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Solar System - Keep on Movin'

Why do the planets remain in orbit around the sun? Does something hold them on their revolutionary path? Why doesn’t gravity pull the planets toward the sun? Why don’t they go flying off into space?

There are forces, or factors, that keep planets, the moon and other objects in orbit.  It all boils down to Sir Isaac Newton’s first law of motion.  Newton’s law says that an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

  • Inertia: Objects like to keep moving the way they are. They won’t stop or change that movement unless something forces it to.
  • Gravity: The force that pulls us and other objects toward the earth so they don’t float around or fly off into space. This also exists between other objects in space.
  • Forward velocity: the forward speed at which objects in space travel.

All of these work together to keep planets in orbit around the sun and keep the moon in orbit around the earth.


  • Bucket with a handle
  • Water 
  • Rope


  1. Fill your bucket half way with water.  Make sure that you can lift it easily.  If you can’t easily lift it, pour out a little of the water until you can.
  2. Tie one end of the rope to the handle of bucket. MAKE SURE YOU TIE A SECURE, DOUBLE KNOT.
  3. Tie another knot in the rope two feet from the handle.  Holding on to this knot with your right hand, begin swinging the bucket. 
  4. As you get up to a good speed, start swinging the bucket over your head like you are doing the backstroke in a swimming pool.


  1. If you are doing this correctly, the water should stay in the bucket. How does this relate to Newton’s Law?
  2. How does this activity relate to why planets stay in orbit?
  3. Predict what would happen if you slowed down
  4. Test your prediction. What happened?
  5. Predict what would happen if you let out more rope
  6. Test your prediction. What happened?
utah state board of education This Sci-ber Text was developed by the Utah State Board of Education and Utah educators.