Weighing the Earth: teacher key

Gravity, grade 8 edition, written for 8th grade. How did people put numbers on gravity? Follow Newton's law, Cavendish's constant and Einstein's answer to Mercury's odd orbit.

Read the book. Print this page for the key.

Questions and answers

1. Why could scientists not find G before Henry Cavendish?
   • They could only work out G times the mass of Earth (answer)
   • Nobody asked
   • Earth had no mass
2. What does the weight of a 1.0-kilogram mass come to on Earth?
   • 1.6 newtons
   • 98 newtons
   • 9.8 newtons (answer)
3. What did Newton's gravity fail to explain?
   • Mercury's orbit slowly moving around the Sun (answer)
   • Why apples fall
   • Tides
4. In general relativity, what tells matter how to move?
   • Curved spacetime (answer)
   • Light
   • Air
5. What did LIGO first detect gravitational waves from?
   • A falling apple
   • Two black holes that collided (answer)
   • The Moon

Words

• constant: fixed number
• precession: orbit slowly turning
• spacetime: space with time
• geodesic: straight line path