📖 Chapter 4 of 14 · Mysterious Space- Advance 1 · Class 8
Chapter 3: Gravity Across the Universe
The simple idea: everything pulls on everything
Gravity is not only an Earth thing. Every object with mass pulls on every other object. You pull on the Earth, and the Earth pulls on you — but the Earth is so vastly bigger that we only feel its pull. The more mass something has, the stronger it pulls; the farther away it is, the weaker the pull becomes.
The technical layer: mass, distance and tides
The Sun contains almost all the mass in the Solar System, so its gravity reaches across billions of kilometres to hold every planet in its orbit. Closer, the Moon's gravity tugs on the Earth's oceans, pulling the water into a slight bulge — and as the Earth turns, that bulge passes as the rising and falling tide. Gravity, the same force that drops an apple, shapes seas and steers worlds.
Story Time — The Argument of the Two Pulls
Meera · Kochi, Kerala
Meera and her friend were arguing on the way home. “The Earth pulls the apple down,” her friend said. “Yes,” said Meera, “but here is the strange part my teacher told me — the apple also pulls the Earth up.” Her friend laughed. “That cannot be true. The Earth does not move for an apple.”
At home, Meera asked her grandfather, who had taught physics long ago. “Your friend is half right and you are wholly right,” he said, delighted. “Gravity always works both ways. The apple does pull the Earth. But the Earth is so enormous that the apple's pull moves it far too little to notice.”
“So the size decides who moves?” Meera asked. “The mass decides,” he corrected gently. “More mass, more pull, and less movement in return. The Sun has so much mass that it holds every planet across the whole Solar System — and yet the planets tug back on the Sun a little, too.”
Meera thought of the backwaters near her home, rising and falling each day. “And the tides?” she asked. “The Moon's gravity,” her grandfather said, “pulling gently on our seas. The same force, from the apple to the ocean to the planets. One rule, everywhere.”
Meera went back to her friend the next day, not to win the argument, but to share it. “Everything pulls on everything,” she explained. “The apple, the Earth, the Moon, the Sun. We only notice the big pulls.” Her friend, this time, listened.
Story Time — The Notebook of the Tides
Bhavesh · Puri, Odisha
Bhavesh had kept, since he was small, a simple notebook of the sea at Puri — when it rose high, when it fell low. Now older, he noticed a pattern he had missed before: the highest tides came when the Moon was full or new, and gentler tides in between.
He showed the notebook to his science teacher. “You have discovered something real,” she said, impressed. “The Moon's gravity pulls our oceans into a bulge. When the Sun and Moon line up — at full and new Moon — their pulls add together, and the tides are strongest.”
“The Sun pulls the sea too?” Bhavesh asked. “It does, though from much farther away, so its effect is smaller than the Moon's. When the two pull in a line, we get the biggest tides. When they pull at an angle, the tides are milder.”
Bhavesh looked at years of his own patient records with new eyes. He had been measuring gravity all along, without knowing its name. “So my little notebook,” he said, “was really a record of the Moon pulling the sea.”
His teacher encouraged him to keep it up, and to add the Moon's shape each day. “Careful records, kept over time, are how real discoveries are made,” she said. “You have the patience of a scientist, Bhavesh.” He returned to the beach that evening and, as always, wrote down the sea.
Solved Examples
Think first, then read the answer.
1. Does gravity act only on the Earth?
Show answer
Answer: No. Every object with mass pulls on every other object, everywhere in the Universe.
2. What two things decide the strength of gravity between objects?
Show answer
Answer: Their mass (more mass, stronger pull) and their distance apart (farther apart, weaker pull).
3. How does the Sun keep the planets in orbit?
Show answer
Answer: The Sun has enormous mass, so its gravity reaches across the Solar System and holds each planet on its path.
4. What causes the ocean tides?
Show answer
Answer: The Moon's gravity pulls the Earth's oceans into a bulge; as the Earth turns, this passes as the rising and falling tide.
5. When are tides the strongest?
Show answer
Answer: When the Sun and Moon line up (at full and new Moon), so their pulls add together.
Check Your Understanding
A. Fill in the blanks
- Gravity is stronger when an object has more _______.
- Gravity is weaker when objects are farther _______.
- The _______ 's gravity causes the ocean tides.
B. Write True or False
- Only the Earth has gravity. ( ___ )
- An apple pulls on the Earth as well. ( ___ )
- Tides are strongest when the Sun and Moon line up. ( ___ )
C. Short answer
- Why do we feel the Earth's pull but not the apple's pull on the Earth?
- Explain in your own words how the Moon causes tides.
D. Think like a scientist
- The Sun is far away yet holds the planets. How can that be?
- How could keeping careful records, like Bhavesh, lead to a discovery?
- Where in the Space Lab did you feel gravity's effect on weight?
Check your understanding
Tap an answer to see if it is correct. This is just for fun — nothing is saved.
1. What makes gravity stronger?
- More mass
- Brighter colour
- Faster spin
Show answer
A. More mass
2. What causes ocean tides?
- The wind only
- The Moon's gravity pulling the sea
- The Earth's heat
Show answer
B. The Moon's gravity pulling the sea
3. Does an apple pull on the Earth?
- No, never
- Yes, but far too little to notice
- Only at night
Show answer
B. Yes, but far too little to notice