Gravity of Planets

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What You Need:

  • Solar_System_size_to_scale.svgDigital scales
  • Weigh boats
  • Modeling clay
  • Planet gravity info


What Do You Do?

  • Take a ball of clay and roll it in a ball and place it in the weigh boat (already on the scale).
  • Place the clay in the weigh boat on the scale and read what the weight is.
  • Use your planet gravity info to either add or take away clay based on the planet you are making.
  • Continue on with a new ball of clay until you finish each planet.
  • Compare the gravity-based sizes of the planets… how are they different compared to the actual size of the planets?

What Happened?

Mass and Weight

Before we get into the subject of gravity and how it acts, it’s important to understand the difference between weight and mass.

We often use the terms “mass” and “weight” interchangeably in our daily speech, but to an astronomer or a physicist they are completely different things. The mass of a body is a measure of how much matter it contains. An object with mass has a quality called inertia. If you shake an object like a stone in your hand, you would notice that it takes a push to get it moving, and another push to stop it again. If the stone is at rest, it wants to remain at rest. Once you’ve got it moving, it wants to stay moving. This quality or “sluggishness” of matter is its inertia. Mass is a measure of how much inertia an object displays.

Weight is an entirely different thing. Every object in the universe with mass attracts every other object with mass. The amount of attraction depends on the size of the masses and how far apart they are. For everyday-sized objects, this gravitational pull is vanishingly small, but the pull between a very large object, like the Earth, and another object, like you, can be easily measured. How? All you have to do is stand on a scale! Scales measure the force of attraction between you and the Earth. This force of attraction between you and the Earth (or any other planet) is called your weight.

If you are in a spaceship far between the stars and you put a scale underneath you, the scale would read zero. Your weight is zero. You are weightless. There is an anvil floating next to you. It’s also weightless. Are you or the anvil mass-less? Absolutely not. If you grabbed the anvil and tried to shake it, you would have to push it to get it going and pull it to get it to stop. It still has inertia, and hence mass, yet it has no weight. See the difference?

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