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Gravity: It Pulls Us Together
How does an invisible force shape everything in the universe?

Gravity is the invisible force that holds everything together, it keeps you on the ground, keeps the Moon circling Earth, and keeps planets orbiting the Sun. In this lesson, your learner will explore what gravity is, how it creates orbits, and why it's one of the most fundamental forces shaping our universe. Through hands-on demonstrations, they'll see that gravity isn't magic: it's physics.

S2S_Lesson04_GRAVITY by Selene

Key Ideas

  • Gravity is a force that pulls objects toward each other. On Earth, gravity pulls things towards Earth’s core
  • The Sun's gravity keeps planets going around it, and Earth's gravity keeps the Moon near us.
  • Planets orbit the Sun due to the balance between gravity and motion: gravity pulls inward while the planet tries to move forward in a straight line.
  • Moons orbit planets the same way planets orbit stars, using gravity and motion.

Spines
  • DK's Science as You've Never Seen it Before: pg. 102-103 (Gravity)
  • Mammoth Science: Gravity pg. 116
✏️  Notebooking Activity
Write two sentences explaining what would happen if gravity suddenly disappeared OR write two sentences explaining why an orbit is like a constant falling motion that never hits the ground.

Discussion Questions
  • What does gravity do? 
    (Sample answer: Gravity pulls things together.)
  • What keeps the planets going around the Sun? 
    (Sample answer: The Sun's gravity keeps the planets going around it.)
  • Why don't planets fly away into space? 
    (Sample answer: Gravity keeps pulling them toward the Sun.)
  • Why don't planets fall straight into the Sun? 
    (Sample answer: Planets are moving forward while gravity pulls them inward, creating a balanced orbit.)
  • How does gravity help create orbits? 
    (Sample answer: Orbits happen when an object is moving forward and gravity pulls it into a curved path.)
  • How do moons orbit planets? 
    (Sample answer: Moons orbit planets the same way planets orbit stars, using gravity and motion.)
Digging Deeper
  • If you traveled to a much smaller planet with less mass than Earth, would you be able to jump higher or lower than you can on Earth? Why do you think that is? 
    (Sample answer: You would jump higher because there is less gravity.)
  • The Sun has a lot more mass than the Earth, but why doesn't it just pull the Moon away from us? How do you think distance changes how strong gravity's pull is? 
    (Sample answer: Gravity is like a magnet: the more mass an object has, the stronger the pull, but distance also matters a lot. Even though the Sun is much bigger than the Earth, it is very, very far away. The Earth is much smaller, but it is right next door to the Moon. Because the Earth is so close, its "tug" on the Moon is stronger than the Sun’s distant pull, keeping the Moon safely in our orbit.)
  • We know stars have gravity, but have you ever heard of a Black Hole? Based on what we know about gravity pulling things in, what do you think makes a Black Hole different from a regular star?
    (Sample answer: Both stars and black holes have gravity, but a black hole is like a star that has been squeezed down into an incredibly tiny, super-dense space. Because all that mass is packed so tightly, the gravity near it becomes so strong that not even light can jump high enough to get away. A regular star is big and "fluffy" by comparison, so light and heat can still escape and reach us.)

Cosmic CalendarWhere we are: No Calendar Date
Lesson 4 is a conceptual lesson. The Cosmic Calendar has no date for gravity. Read the script below to explain this to your learner.

Read aloud:You might notice we’re not marking a date on our Cosmic Calendar today, and that’s intentional. Gravity doesn’t have a birthday. It wasn’t discovered at some moment in time. It has been present since the very first second of the universe, working quietly in the background of every single event we’ve studied and every event we’ll study for the rest of the year. Some of the most important ideas in science aren’t events. They’re forces and patterns that run through everything. Gravity is one of those. So today we’re stepping back from the timeline to look at the invisible hand that has been shaping our story from the very beginning. We’ll be back on the calendar next lesson. 
Timeline Page AlternativeThere is no timeline date for the next page, but you could add an optional page titled “Gravity: The Universal Pull.” The workbook prompt asks learners to draw objects being pulled toward each other with curved lines showing gravitational pull.

VocabularyGravity — An invisible force that pulls objects with mass toward each other; the greater the mass, the stronger the pull.
Mass — The amount of matter in an object; mass determines how strong an object's gravitational pull is.
Orbit — The path an object follows as it continuously falls around another object due to gravity.
Weight — The force of gravity pulling on a mass; weight changes with gravity's strength, while mass stays the same.

SCIENTIST SPOTLIGHT: Subrahmanyan Chandrasekhar
Subrahmanyan Chandrasekhar was an Indian-American astrophysicist who used mathematics to figure out what happens to stars when gravity finally wins. In 1930, when he was just nineteen years old and traveling by ship from India to England, he calculated that stars above a certain mass would not simply fade out when they died but would collapse catastrophically under their own gravity, eventually becoming what we now call neutron stars or black holes. The leading British astronomer of the day publicly ridiculed his work. Chandrasekhar was so discouraged that he left the field of stellar physics entirely for a time. He was eventually awarded the Nobel Prize in 1983, more than fifty years after his original discovery.

Videos:
Subrahmanyan Chandrasekhar: The Genius Who Explained How Stars Die
1983 Nobel Laureate in Physics, Subramanyan Chandrasekhar

Digging Deeper Activity:
What is the Chandrasekhar Limit? Research what happens to stars of different masses when they reach the end of their lives. Make a diagram showing the different fates of small, medium, and large stars.

Sources

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