# Which Way Is Down?

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THE VSAUCE CURIOSITY BOX: https://www.curiositybox.com/
Links to sources and to learn more below!

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Thanks to Eric Langlay (https://www.youtube.com/user/ericdraven30) for producing, editing, and animating this episode with me. Thanks also to Henry Reich (https://www.youtube.com/user/minutephysics) for his advice and guidance.

Mass vs weight:

https://en.wikipedia.org/wiki/Mass_versus_weight
Great Veritasium video: https://www.youtube.com/watch?v=_Z0X0yE8Ioc
two other great videos: https://www.youtube.com/watch?v=6ccIjRwYO9U and https://www.youtube.com/watch?v=aCqQzrPCcFM

baseballs coming together under gravitational attraction can be simulated in Universe Sandbox 2. More math behind it can be found here: https://www.reddit.com/r/explainlikeimfive/comments/2vyxl7/eli5_will_two_baseballs_a_foot_apart_in_deep/

Weight to mass (on surface of Earth) convertor: https://www.convertunits.com/from/kilograms/to/newton

NASA HD footage: https://archive.org/details/NASA-Ultra-High-Definition

Earth’s spin and its effect on ‘down’: http://www.freemars.org/jeff/Earth/down.htm

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: https://www.reddit.com/r/askscience/comments/4qfl52/do_you_get_lighter_the_further_underground_that/

vertical deflection:

https://en.wikipedia.org/wiki/Vertical_deflection

practical uses of measuring gravity: https://en.wikipedia.org/wiki/Gravimetry

geoid:

Interactive Earth geoid: https://experiments.withgoogle.com/chrome/geoid-viewer

your weight when moon is overhead: https://www.thenakedscientists.com/forum/index.php?topic=11639.0

Hammer and feather drop on moon: https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html

Wolfram Alpha cone geodesic tool: http://demonstrations.wolfram.com/ConeGeodesics/

General Relativity:

simple animation showing geodesic on cone and how it causes motion DOWN in space: https://www.youtube.com/watch?v=DdC0QN6f3G4
GREAT pbs spacetime video (watch the whole channel): https://www.youtube.com/watch?v=AwhKZ3fd9JA
http://www.askamathematician.com/2009/10/q-why-does-curved-space-time-cause-gravity/
https://www.av8n.com/physics/geodesics.htm#sec-straight
http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/spacetime/
http://www.einstein-online.info/spotlights/geometry_force

time and gravity:

tests of general relativity: https://en.wikipedia.org/wiki/Tests_of_general_relativity

RECOMENDED BOOKS:

great introductory texts:

“Relativity Visualized” by Lewis Carroll Epstein https://www.amazon.com/Relativity-Explained-Classics-Science-Mathematics/dp/0486293157

There’s also this PDF that takes Epstein’s diagrams into more detail: http://www.relativity.li/uploads/pdf/English/Epstein_en.pdf

“Relativity Simply Explained” by Martin Gardener https://www.amazon.com/Relativity-Explained-Classics-Science-Mathematics/dp/0486293157

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 https://www.amazon.com/Most-Incomprehensible-Thing-Introduction-Mathematics/dp/0957389469/

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

“Spacetime And Geometry: An Introduction To General Relativity” by Sean Carrol https://www.amazon.com/gp/product/9332571651/

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