# B Feeling the Earth rotate

#### Thecla

In the early 17 th century people argued that the earth can't be rotating because the don't feel any movement. How fast would the earth have to rotate for us to feel the effect of rotation and have no doubt of its rotation?

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#### Henryk

Gold Member
This is a question of how sensitive you are to rotation. An ordinary person is not sensitive to Earth's rotation because it is 'always there', just as he is not sensitive to his heart beat. That is if you wan to feel it directly with your senses. However, the object of science is not to rely on our subjective sensual feelings but to look for objective evidences. In the case of the Earth rotation, one could look at the pattern of a hurricanes - swirling around instead of radially converging. Another example of sensing the Earth rotation if Foulcault's pendulum.

#### Janus

Staff Emeritus
Gold Member
Let's put it this way: If the Earth was spinning fast enough so that it attained orbital speed at the equator, then objects at the equator would be "weightless". However this still only results in the Earth making 1 rotation in roughly 90 min. Could you easily detect that you were rotating that slow?.
So while you could deduce the spin of the Earth by traveling North and South and noting the difference in gravity as you did so, and this difference would be easier to measure as the speed of rotation increased, you still wouldn't be able to "feel" it when standing in one spot.

#### Tghu Verd

How fast would the earth have to rotate for us to feel the effect of rotation and have no doubt of its rotation?
Were you thinking of physical effects such as inner ear confusion when you asked? In that case, you can use SpinCalc to determine at what point the Earth's rotation falls outside 'comfort zone'.

#### sophiecentaur

Gold Member
How fast would the earth have to rotate for us to feel the effect of rotation and have no doubt of its rotation?
There is a possible way of estimating this - or at least putting a lower limit on it. Consider life on a ship. The crew (even on a very small one) very soon get used to the curved courses that ships take and they easily cancel it out in their brains. The rate of turn on a large tanker is a lot faster than the Earth's motion and, more importantly, it is not uniform over time but I wouldn't mind betting that experienced crew would be able to detect some very slight course adjustments. To put a figure on it, perhaps 360°/hour would be too slow to be detectable (compared with 24hours we all experience).
But I like to bring these questions down to good old Evolution and ask what would be the point in being excessively sensitive to rotation. According to lore, we tend to walk in circles when left with no obvious direction reference so that implies we are actually not very sensitive at all to this sort of thing. Perhaps that's not a fair test, in view of all the other clues (moss on N side of trees etc.) but those are 'intellectual' clues and not innate. Migrating animals have to make use of the Earth's Magnetic field and not (afaik) Coriolis force.

#### anorlunda

Mentor
Gold Member
This question has much to do with human physiology and little to do with physics.

Human perception of acceleration has a big deadband. Flight simulators take advantage of that to give pilot trainees the illusion of unlimited motion even though the actual motion of the simulator is limited.

#### russ_watters

Mentor
Let's put it this way: If the Earth was spinning fast enough so that it attained orbital speed at the equator, then objects at the equator would be "weightless". However this still only results in the Earth making 1 rotation in roughly 90 min. Could you easily detect that you were rotating that slow?.
So while you could deduce the spin of the Earth by traveling North and South and noting the difference in gravity as you did so, and this difference would be easier to measure as the speed of rotation increased, you still wouldn't be able to "feel" it when standing in one spot.
Right: there is nothing to "feel" except the magnitude of the acceleration and there is no way to tell the difference between gravitational and rotational. If you're on a different planet with a 10% different apparent g, you might notice, but there's no way to tell by "feel" how much of that is gravity and how much is rotation.

(Note: oblate sphereoid = uniform apparent g).

#### russ_watters

Mentor
What do you mean?
Earth's surface is equipotential to the extent it fits the theoretical geoid. The g you measure with an accelerometer while sitting on Earth's surface does not vary with latitude:

Earth's shape is caused by that g.

#### jbriggs444

Homework Helper
Earth's surface is equipotential to the extent it fits the theoretical geoid. The g you measure with an accelerometer while sitting on Earth's surface does not vary with latitude:
Wait a minute. Potential does not change with latitude. But the g you measure with an accelerometer is not potential. It is the [magnitude of the] gradient of the potential.

The g you measure with an acceleometer while sitting on the Earth's surface most certainly does vary with latitude.

Earth's shape is caused by that g.
Yes. The geoid will always be at right angles to the local direction of $\vec g$. That's not the same thing as saying that it is a surface where local $\vec g$ has a fixed magnitude.

Mentor

#### sophiecentaur

Gold Member
If you're on a different planet with a 10% different apparent g, you might notice,
Definitely - you notice very small slopes when cycling (and running) because of the different rate you have to work at.

#### jbriggs444

Homework Helper
Definitely - you notice very small slopes when cycling (and running) because of the different rate you have to work at.
But we are not talking about any slopes. Just an equipotential surface -- flat and level.

#### sophiecentaur

Gold Member
But we are not talking about any slopes. Just an equipotential surface -- flat and level.
I was thinking that mgh (Work) would be noticeable when lifting and climbing. Going up a slope is the equivalent of a bigger g. (I think that must be right) I wouldn't just be Weight Force that counts in general 'getting around' in a different g. But it al depends what the rules are to this particular game, I think.

#### jbriggs444

Homework Helper
Going up a slope is the equivalent of a bigger g.
When cycling, the bicycle takes care of the vertical load. Bigger g is irrelevant. Slope and wind resistance are the enemies. Good wheel bearings and properly inflated tires make rolling resistance (which can scale with g) largely irrelevant.

Agree that we're likely quibbling about rules :-)

#### sophiecentaur

Gold Member
Bigger g is irrelevant
I see what you mean - I shouldn't compare flat cycling with cycling up a slope. But two different slopes would be a valid analogue with the broader experience in different g conditions but we would have to define more precisely what we are comparing with what (before it's handbags at dawn time). Power is actually what counts in getting you exhausted and mg dh/dt has contributions from g and dh/dt. Or, if you carry extra weight up hill, of course. I know from my limited cycling experience that changes in slope are very noticeable, even when you are fit.
The suggested 10% would definitely be noticeable. I was at the gym today and 10% extra load is very noticeable after a set of 12 reps.

#### Ralph Dratman

Great answer! What a fascinating thought.

#### zanick

Is there a change of velocity for us circuling the sun? (as small as it might be, since we are talking about lack of changes of velocity being detected of us on the earth as far as rotation).
the earth rotates the sun every 365ish days, at a velocity near 66,000mph. because we spin a the equator at near 1000mph, does this mean we have a 1000mph change of velocity around the sun as viewed from the surface of the earth relative to the sun? (i.e. 67,000mph at night and 65,000mph at night). Is this another case of the percentage of velocity change being so small that we cant feel it?

#### jbriggs444

Homework Helper
Is this another case of the percentage of velocity change being so small that we cant feel it?
Percentage velocity change is neither well defined nor observable. That's basic Galilean relativity.

#### zanick

Percentage velocity change is neither well defined nor observable. That's basic Galilean relativity.
True... so the change of velocity of 1000mph, over 12 hours I so slight of an acceleration that we couldn't feel it. is that better? (1.3mph per min, ave rate of change)

#### jbriggs444

Homework Helper
True... so the change of velocity of 1000mph, over 12 hours I so slight of an acceleration that we couldn't feel it. is that better? (1.3mph per min, ave rate of change)
Yes, that is better.

Back of the envelope: 1.3 miles per hour per minute is around 1.5 feet per second per minute. 1.5 feet per second per second would be about 1/20 of a gee. We are talking about 60 times less than that. So about one tenth of one percent of a gee.

Meanwhile you are busy standing, sitting and walking around in one gee. A tenth of a percent is in the noise. Your bathroom scale cannot detect it either.

#### russ_watters

Mentor
Hang on; Is there actually anything to feel here at all? The acceleration is constant and circular, not linear and reciprocating. I don't think the issue is that it's too small to be noticeable but that there is nothing to notice, not even in theory....Per post #19.

The only thing that I can see changing is due to solar tides, not the change in a person's linear speed around the sun.

I know I was wrong about g changing with latitude, but...

#### mfb

Mentor
You can't feel or measure that your velocity relative to the Sun is changing without astronomical observations. That is one of the key results of physics: There is no absolute velocity. The velocity relative to X can only be measured if you have a way to compare yourself to X (e.g. use astronomical measurements).

What you can measure is a small change in the apparent gravitational acceleration (relative to the surface of Earth) during a day. At noon it is slightly smaller as you are pulled more towards the Sun than Earth, at sunset and sunrise it takes its largest values, at midnight it is slightly smaller as the Earth is pulled more towards the Sun than you. The Moon has a similar (and stronger) effect. Both together create the tides on Earth due to this.

#### CWatters

Homework Helper
Gold Member
As I recall the weight difference attributed to the earths rotation is about 0.3% at the equator. So a man weighing 100kg weighs about 300g less at the equator due to the rotation. You don't feel that much lighter because its constant so you have nothing to compare it with. Do you feel 300g heavier after drinking a can of soda?

#### ZapperZ

Staff Emeritus
2018 Award
This question has much to do with human physiology and little to do with physics.

Human perception of acceleration has a big deadband. Flight simulators take advantage of that to give pilot trainees the illusion of unlimited motion even though the actual motion of the simulator is limited.

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I very much agree with this. This not a physics question.

Besides, no matter how good we think our senses are, humans have a very poor and very limited range of ability as a detector of many things.

Zz.

"Feeling the Earth rotate"

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