# Why is the earth constantly rotating, but we as humans can never feel it?

1. Jul 26, 2006

### JERGLOVE

I don't understand why we can still go about with our lives, and stay stuck on this earth while the earth is actually moving. Why does this not affect us?

2. Jul 27, 2006

### Outlandish_Existence

I can feel it every night that I peak up at the beautiful stars and watch them slowly seem to drift across the sky... when in my mind I know that the earth is really rotating. There is a force that hasn't fully been figured out yet called gravity that keeps us grounded. It does effect us... from seasons to night and day and beyond.

Last edited: Jul 27, 2006
3. Jul 27, 2006

### Mickey

I hear that the Earth's crust is also divided up into sections that move around, but I don't feel them moving. I'm in the Midwestern US, and I have never even felt the slightest tremor.

I guess it's because I'm on top of one of the sections at an area where it's very stable and I move along with it. And as I move along with the section, the stability goes with me!

4. Jul 31, 2006

### Mk

Everything is relative. If you are moving at forty-five miles per hour in your car, do you feel like you are moving? Only when you accelerate to it. This is because you are moving forty-five miles per hour along with your car, so the difference is zero. You feel zero. When you are sitting in front of your computer screen on Earth, you are moving at the same speed in the same direction as the ground. Therefore the difference is zero. You feel zero.

5. Aug 2, 2006

### matthyaouw

The sections (Plates) move at about the speed your fingernails grow, so unless they go with a bit of a jolt (ie. an earthquake) you're not going to feel a thing.

6. Aug 2, 2006

### DaveC426913

1] We don't feel constant motion (such as in a train going straight), but we do feel accelerated motion (such as in a train going around a curve).

2] While the Earth's straight-line motion is pretty fast (you are moving East at between 500 and 1000 mph), it's angular motion is pretty small because Earth is so large. Pick up a tennis ball and rotate it as fast as the Earth turns: after an hour you will have only turned it by a half inch.

3] The only way to feel this kind of motion is to be very large. Air masses are large enough to feel this rotation - that's why they curve into rotating storms. This is called the Coriolis Force.

7. Aug 2, 2006

### Mickey

But I still don't feel earthquakes when they happen, since I'm no where near the edge of the plate. It's like Galileo on a ship with a bunch of butterflies in boxes and... oh nevermind.

8. Aug 6, 2006

### Pythagorean

Does the earth's rotation speed vary? Wouldn't we feel it if the earth accelerated enough?

9. Aug 6, 2006

### Andre

Is there anything constant in the movements of the Earth? The spinning for instance, or the length of day is changing in cycles with micro seconds a day

http://www.terrapub.co.jp/journals/EPS/pdf/5211/52110989.pdf

But forces associated with those perturbations are a few dozen orders of magnitude smaller than gravity, so there is nothing to notice.

10. Aug 13, 2006

### Pythagorean

but is the buttefly effect applicable? What if, at a peak in the magnitude of the jerks, I was involved in an extremely calculating coordination, like spinning around and making a shot around an opponents arms, and the little disturbance is enough to throw me off and knock me over, and i'm like "man I must be off my game today" but it's really the earth's fault. I KNEW IT!

11. Aug 13, 2006

### DaveC426913

In a word, no.

And even if it actually could happen, it wouldn't seem like "a little disturbance". Think about how the energy of that jerk would be transferred through the Earth's mass. No, it wouldn't be a little disturbance, it would be the mother of all Earthquakes.

12. Aug 30, 2006

### chance

Indeed, relative motion is the key.
To actually feel/see the difference between equatorial an polar earth rotation speed, you need sensitive instruments like an accelerometers. Then rapid North-South motion will defiantly register on an East-West mounted accelerometer.

13. Nov 22, 2006

### Andrew Mason

The earth is in orbital freefall around the sun (and around the earth moon centre of mass). According to Einstein's principle of equivalence, gravitational freefall is equivalent to an inertial (non-accelerating) frame. So there is nothing to feel.

AM

14. Nov 23, 2006

Staff Emeritus
Acually free fall only equals inertial in the small limit. For extended objects there are tidal forces. And the earth is also spinning which is quite noninertial and which generates forces (centrifugal and coriolis) that we DO feel! We feel coriolis, when combined with the solar input and the thermodynamics of the atmosphere, as weather. Think of that when you're shoveling four inches of earth-spin off your driveway.

15. Nov 24, 2006

### billiards

You don't just get e'quakes at plate boundaries ya know (okay predominantly you do but..) about a hundred years ago therre were some pretty big tremors in the New Madrid region in central USA, you anywhere near there coz some think it might happen again.

16. Jan 6, 2007

### mikewashere

Its like when you are driving at the same speed as other cars on the highway, they are all standing still with respect to your car.. Everything else thats not spinning with us, is just so far away that it doesn't create for much of a motion blurring effect like youd expect going at thousands of miles an hour.

17. Jan 7, 2007

### DaveC426913

What??
10 char

18. Jan 7, 2007

### D H

Staff Emeritus
You do feel the effects of the Earth rotating on its axis. It reduces the gravitational acceleration toward the Earth by a small amount. A person is heavier at the North and South Poles than at the equator. g is 9.78039 m/sec^2 at the equator, 9.83217 m/sec^2 at the poles. The rotational effect at the equator, 0.034 m/sec^2, accounts for about 2/3 of the difference in g values.

19. Jan 7, 2007

### HallsofIvy

That's not quite the situation. Rotation is accelertion (rotational speed is constant but velocity direction changes) so we do "feel" the earth's rotation. As D H said, it causes us to feel slightly lighter.

20. Jan 8, 2007

### DaveC426913

If this were true, all the water in the world would pool around the equator.

21. Jan 8, 2007

### billiards

What do you mean 'pool'?

But D H's right, I knew gravity was stronger at the poles but I was surprised the differences were that much so I checked, it turned out he was quite correct. The additional 1/3 of the difference is accounted for by the fact that the earth's radius is greatest at the equator and least at the poles, and we all know that gravity decreases with distance.

22. Jan 9, 2007

### Andrew Mason

And it does, actually. But only so far because pooling would increase sea levels and increase the gravitational potential of sea water. So the surface of the oceans represents an equipotential surface - similar to charge on a conducting hollow sphere.

AM

23. Jan 9, 2007

### DaveC426913

This is not as simple as you might think. Despite being off-round, and despite the expectation that g is not the same at differnt places oin the Earth, the water levels are an extant indication of the fact that it all balances out. Water does not "flow downhill" from the poles to the equator as would be the case if one of obth of the above two items were true.

The fact is, the mouth of the Mississippi (near the equator) is several miles farther from the centre of the Earth than its headwaters (nearer the pole), - yet - it still flows toward the equator. As far as the Mississippi is concerned, the Earth is both spherical (on a large scale - it only flows downhill on a local/terrain scale) AND non-rotating.

24. Jan 9, 2007

### Andre

And it's even slightly more complicated. Yes, gravity is generally strongest at the poles and the reason why water doesn't flow to the poles or equator is because gravity is at right angles with the earth surface. There cannot be a horizontal component.

But the difference between poles and equator is slightly less than subtracting centrifugal forces at the equator because Earth is not a sphere and the simple form of Newtons gravity law assumes point masses and spheres. The latter is a complex intergral of all seperate components of the mass in the sphere. The equatorial bulge causes to have more mass directly underneath you at the equator, increasing the effective gravity in the directly downward components. while for the poles, the eq bulge causes those components of the gravity to point more outwards instead of downwards, decreasing gravity slightly but the effect is negliglible compared to the centrifugal force and the assymetric masses within Earth.

Have a look at Grace

http://www.csr.utexas.edu/grace/
http://www.gfz-potsdam.de/GRACE/results/grav/g001_eigen-grace01s.html

Last edited: Jan 9, 2007
25. Jan 9, 2007

### D H

Staff Emeritus
You are confusing force and potential energy. The mean sea level exemplifies Hamilton's Principle: the mean sea level is an equipotential surface, not an equal force surface. The potential field in question is the sum of the potential due to gravitation and the potential due to rotation: (http://en.wikipedia.org/wiki/Geopotential" [Broken])
For geophysical applications, gravity is distinguished from gravitation. Gravity is defined as the resultant of gravitation and the centrifugal force caused by the Earth's rotation. The global mean sea surface is close to one of the equipotential surfaces of the geopotential of gravity. This equipotential surface, or surface of constant geopotential, is called the geoid.

To illustrate, imagine what would happen
• if the Earth was spherical but maintained its rotation
• if the Earth stopped rotating but maintained its current shape.

• If the Earth Was Spherical
This change in geometry to a spheroid geometry with its altered gravity would cause the global ocean to change. The polar zones would be relatively farther from the center of the earth, and these new higher altitudes would force seawater toward the equator. By the same token, the equatorial region would be relatively closer to the earth's center and would be more strongly affected by gravity. Increased gravitational force in the equatorial zone would pull oceanic water toward the equator and form a global equatorial ocean.
• If the Earth Stood Still
What would happen if the earth stopped spinning and the centrifugal effect ceased to force oceans to accumulate around the equator? It appears that the world's ocean would split into two polar oceans and leave the equatorial area totally dry. To model this hypothesis, a value of 6,371,146 metersâ€”the distance from the earth's center indicates the approximate elevation of the sea level on the reference ellipsoidâ€”was specified to separate water from land. For this "what if" simulation, the elevation of the sea level was based on the assumption that the volume of ocean water would be about the same as it is today.

The attached images illustrate the above thought experiments.

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Last edited by a moderator: May 2, 2017