Lesson 3: Gravity

Gravity pulls objects toward Earth's center and provides a reference for vertical and horizontal. Plumb bobs, levels, and simple balancing experiments connect this force with building and everyday motion.

Key Ideas

  • Earth’s gravity pulls objects toward the center of Earth. We call that direction “down.”
  • Gravity keeps pulling even when something is not falling. A table or the ground can hold an object up.
  • Things that float or fly are still pulled by gravity. An upward push can keep them in the air.
  • Vertical means straight up and down. Horizontal means level. A hanging weight shows vertical, and a builder’s level checks horizontal.

Vocabulary

  • Gravity: A pulling force. Earth's gravity pulls objects toward Earth's center.
  • Vertical: Straight up and down, like a still weight hanging on a string.
  • Horizontal: Level, like a shelf that does not tilt.
  • Plumb Bob: A weight on a string. When it hangs still, it shows straight up and down.

Discussion Questions

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

Supplies for Live Class

  • Something to drop (literally anything that isn’t going to break)
  • Clear bottle or cup with water in it
  • Some type of oil (vegetable oil works well)
  • Blocks or something to stack that can fall without breaking
  • Optional:
    • Plumb bob (or a weight tied to the end of a string)
    • Carpenter's level

Hands-On Activity: Block Tower Challenge

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 still pulled downward. A freestanding tower can tip when its center of gravity moves beyond its supporting base.
  • 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 check alignment and the position of a structure's center of gravity? How can supports and design help a structure remain stable?

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