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Ask A Scientist

Does gravity get stronger the closer you get to the earth's core?

Asked by: James Woolson
School: Thomas Jefferson Elementary School
Grade: 4
Teacher: Mrs. Carter
Hobbies/Interests: Soccer, video games, reading, origami
Career Interest: Paleontologist

Answer from Alan Jones

Adjunct Professor, Binghamton University

 Research area: Writing software for educational seismology PhD school: Purdue, 1964 Interests/hobbies: Camping, canoeing, genealogy, software for scoring running races Family: Wife Barbara, grown children: Kendra, Clain, and Adele Web page address: http://bingweb.binghamton.edu/~ajones

 James, that is a reasonable question since we know that if we were out in space, far from a body such as the earth, we would experience very little gravity. If we could, somehow, lower ourselves slowly toward the surface of the earth, the pull of gravity would become greater and greater until we were on the surface. At that point we would experience what we call “1-G”. This 1-G is represented mathematically as 9.8 m/s2 (meters per second-squared) or 32.2 ft/s2. It means that if you were standing on the surface of the earth and dropped something, it would accelerate in such a way that its speed would increase by 9.8 meters/sec for each second it was falling. This would only happen if the object were falling in a vacuum or was significantly dense so that the effect of air resistance was negligible.

You might think if we were somehow able to go down a tube toward the center of the earth that this gravitational pull would get greater. However, as we descended our fictitious tube, some of the earth\'s mass would now be above us and would pull upward on us. Let\'s say you descended this tube so that you are now part way toward the center of the earth. Picture a sphere with its center at the center of the earth and its surface exactly where you are standing. All of the mass inside that sphere will contribute toward your weight. If you sum up the gravitational pull of all the mass outside that sphere up to the surface of the earth, it turns out that it contributes nothing to your weight since the pull is in different directions and from different distances. So, as you continue your imaginary journey downward, you will become weightless when you reach the center of the earth since the distributed mass of the earth is all around you equally in all directions.

This thought experiment we just performed encourages me to point out to you the difference between your mass and your weight. As you travel into the earth or travel out into space, your mass remains the same. However, your weight, that number you see when you stand on a scale, is how much force gravity is pulling on you.

Last Updated: 9/18/13