http://www.ck12.org Chapter 4. Motion and Forces
4.14 Gravity
- Define gravity.
- Describe Earth’s gravity.
- Relate gravity and weight.
Long, long ago, when the universe was still young, an incredible force caused dust and gas particles to pull together
to form the objects in our solar system. From the smallest moon to our enormous sun, this force created not only our
solar system, but all the solar systems in all the galaxies of the universe. The force is gravity.
Defining Gravity
Gravityhas traditionally been defined as a force of attraction between things that have mass. According to this
conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. Gravity can act
between objects that are not even touching. In fact, gravity can act over very long distances. However, the farther
two objects are from each other, the weaker is the force of gravity between them. Less massive objects also have
less gravity than more massive objects.
Earth’s Gravity
You are already very familiar with Earth’s gravity. It constantly pulls you toward the center of the planet. It prevents
you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects
that are above the surface—from meteors to skydivers—down to the ground. Gravity between Earth and the moon
and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth and
the other planets moving around the much more massive sun.
Q: There is a force of gravity between Earth and you and also between you and all the objects around you. When
you drop a paper clip, why doesn’t it fall toward you instead of toward Earth?
A: Earth is so much more massive than you that its gravitational pull on the paper clip is immensely greater.