth. To get our weight, we would have to multiply our mass by how much gravity our “ground” has. Since we all live on earth and have nearly the same amount of gravity tugging on us, we forget the fact that weight and mass are different things.

Interestingly, if there were no floor or surface to stand on, you wouldn’t feel weight at all. If the floor wasn’t “pushing” back on you as hard as gravity is pulling you toward the center of the earth, you would just fall in, feeling no weight at all. This is “free fall” or “weightlessness.” It’s hard to do on earth, but the astronauts in orbit are very familiar with it. The astronauts and the space station might not have any “weight” in orbit, but they certainly have mass, which takes energy to move. That’s why it takes a lot of fuel to move stuff around even in “weightlessness.”

The fun comes when we figure out how much we’d weigh on other planets. Which would you choose, big planet or small? A 150 pound person would weigh 57 pounds on Mars. On the moon, you’d weigh a mere 25 pounds. But things get weird when you go to one of the giant, gassy planets. You’d think a mammoth planet like Saturn, a planet that could engulf earth 760 times, would have an enormous amount gravity. It does, but the farther you get from the center, the force of gravity lessens exponentially. Saturn has about 95 times more mass than earth, but its radius is 9.4 times that of earth. The math works out that on the “surface” or visible gassy outer atmosphere of Saturn, you would weigh approximately what you do on earth. The same is true of Neptune and Uranus. Jupiter is the most massive planet, 318 times more than earth, but its radius is over 11 times earth’s. Its surface gravity turns out to be about 2.5 times that of earth’s. Our 150 pound person would only weigh 375 pounds on Jupiter. Not bad for the biggest planet of them all. On Saturn, this same person would weigh a svelte 137 pounds. So you can actually “lose” weight by going to Saturn. Not to mention the weight you’d lose by eating freeze-dried peas for the three years it would take to get there.

If you’d like to check your own weight on the planets, you can go to: http://www.exploratorium.edu/ronh/weight/index.html to plan your weight loss/gain itinerary.

Until next week, my friends, enjoy the view.