Week 6: Gravity Never Clocks Out

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Drag fights a rocket only while air is thick enough to matter. Gravity never takes a break, from the moment of liftoff to the moment of touchdown.

History

Isaac Newton formalized universal gravitation in his 1687 work, the Principia Mathematica — one of the most important books in the history of science. Every trajectory calculation in this series, from here to Week 25, ultimately traces back to the ideas in that one book.

Forces

This week draws a distinction that's easy to blur: mass is how much stuff something is made of, while weight is the force gravity exerts on that stuff. Mass stays the same everywhere; weight changes depending on how strong gravity is where you're standing.

Math

W = m·g

Worked example (metric): a rocket with a mass of 0.1 kg has a weight of W = 0.1 × 9.81 = 0.981 N on Earth.

Key Date

Newton's Principia Mathematica was published in 1687.

Safety Note

This is exactly why we compute expected altitude and descent time before choosing a launch site — if you don't know roughly where gravity is going to bring your rocket back down, you can't pick a recovery field that's actually big enough.