Links to sources and to learn more below!

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Thanks to Eric Langlay ( for producing, editing, and animating this episode with me. Thanks also to Henry Reich ( for his advice and guidance.

Mass vs weight:
Great Veritasium video:
two other great videos: and

baseballs coming together under gravitational attraction can be simulated in Universe Sandbox 2. More math behind it can be found here:

Weight to mass (on surface of Earth) convertor:

pencil and Earth falling numbers:

NASA HD footage:


Earth’s spin and its effect on ‘down’:

The measurement of Earth and its gravity:

Movement of Earth’s center of mass:

you get heavier before you get lighter as you descend into Earth:

vertical deflection:

practical uses of measuring gravity:


Interactive Earth geoid:

your weight when moon is overhead:

Hammer and feather drop on moon:

Why things fall at the same rate:

Wolfram Alpha cone geodesic tool:

General Relativity:

simple animation showing geodesic on cone and how it causes motion DOWN in space:
GREAT pbs spacetime video (watch the whole channel):

time and gravity:

tests of general relativity:


great introductory texts:

“Relativity Visualized” by Lewis Carroll Epstein

There’s also this PDF that takes Epstein’s diagrams into more detail:

“Relativity Simply Explained” by Martin Gardener

great intro to the math of general relativity:

“A Most Incomprehensible Thing: Notes Towards a Very Gentle Introduction to the Mathematics of Relativity” by Peter Collier

Requires some background in relevant math topics (see above) but very very good:

“Spacetime And Geometry: An Introduction To General Relativity” by Sean Carrol

Altri video