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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

Gravity has 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.

Gravity and Weight

Weight measures the force of gravity pulling downward on an object. The SI unit for weight, like other forces, is the Newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 Newtons because of the pull of Earth’s gravity. On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale shown in the Figure below . The scale measures the force with which gravity pulls an object downward. To delve a little deeper into weight and gravity, watch this video:



  • Gravity has traditionally been defined as a force of attraction between things that have mass. The strength of gravity between two objects depends on their mass and their distance apart.
  • Earth’s gravity constantly pulls matter toward the center of the planet. It also keeps moons and satellites orbiting Earth and Earth orbiting the sun.
  • Weight measures the force of gravity pulling on an object. The SI unit for weight is the Newton (N).


  • gravity : As traditionally defined, force of attraction between things that have mass.


At the following URL, read about gravity and tides. Watch the animation and look closely at the diagrams. Then answer the questions below.


  1. What causes tides?
  2. Which has a greater influence on tides, the moon or the sun? Why?
  3. Why is there a tidal bulge of water on the opposite side of Earth from the moon?
  4. When are tides highest? What causes these tides to be highest?
  5. When are tides lowest? What causes these tides to be lowest?


  1. What is the traditional definition of gravity?
  2. Identify factors that influence the strength of gravity between two objects.
  3. Define weight. What is the SI unit for weight?
  4. Explain why an astronaut would weigh less on the moon than on Earth.

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