MadSci Network: Astronomy

Re: What is the gravitational pull on Mercury compared to Earth?

Date: Fri Mar 16 08:52:24 2001
Posted By: Steve Furlanetto, Grad student, Astronomy, Harvard-Smithsonian Center for Astrophysics
Area of science: Astronomy
ID: 983388146.As

First of all, let me try to clarify your question. You ask what the equivalent of 1 kilogram would be. Kilograms actually measure how much mass an object has. Mass just measures how much matter is in the object. Because the amount of matter in (for example) your body won't change whether you are on Earth or Mercury, mass is constant everywhere. Thus, technically, the equivalent of 1 kilogram on Mercury is just 1 kilogram!

But I'm guessing you are asking how much one kilogram would weigh on Mercury. The weight of an object reflects how strongly gravity pulls on it. This will change, because factors other than the mass of the object affect the pull of gravity: on Earth, 1 kilogram weighs 2.2 pounds, while on Mercury, a 1 kilogram object will have the same mass but a different weight. The other factors that affect gravity's pull are the mass of the planet (the more matter there is to attract you, the stronger gravity will be) and the radius of the planet (because the closer the matter in the planet is to you, the stronger gravity gets). There's a formula for this, first discovered by Isaac Newton, that describes the strength of the gravitational attraction between any two objects, depending on their masses and the distance between them. You'll probably learn this equation, and how to use it, in school in a few years.

The numbers you need are these: the mass of Mercury is only 5.5% that of earth, and Mercury's diameter is 38% of Earth's (you can find these numbers at The Nine Planets Web site. When you plug the numbers into the formula for gravity, you find that gravity is only 38% as strong on Mercury as it is on Earth. Therefore, a 1 kilogram object will weigh 2.2*0.38 = 0.83 pounds on Mercury.

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