Hilary-Ann B. (9) and her father, Russell of Tacoma, Washington, have emailed me the following question: "Why would I weigh a certain amount on Earth, but on another planet I would weigh a different amount?"
Hilary-Ann, let me say that while the answer is fascinating, I am sorry to hear you're concerned about your weight on... well, any planet. However, after talking to your Mom and Dad over email too, we know this is curiosity about gravity, not calories!
In simple terms, gravity is the force of attraction between objects. Gravitational pull is what makes the Earth orbit the sun, or the moon orbit the Earth. Suns, moons and planets are all surrounded by fields of gravity. These fields will be different, depending on things like planet size, mass, speed, its location in any solar system, and any other objects around it in space.
If Earth's force of gravity is measured at 1.00, force on other planets would be:
We'll include Pluto for the purists. And so I don't have to listen to any arguments.
At any rate, multiply your weight by any of these numbers, and you will see what you weigh on that planet. For instance, someone who weighs 100 pounds on Earth will weigh about 106 pounds on Saturn, but only 37 pounds on Mercury. I imagine the average 9-year-old weighs about 50 pounds, so you could just cut all those in half. Oh goody, math homework!
More complex components of gravity come into play, but in general: the larger the object, the greater the gravity. However, the further away you travel from an object, the less you are affected by its gravitational field. Jupiter is the largest planet in our solar system, so it has the strongest field (except for our Sun, which is over 27+!). You cannot stand on Jupiter, because it's mostly gas. However, if it had a surface, the force holding your body on the planet would be much greater. This increases your weight, even though your mass remains the same.
There is a great contraption at the San Francisco Exploratorium, where you can step on a large scale and see what you weigh on all the planets. I've been on it, and it's quite a fun experience. They also have their calculator on a web page, so you can see Your Weight On Other Worlds.
Needless to say there will never be a book fad known as "The Jupiter Diet."
And watch your mailbox, Hilary... a special NASA treat is on the way to you! :)
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