What Goes Up…
Have you heard the story about Isaac Newton and the apple that almost struck him as it fell to the ground? He wondered why it didn't remain in the air or even go up into the sky! Newton called the force that pulled the apple to the ground gravity.
After someone told you that the world was spinning, didn't it make you wonder why you weren't dizzy or why you didn't fly out into space when you jumped up in the air? Have you ever ridden on the flying swings at an amusement park? Before the ride starts, the swings hang down, but as soon the machine starts spinning, the swings move upward. The rider feels as though she is flying off into space; scientists call this centrifugal force. As the ride stops, gravity pulls the swings back down to the ground. How many other rides can you name that use centrifugal force and gravity to make you feel like you're in another world?
Newton believed that gravity was also what held the moon close to the earth and the earth close to the sun. Like the swing on the ride, the moon is slowly moving out and away from our planet. The earth's rotation is gradually slowing, its gravitational pull is less, and the moon's spinning motion is moving it further out from the earth. Over time this has made our days longer. Scientists believe that, long ago, a day lasted several hours less than it does now. Gravity affects everything we do each day. Turning a corner too fast on your bicycle and landing on the ground afterwards is just one example of centrifugal force and gravity at work. Can you think of others?
A Real Balancing Act
Can you balance a ball on the palm of your hand? How about spinning it on the tip of your finger? It might be easier if you try twirling a coin on a tabletop. Balance is very important to all things in the world and in space. There needs to be just enough rain and sunshine for crops to grow, and there must also be a balance between the day and night. The universe seems to be based on a balance between push and pull. If gravity was too strong here on Earth, you could barely lift your feet to walk, which would be like walking through water. If we didn't have enough gravity, you might have to wear weights to keep you on the ground. Ask your family if you can wear a pair of their big boots or heavy shoes around for a few minutes to see how it would feel to have more weight pulling down on you. Having balance, especially balance in our gravity, is a wonderful thing.
Gravity is the force that pulls things toward the earth. There is also a gravitational force in many other areas throughout the universe that seems to hold things together.
One in a Billion
With billions of stars in the sky, it's not easy to find one star in particular! Can you find the five-pointed star that is hiding in this corner of the galaxy?
Taking the Long Way Around
Many galaxies are elliptical, shaped more like a racetrack than a circle. If you want to see if the stars on the inside edge of a galaxy would go around faster than the stars on the outside edge, challenge one of your friends at a nearby racetrack! Go around once and see if the one on the inside track wins. Then trade places to see if the one on the inside track still wins.