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# Gravity

## A force of attraction between things that have mass and how it relates to weight.

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Practice Gravity

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Gravity and Microgravity

### 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.

### Gravity and Weight

Weight measures the force of gravity pulling downward on an object. 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:

### Microgravity

A common misconception is that there is no gravity in space.  In orbit, there is still aobut 80% of the gravity as there is on Earth.  However a feeling of weightlessness occurs. Lets use an example to help us understand this. If you drop an apple on Earth, it falls. If an astronaut on the space station drops an apple, it falls too. It just doesn't look like it's falling. That's because they're all falling together: the apple, the astronaut and the station.

Inforamation on microgavity obtained from: http://www.nasa.gov/centers/glenn/shuttlestation/station/microgex.html

### Summary

• 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.

Review Questions

1. What is the traditional definition of gravity?
2. Identify factors that influence the strength of gravity between two objects.
3. Define weight.
4. Explain why an astronaut would weigh less on the moon than on Earth.
5. Why do astronauts float in space if there is still gravity?

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