Gravity Concepts


A quick summary to help you think about gravity

By Marc Reif

Gravity is a mystery about which relatively little is known. We can describe how gravity behaves very precisely, but what it is, that is harder to say.

Gravity is a force of attraction between any two objects with mass. Since all objects in the universe have mass, everything is attracting to everything else. The earth is attracted towards you, the same as you are attracted toward the earth.

Since gravity is a force, it is measured in Newtons. Or Pounds, if you're in the archaic English system.

The attraction is equal and opposite (by Newton's 3rd Law). The earth is attracted to you by the same amount that you are attracted to the earth.

Why doesn't the earth fall up to meet you when you jump up (the inverse of you falling down to meet it)? The earth has a mass of about 6 x 1024 kg. So what does that mean? It (the earth) does fall up, just with such a small acceleration that you could never measure it.

The farther away you are from an object, the smaller the gravitational force between the two objects. In precise form: Gravity decreases inversely with the square of the distance between the two objects.

Gravity is the weakest force in the universe. This is why you can't measure or feel the force of gravity between you and your pencil, or even the force of gravity between you and the Queen Mary.

When we talk about weight, what we usually mean is the force of the earth's gravitational pull on an object. Since an object in freefall is weightless, the way that you experience weight is the surface force.

All falling objects on earth accelerate at approximately the same rate, 9.81 m/s2, if air resistance is not a factor. We call this number "the acceleration due to gravity." It is symbolized by the lower case "g."

Students are often confused about the difference between mass and weight (same as force of earth's gravity on an object). Mass is a measure of an objects resistance to acceleration, or inertia. Weight is the pull of the earth's gravity on the object. The ratio between weight and mass is 9.81 N/kg, also symbolized by a lower case "g." This is often called the "gravitational field strength." Why, two names for one symbol? They're really the same thing, and the units are the same, too. It's just two ways of looking at the same thing.

We calculate force of gravity (Fg) by the relationship Fg= mass* g. Memorize this relationship!

19 April 2005