Are we lighter in the daytime?

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Re: Are we lighter in the daytime?

#81  Postby Pulsar » Jan 01, 2015 4:46 pm

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Re: Are we lighter in the daytime?

#82  Postby Veida » Jan 01, 2015 5:18 pm

Ok, so you realize that we're in free fall around the Sun.

You also realize that the discus trower is a poor analogy.

Then I can't figure out where your mistake is.

Why do you think that there would a be weight decrease only at daytime and not at night?
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Re: Are we lighter in the daytime?

#83  Postby kennyc » Jan 01, 2015 5:36 pm

New Year's Day advice, you'll learn that David is best ignored.
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Re: Are we lighter in the daytime?

#84  Postby Veida » Jan 01, 2015 5:50 pm

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Re: Are we lighter in the daytime?

#85  Postby DavidMcC » Jan 01, 2015 7:16 pm

Veida's link wrote:The fact that the Earth rotates around the CM or on itself, unfortunately throws dust into our sight. Those effects do not produce any tidal deformation, they just confuse us.

Ha ha! I''m sorry to disillusion the confused Paulo Sirtoli, but his physics is off-beam.
Perhaps you should find a peer reviewed physics journal next time.
The earth's rotaion does not change the gravitational force itself (except in a very small way, due to GR), but certiainly does impact the contect force that is our effective weight. That contact force is given by m*r* omega2, where omega is the angular velocity of the earth's spin about its own axis.
I rememember putting the numbers into this exprssion the last time this issue came up on this site, and it comes out as much larger than the moon's pull, at least when you are at the equator.
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Re: Are we lighter in the daytime?

#86  Postby DavidMcC » Jan 01, 2015 7:20 pm

... Hint: Try looking up "rotating reference frames".
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Re: Are we lighter in the daytime?

#87  Postby Veida » Jan 01, 2015 7:44 pm

DavidMcC wrote:
Veida's link wrote:The fact that the Earth rotates around the CM or on itself, unfortunately throws dust into our sight. Those effects do not produce any tidal deformation, they just confuse us.

Ha ha! I''m sorry to disillusion the confused Paulo Sirtoli, but his physics is off-beam.
How?

DavidMcC wrote:The earth's rotaion does not change the gravitational force itself (except in a very small way, due to GR), but certiainly does impact the contect force that is our effective weight. That contact force is given by m*r* omega2, where omega is the angular velocity of the earth's spin about its own axis.
I rememember putting the numbers into this exprssion the last time this issue came up on this site, and it comes out as much larger than the moon's pull, at least when you are at the equator.


That's irrelevant, as it does not change between day and night.

The question you appear to be avoiding is why you think that the weight would decrease only at daytime, and not at night?
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Re: Are we lighter in the daytime?

#88  Postby DavidMcC » Jan 01, 2015 7:48 pm

Veida wrote:
DavidMcC wrote:
Veida's link wrote:The fact that the Earth rotates around the CM or on itself, unfortunately throws dust into our sight. Those effects do not produce any tidal deformation, they just confuse us.

Ha ha! I''m sorry to disillusion the confused Paulo Sirtoli, but his physics is off-beam.
How?


DavidMcC wrote:The earth's rotaion does not change the gravitational force itself (except in a very small way, due to GR), but certiainly does impact the contect force that is our effective weight. That contact force is given by m*r* omega2, where omega is the angular velocity of the earth's spin about its own axis.
I rememember putting the numbers into this exprssion the last time this issue came up on this site, and it comes out as much larger than the moon's pull, at least when you are at the equator.


That's irrelevant, as it does not change between day and night.

The question you appear to be avoiding is why you think that the weight would decrease only at daytime, and not at night?

I thought I'd already explained that the effect is NOT related to the day-night difference (perhaps you should read the thread more carefully), but may still be of interest to the OP, as it is a non-gravitational effect that changes your weight. Unfortunately, jamest hasn't seen fit to comment.
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Re: Are we lighter in the daytime?

#89  Postby Veida » Jan 01, 2015 7:52 pm

DavidMcC wrote:
I thought I'd already explained that the effect is NOT related to the day-night difference (perhaps you should read the thread more carefully), but may still be of interest to the OP, as it is a non-gravitational effect that changes your weight.


It does not change your weight.
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Re: Are we lighter in the daytime?

#90  Postby Thommo » Jan 01, 2015 7:52 pm

Ahem.

DavidMcC wrote:It isn't quite that simple. Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.
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Re: Are we lighter in the daytime?

#91  Postby DavidMcC » Jan 01, 2015 7:55 pm


Pulsar, you appear to be having the same comprehension problem that Veida has. I have already explained the tides on both sides of the earth as being due to conservation of angular momentum (and the incompressbility of water).

EDIT: Perhaps you also need to look up rotating frames of reference. :roll:
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Re: Are we lighter in the daytime?

#92  Postby DavidMcC » Jan 01, 2015 7:59 pm

Thommo wrote:Ahem.

DavidMcC wrote:It isn't quite that simple. Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.

Ahem, what, Thommo? The tides go up and down twice a day for the same reason as our contact force with the earth is affected by being in a rotating frame of reference.
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Re: Are we lighter in the daytime?

#93  Postby Veida » Jan 01, 2015 8:05 pm

DavidMcC wrote:
Veida wrote:I'm afraid you are wrong. The Earth's rotation has nothing to do with the tidal forces, other than moving the Earth so that they move around on the surface. It the Earth would not rotate at all around its axis, so that an Earth day would be the same as an Earth year, the tidal forces would be the same: a maximum in the middle of the day, and a maximum at the middle of the night.

I do know that this is a common misconception. Nevertheless, it is a misconception.

You misunderstand. I didn't say that earth's rotation was part of tidal forces, only that it modifies their effect on our weight. Think of a discus thrower, exploiting angular momentum and mass (an so weight) to his/her advantage.


This is the statement you've made, DavidMC, that I would like you to elaborate on. You are saying that the Earth's rotation modifies the effects of the tidal forces on weight.

Elsewhere you said the following, which seems to be related to what you say above.
Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.


Care to explain that? How would our weight increase at night if the Earth didn't spin about its axis?

(It wouldn't. I'm asking only in the hope that you'll explain yourself so I/we can pinpoint where you went wrong.)
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Re: Are we lighter in the daytime?

#94  Postby DavidMcC » Jan 01, 2015 8:19 pm

Veida wrote:
DavidMcC wrote:
Veida wrote:I'm afraid you are wrong. The Earth's rotation has nothing to do with the tidal forces, other than moving the Earth so that they move around on the surface. It the Earth would not rotate at all around its axis, so that an Earth day would be the same as an Earth year, the tidal forces would be the same: a maximum in the middle of the day, and a maximum at the middle of the night.

I do know that this is a common misconception. Nevertheless, it is a misconception.

You misunderstand. I didn't say that earth's rotation was part of tidal forces, only that it modifies their effect on our weight. Think of a discus thrower, exploiting angular momentum and mass (an so weight) to his/her advantage.


This is the statement you've made, DavidMC, that I would like you to elaborate on. You are saying that the Earth's rotation modifies the effects of the tidal forces on weight.

Sorry about the wording! :oops: I should have said that the earth's rotation about its own axis makes its own contribution to the contact force that determines your apparent weight, and tyhis depends mainly on your latitude. It is a much stronger effect than the moon's pull.
Elsewhere you said the following, which seems to be related to what you say above.
Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.


Care to explain that? How would our weight increase at night if the Earth didn't spin about its axis?

(It wouldn't. I'm asking only in the hope that you'll explain yourself so I/we can pinpoint where you went wrong.)

I don't think I did go wrong, Veida. Without the earth's rotaion, all that would matter would be the combined pulls of earth, sun and moon, which would combine differently at night (when the sun's pull would add to that of the earth, but the moon's pull would depend on where the moon was (it can be above the horizon, or below)).
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Re: Are we lighter in the daytime?

#95  Postby Macdoc » Jan 01, 2015 9:15 pm

Ummm there would be no night and day as we understand it....there would be a darkside and a light side as the it's the earth's rotation that creates night and day.

You are postulating an earth tidal locked to the sun the way the moon is to earth with no rotation.
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Re: Are we lighter in the daytime?

#96  Postby Thommo » Jan 01, 2015 9:41 pm

DavidMcC wrote:
Thommo wrote:Ahem.

DavidMcC wrote:It isn't quite that simple. Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.

Ahem, what, Thommo? The tides go up and down twice a day for the same reason as our contact force with the earth is affected by being in a rotating frame of reference.


If the Earth didn't spin the tidal bulge would still lead to a reduction in weight due to the sun at the point directly opposite the sun. This statement of yours is both clear, and clearly wrong. The Earth's rotation clearly has no impact on the case when the Earth is not rotating.

The gradient of gravity across the Earth (due to the sun) would still exist, with the pull on the Earth's centre of mass being slightly greater than the pull on a person standing at the diametrically opposed point to the sun, on the Earth's surface. Since the Earth experiences more force/acceleration than the person the experienced gravity is lower, not higher. This is explained in Veida's posts, my own and the ample links kindly provided by Pulsar and Veida.
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Re: Are we lighter in the daytime?

#97  Postby Veida » Jan 01, 2015 9:52 pm

DavidMcC wrote:
Veida wrote:
DavidMcC wrote:
Veida wrote:I'm afraid you are wrong. The Earth's rotation has nothing to do with the tidal forces, other than moving the Earth so that they move around on the surface. It the Earth would not rotate at all around its axis, so that an Earth day would be the same as an Earth year, the tidal forces would be the same: a maximum in the middle of the day, and a maximum at the middle of the night.

I do know that this is a common misconception. Nevertheless, it is a misconception.

You misunderstand. I didn't say that earth's rotation was part of tidal forces, only that it modifies their effect on our weight. Think of a discus thrower, exploiting angular momentum and mass (an so weight) to his/her advantage.


This is the statement you've made, DavidMC, that I would like you to elaborate on. You are saying that the Earth's rotation modifies the effects of the tidal forces on weight.

Sorry about the wording! :oops: I should have said that the earth's rotation about its own axis makes its own contribution to the contact force that determines your apparent weight, and tyhis depends mainly on your latitude. It is a much stronger effect than the moon's pull.


Ok, that is true.
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Re: Are we lighter in the daytime?

#98  Postby Veida » Jan 01, 2015 9:59 pm

DavidMcC wrote:
Elsewhere you said the following, which seems to be related to what you say above.
Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.


Care to explain that? How would our weight increase at night if the Earth didn't spin about its axis?

(It wouldn't. I'm asking only in the hope that you'll explain yourself so I/we can pinpoint where you went wrong.)

I don't think I did go wrong, Veida. Without the earth's rotaion, all that would matter would be the combined pulls of earth, sun and moon, which would combine differently at night (when the sun's pull would add to that of the earth, but the moon's pull would depend on where the moon was (it can be above the horizon, or below)).


Ah. I think I see where you went wrong. You seem to think that the weight difference due to tidal forces is a first order effect from gravitation.

It isn't. It is a second order effect, that is due to the gravitation from e.g. the sun not being uniform - it varies slightly in strength between dayside and nightside, and it varies slightly in direction between dawnside and duskside.

All first-order effects are cancelled out due to the Earth being in free fall.
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Re: Are we lighter in the daytime?

#99  Postby Pulsar » Jan 01, 2015 10:24 pm

Veida wrote:Right.

The major point is that the effect on weight from the sun on a person at the surface of the Earth is that it ever so slightly decreases weight when it is highest in the sky and when it is lowest under the horizon. On most places that is roughly at midday and at midnight. This means that the sun does not make us lighter in the day compared to at night.

One correction: due to the tilt of the Earth's axis, the altitude of the Sun during the day is not symmetrical to its altitude at night, at a specific location. The same applies to the Moon.

Image

So there is a difference between the semi-diurnal tides, which depends on the latitude of the location, and the position of the Sun and Moon with respect to the equator.

Image

More info: Leading-order cause of diurnal (not semidiurnal) variations in g
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Re: Are we lighter in the daytime?

#100  Postby Veida » Jan 01, 2015 10:47 pm

Yes. Nice.

If you average the daytime maximum and the nighttime maximum over a year, however, they would be the same - right? (Ignoring variations in tilt.)

There would be no difference between the minimums at day and night at fall and spring equinox. The strength of the daytime minimum would be larger in summer and smaller in winter.
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