# The Science Guys

### January 2003

What is gravity and where is it the strongest in the United States?

Of the four fundamental forces - gravity, electromagnetic, strong nuclear, and weak nuclear - gravity is the one with which people are most familiar. Isaac Newton is credited with ’discovering gravity’. What he actually did was give us an analytical interpretation of gravity, that is, describe the quantities that determine the gravitational force on an object.

According to Newton, any two objects have an attractive force trying to pull them together. The magnitude of this force depends upon the mass of each object and the distance between the centers of the two objects. Mathematically, we say the force of gravity depends directly upon the masses of the objects and inversely upon the distance between the objects squared. [ F = G M1 M2 / D2 ] The G in the relationship is a constant that is called the universal gravitational constant.

For everyday objects like people, cars, balls, and planes the force of gravity between any two of these objects is so tiny it is insignificant. However when one of the objects is very massive, such as the Earth, then the force of gravity becomes significant. Your weight is actually just the force of gravity between your body's mass and the Earth's mass. We feel the Earth pulling on us with a force that we call gravity.

The ’force of gravity’ is often expressed in terms of the acceleration the gravitational force will give an object when the object is dropped. This acceleration of gravity is written as a small g and is used to describe the strength of gravity at different locations on Earth as well as on other planets.

In general, the closer the centers of two objects, the greater the force of gravity becomes. Therefore, you would expect gravity in the United States to be stronger wherever you are closest to the center of the Earth. The lowest spot in the U.S. is Death Valley, so gravity would be expected to be stronger there (where you are closest to the Earth’s center). However, the Earth is not uniform material throughout its interior and gravity depends upon exactly how much mass is between you and the center of the Earth.

Therefore as you move around the U.S. the acceleration due to gravity (g) varies from about 9.79 to 9.81 meters per second squared. The Earth’s average is 9.80 m/s2 (32 ft/s2) which is generally reported as the acceleration of gravity on Earth. This means a dropped object will speed up 32ft/s every second it falls assuming no air resistance. For comparison, the acceleration of gravity on the Moon is only 1.63 m/s2 due to its composition and smaller size.

Newton’s explanation of gravity is adequate for everyday life. However, to understand the most elaborate details of gravity one must include Einstein’s gravitational theory, which is mathematically complex.