Lesson ​3
Gravity Introduction
Learners discover what gravity is, explore why it never turns off, and connect it to the ideas of horizontal and vertical. Hands-on experiments show how gravity defines direction and why builders depend on it.

Key Ideas
  • Gravity is a force that pulls everything toward the center of the Earth. It never stops. Things that float or rise are not escaping gravity; something heavier is pushing them upward.
  • On a round Earth, 'down' always points toward Earth's center no matter where you are standing. People in Australia are not upside down.
  • VERTICAL means exactly aligned with the direction of gravity's pull. A plumb bob (any weight hung on a string) always hangs perfectly vertical. Buildings must be vertical, or gravity will pull them further in the direction of the lean.
  • HORIZONTAL means perfectly perpendicular to vertical. Still water is always horizontal. A carpenter's level works on this same principle.

Vocabulary

  • Gravity: A force that pulls everything toward the center of the Earth. Every object with mass has gravitational pull.
  • Force: A push or pull that acts on an object.
  • Vertical: Exactly aligned with the direction of gravity's pull; straight up and down.
  • Horizontal: Perfectly perpendicular to vertical; perfectly level in every direction.
  • Plumb bob: A weight hung on a string that always hangs exactly vertical because gravity pulls it straight down.

Discussion Questions
  • If gravity defines 'down,' what does 'down' mean for someone standing on the opposite side of the Earth from you right now?
  • If you were an astronaut on the International Space Station, would you still experience gravity? Why do so many people assume you would not?
  • Why do engineers still use plumb bobs and levels when modern technology like lasers and computers exists?

Hands-On Activity: Block Tower Challenge

Lab Sheet 
Supply List
  • Building blocks, hardcover books, or similarly stackable objects
  • A piece of string about 12 inches long with a small heavy object tied to one end (plumb bob)
  • A flat surface
  • One small object to prop a book at an angle (creating a tilted surface)
  • Notebook and pencil

Instructions
  • Stack blocks on the flat surface as high as you can. Observe which direction the tower falls when it goes.
  • Discuss: why did it fall that way? Gravity kept pulling it further in the direction of the lean.
  • Now prop one end of a book up slightly to create a tilted surface. Stack blocks on it. Which direction does the tower want to fall?
  • Hold the plumb bob next to your tower. Does the tower line up with the hanging string, or does it lean away from it?
  • Try to build a tower on the tilted surface that stays standing. What do you have to do differently to make it work?
  • Discuss: why do builders always check that walls are vertical before finishing construction? What happens over time to a building that is even slightly off vertical?


Sources

D-1 Gravity I by Selene
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