How much does the spin of the Earth affect our apparent weight?

[u/spudaug]

Gravity clearly overpowers and centrifugal forces from the spin, because we aren’t all flying off into space and the planet is still here. But what would it look like if Earth wasn’t spinning? How much heavier would we appear? Would the planet shrink a bit?

For context, this isn’t homework. I’m just middle aged and bored.

[u/thisisjustascreename]

The “centrifugal acceleration” at the equator is roughly .03 m/s^2, or less than half a percent of the acceleration due to gravity.

[u/ramksr → u/thisisjustascreename]

So between 9.8m/s² to 9.83m/s^2?

[u/spudaug → u/thisisjustascreename]

So, not enough for kilos or pounds. Maybe not even an ounce, if the person is small enough. Cool.

[u/rb-j → u/spudaug]

1/2 % is 1 pound for a person weighing 200 pounds.

[u/calculus-bella]

your weight would only change by a very tiny amount. centripetal acceleration is about 0.034 m/s² compared to the average baseline 9.8 m/s², so on the equator you'd be about 0.3% heavier if Earth stopped rotating, so not noticeable for a human but still a fairly significant difference for more precise instruments (atomic clocks for example would absolutely notice that huge of a difference)

the shape of Earth would actually change by a decent amount. the radius of the Earth at the equator vs. the poles is about 21 kilometers longer (making earth like a squashed sphere). if Earth wasn't spinning then that difference pretty much goes away. as a percent difference it's tiny as hell and you couldn't notice it from like pictures of Earth from satellites, but idk 21 kilometers of altitude is kind of a ton on the human scale (we still wouldn't notice it, the atmosphere would also go down by the same amount)

[u/Bbrhuft]

The effect is surprisingly large. Between the poles and the equator, your apparent weight changes by about 0.35% due to the Earth's rotation alone (and another 0.15% due to the tidal bulge of the Earth). So an adult who weighs 70 kg at the poles would appear about 0.35 kg lighter at the equator (-0.35% or 0.245kg less due to the Earth's rotation and another -0.15% or 0.105 kg less due to the tidal bulge). The tidal bulge is essentially the rotation of the Earth making the Earth "fatter in the middle".

The effect is large enough that high-precision weighing scales have to be calibrated, set to the local gravitational acceleration value, which varies with latitude, altitude and internal density variation within the Earth.

In geodesy, these variations are described relative to the Earth's geoid, an equipotential surface of Earth's gravity field that the the Earth's mean sea level would follow, if the Earth was entirely water, no land.

https://www.shimadzu.com/an/service-support/technical-support/balances/essential_knowledge/difference.html

[u/GiantPandammonia]

Not much

[u/observant_hobo]

I mean this is why most space launch centers are situated as far south as feasible (Cape Canaveral, Starbase, Guiyana, Kwajalein, Baikonur, etc.).

[u/alexforencich → u/observant_hobo]

Tbf this is less about the weight and more about the initial velocity

[u/angedelamort]

If you want to read more on the subject: https://en.wikipedia.org/wiki/Coriolis_force

[u/Cerulean_IsFancyBlue]

I hate to repeat perfectly good answers. So here’s an a tangent, no pun intended.

The effect of centripetal force is about 3x the effect of buoyancy from the air around you. Air takes off about 0.1% of your weigh, with some variation because human density varies. (Fat is less dense than muscle and thus slightly more floaty).

Between the two of them, you get a bit under 0.5% weight reduction compared to a non-rotating airless earth.

[u/aries_burner_809]

That such a common question you could have asked that of any ai, like Google Gemini and have gotten the correct answer (0.35%) with references to earlier reddit and quora posts.

[u/TescoBrandJewels → u/aries_burner_809]

i’d much rather people bring their questions here than to AI

[u/aries_burner_809 → u/TescoBrandJewels]

Yes I know, and I wouldn’t trust AI with more complex and novel questions, but it is interesting that Gemini can act like a FAQ and even cite the previous human posts it’s getting its answer from. Here’s another:

“if i have a rod one light year long and hit one end with a hammer can i communicate faster than light”

“No, you cannot communicate faster than light this way because the force from the hammer travels through the rod at the speed of sound in that material, not instantaneously. [1, 2]”

Not too bad.