What Goes Up: Gravity Around Us - First Grade Science
Have you ever tossed a bright red rubber ball high into the sky? No matter how hard or high you throw it, the ball reaches a peak, pauses for a split second, and then plunges straight back down into your waiting hands. You jump up off the ground, and your feet immediately land back on the floor. Leaves falling from autumn branches flutter down to the soil, and raindrops fall from clouds to the pavement. Why doesn't anything float away into outer space? The answer is gravity, one of the most powerful and universal invisible forces in the universe.
What is Gravity?
Gravity is an invisible pulling force that draws objects toward one another. On our planet, Earth's immense gravity pulls everything downward toward the absolute center of the Earth.
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| WHAT GRAVITY DOES |
| |
| ( Ball Tossed Up ) |
| ^ |
| | Muscular throw pushes ball UP |
| | |
| * * * * * |
| * * |
| * * |
| | |
| v GRAVITY pulls ball DOWN! |
| |
| ================= Ground Level |
| | |
| v Toward Center of the Earth |
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Gravity is Always Pulling
Unlike your hands, which can choose to push or pull, gravity only pulls! Gravity never pushes objects away. Gravity is always working 24 hours a day, 7 days a week. It never takes a break, never turns off, and acts on every single piece of matter on Earth, whether it is a grain of sand, an elephant, or you!
Why Does Earth Have So Much Gravity?
Every object that has mass possesses its own gravity. You have a tiny bit of gravity, your pencil has gravity, and a tree has gravity. But because you and your pencil are small, your gravitational pull is far too weak to notice.
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| MASS AND GRAVITY |
| |
| SMALL MASS (Tiny Gravity) COLOSSAL MASS (Giant Gravity) |
| A rock or human has almost zero Planet Earth weighs sextillions |
| detectable gravitational pull. of tons; its gravity holds |
| oceans, air, and buildings down!|
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Earth is gigantic—it weighs approximately six sextillion tons! Because Earth has such massive bulk, its gravitational pull is powerful enough to keep our feet firmly on the ground, keep ocean waters in their basins, and hold our blanket of air (the atmosphere) wrapped tightly around the planet so we can breathe.
Does Heavy Fall Faster Than Light?
A famous question in science is: "Does a heavy object fall faster than a light object?" Many people assume that a heavy bowling ball must drop much faster than a small pebble.
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| THE GALILEO EXPERIMENT: SAME SPEED! |
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| DROP A HEAVY BASEBALL AND A LIGHT TENNIS BALL TOGETHER: |
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| [BASEBALL: Heavy] [TENNIS BALL: Light] |
| | | |
| v v |
| (Falls at) (Falls at) |
| (Same Rate) (Same Rate) |
| | | |
| v v |
| ================================================================= |
| RESULT: BOTH HIT THE FLOOR AT THE EXACT SAME INSTANT! |
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Gravity Pulls All Objects Equally
More than 400 years ago, Italian scientist Galileo Galilei proved that gravity pulls all objects toward Earth at the exact same rate of acceleration, regardless of their weight! If you drop a heavy wooden block and a lightweight plastic block from the same height at the same time, they strike the ground together.
What About a Feather? Air Resistance!
If you drop a flat sheet of paper or a bird's feather, it flutters and floats down slowly. Does this break the rule of gravity? No! The paper floats slowly because of air resistance. Air particles push upward against the wide, flat surface of the falling paper like a parachute. If you crumple that exact same piece of paper into a tight little ball, it cuts cleanly through the air and drops just as fast as a stone!
Gravity on the Moon: Leaping High!
Because the moon is much smaller and has less mass than Earth, its gravity is only 1/6th as strong as Earth's gravity. When Apollo astronauts walked on the moon, they weighed six times less than they did on Earth! Even wearing heavy spacesuits, they could bounce and leap high across the dusty craters like superheroes!
Chapter Review Exercises
Question 1: What is gravity, and in which direction does Earth's gravity pull objects?
Question 2: Why does planet Earth have such a strong gravitational pull compared to a person or a car?
Question 3: If you drop a heavy baseball and a lightweight tennis ball from the same height at the same moment, which one will hit the floor first? Explain why.
Question 4: Why does a flat sheet of notebook paper fall slowly to the ground, but falls quickly when crumpled into a tight ball?
Solutions and Explanations
Solution 1: Gravity is an invisible pulling force of attraction between masses. Earth's gravity pulls all objects downward toward the center of the Earth.
Solution 2: Gravitational pull depends directly on mass. Planet Earth has such an enormous amount of mass compared to a person or car that its gravitational pull is strong enough to keep everything anchored firmly to its surface.
Solution 3: Both balls will hit the floor at the exact same instant. Gravity pulls all objects downward at the exact same rate of acceleration, regardless of their mass or weight.
Solution 4: A flat sheet of paper has a large surface area that collides with air molecules, creating upward air resistance that slows its descent. When crumpled into a compact ball, its surface area decreases drastically, minimizing air resistance so gravity pulls it down rapidly.