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Earth and Space 6-8 / Week 05 / Tuesday
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Week 05 · Gravity Holds the System

Tuesday

Falling around Earth
// One pull keeps the whole solar system together
⏱ about 20 min

Tuesday: Falling Around Earth

Rocket tosses a ball gently across the Sky Deck. It curves down and lands near Raven's feet.

He throws harder. The ball curves less and lands by the fence. "Faster means farther before it hits," he says.

Raven draws the two curved paths in her Sky Log. "What do you notice? Both paths bend toward the ground."

"Gravity bends them," Rocket says. "So what if I threw it so fast it bent all the way around Earth?"

Nova dims her light and hums. "A scientist named Newton asked the very same question about the Moon."

"The Moon is falling?" Rocket asks. Nova tilts. "Falling, yes. But not falling down. Think about where it lands."

Newton's big idea

OpenStax Astronomy tells the story. In Newton's time, people thought of gravity as something that belonged to Earth alone.

Everyone knew that if you drop something, it speeds up toward Earth as it falls.

Newton's insight was that Earth's gravity might reach all the way to the Moon. That pull could bend the Moon's path from a straight line.

He went further. Gravity is not limited to Earth. There is a general force of attraction between all objects that have mass.

If so, the pull between the Sun and each planet could keep the planets in their orbits.

Why the Moon does not fall down

Objects at rest stay at rest. Objects in motion keep moving in a straight line unless a force acts on them.

The Moon is moving. Without gravity it would travel in a straight line and leave Earth behind.

Earth's gravity pulls the Moon toward Earth's center. The pull bends the Moon's straight path into a curve.

OpenStax puts it this way: the Moon does not fall to Earth. It falls around Earth. That curved fall is an orbit.

Astronauts in orbit feel weightless for the same reason. They and their spacecraft are falling around Earth together.

The Moon moves sideways
Earth's gravity pulls it toward Earth's center
Its straight path bends into a curve
It keeps falling around Earth
That endless curved fall is an orbit
ORBITS
  • Read the question.
  • Tap your answer.
Without gravity, how would the Moon move?
What does Earth's gravity do to the Moon's path?
Why do astronauts in orbit feel weightless?
ROCKET'S THROWS, SLOWEST TO FASTEST
  • ?A fast enough throw would fall all the way around Earth
  • ?A much harder throw would land far outside the yard
  • ?A hard throw lands by the fence
  • ?A gentle toss lands near Raven's feet
WHY THIS EXERCISENewton reasoned the same way. Fast enough, and a falling object misses the ground forever.
The Moon does not fall to Earth. It falls ____ Earth.
Without a pull, a moving object keeps going in a ____ line.
Try it
Toss a soft ball gently, then a little harder, across a room or the yard. Watch where each one lands.
Sketch the two curved paths in your Sky Log. Label which throw was faster.
Safety first
Use a soft ball, like a foam or tennis ball, never a hard one.
Throw away from windows, people and pets, in an open space.
Draw Earth as a circle. Draw the Moon with an arrow for its sideways motion and a second arrow pointing toward Earth. Draw the curved path that results.
Try this
Roll a ball slowly across the floor. Now imagine the floor curving away below it faster than it drops. Would it ever land?
Write one sentence that uses the words sideways, pull and curve.

Clear thinking. Tomorrow is Sky Deck Lab: a marble in a bowl becomes a planet in orbit.

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