Rocket rolls a ball up the driveway slope. It slows, stops and rolls back to his feet.
"Gravity again," he says. "It is in everything. Jumps, throws, bounces, even this."
Raven looks up at the pale Moon over the tree. "And it is what keeps that up there."
"Wait," Rocket says. "If gravity pulls the Moon, why does the Moon not fall on us?"
Nova circles slowly. "What else does the Moon do, besides being pulled?"
"It moves," Rocket says. "Sideways. Fast. So it keeps missing us."
Raven smiles. "What do you notice? The same pull that drops a ball holds a Moon in orbit."
Every jump ends on the ground because gravity pulls the whole time, up and down. The higher the jump, the longer the fall.
Every thrown or kicked ball curves back to Earth. Next to the forward motion from the throw, gravity adds a steady downward pull.
A ball rolled up a slope slows, stops and rolls back. Gravity pulls it toward the low end.
A university physics text notes that a thrown ball is in free fall once it leaves the hand. Only gravity pulls on it.
The weight you feel lifting a heavy ball is Earth's gravity pulling on the ball's mass.
NASA Space Place says gravity holds the planets in orbit around the Sun and keeps the Moon in orbit around Earth.
The Moon's gravity pulls the seas toward it, causing the ocean tides. Gravity also holds down the air we breathe.
NASA says the Moon is an average of 384,400 kilometres away. Earth's gravity still reaches it, weaker but never zero.
An OpenStax text says Galileo argued that falling bodies and planetary motions have the same cause. Newton then described that cause as gravity.
| Week review | True or false? |
|---|---|
| Gravity is always attractive. | ? |
| Objects with more mass have more gravity. | ? |
| A falling ball keeps the same speed all the way down. | ? |
| Without air resistance, a heavy ball and a light ball fall together. | ? |
| Gravity gets stronger with distance. | ? |
Great week. Tomorrow is Game Day: a family drop race with rules from NASA.