Are we lighter in the daytime?

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Re: Are we lighter in the daytime?

#61  Postby Veida » Jan 01, 2015 2:17 pm

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.
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Re: Are we lighter in the daytime?

#62  Postby DavidMcC » Jan 01, 2015 2:25 pm

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.
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Re: Are we lighter in the daytime?

#63  Postby Veida » Jan 01, 2015 2:32 pm

That's completely irrelevant.
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Re: Are we lighter in the daytime?

#64  Postby DavidMcC » Jan 01, 2015 2:36 pm

Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.
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Re: Are we lighter in the daytime?

#65  Postby DavidMcC » Jan 01, 2015 2:40 pm

If you accept that "weight" = "contact force with the earth" when in steady contact with it, then you must include dynamic effects on it.
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Re: Are we lighter in the daytime?

#66  Postby Thommo » Jan 01, 2015 3:17 pm

DavidMcC wrote:
Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.


The sun's pull also acts on the Earth.

The part remaining is the tidal force due to the gradient (the difference between the force at a distance from the sun of the Earth's surface and the the force at a distance from the sun of the Earth's centre of mass), which in the case of the sun is miniscule as the Earth's radius is << compared to the Earth's distance from the sun. The amount you are lighter in the day due to this tidal force is nowhere near even the small amount of the sun's gravitational force on an object at a distance of the Earth from the sun which was calculated by Klazmon on p1.

This is what Veida has correctly been saying all along. The sun's gravitation can be ignored for objects in free fall, which to a very close approximation both you and the Earth are.
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Re: Are we lighter in the daytime?

#67  Postby DavidMcC » Jan 01, 2015 3:21 pm

Thommo wrote:
DavidMcC wrote:
Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.


The sun's pull also acts on the Earth.

...

You don't say! :shock:
You are as confused as Veida is, it seems.
If we were in free fall around the earth, you might have a point.
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Re: Are we lighter in the daytime?

#68  Postby DavidMcC » Jan 01, 2015 3:30 pm

Perhaps I need to repeat that your weight is the contact force between you and the earth, when you are stationary on the earth. This contract force is affected by the earth's rotation, but not in such a way that there is a day-night difference, only a latitude difference
Maybe that makes this "irrelevant" to the rather narow issue of the OP, but relevant to the broader issue of weight.
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Re: Are we lighter in the daytime?

#69  Postby Veida » Jan 01, 2015 3:49 pm

DavidMcC wrote:If you accept that "weight" = "contact force with the earth" when in steady contact with it, then you must include dynamic effects on it.

Even if I did, the rotation of the Earth around its own axis does not produce any dynamic effect on weight (as you define it). It's irrelevant to the question at hand.
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Re: Are we lighter in the daytime?

#70  Postby Thommo » Jan 01, 2015 3:50 pm

DavidMcC wrote:You don't say! :shock:
You are as confused as Veida is, it seems.
If we were in free fall around the earth, you might have a point.


Sure David.
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.


:tehe:

Only you could think that calling me "as confused" as the most lucid poster in the thread is a meaningful retort.

If the Earth did not spin, the sun would not increase our night time weight for the reason Veida gave.

The "centrifugal force" does decrease your weight everywhere except at the poles, by varying amounts, but it is a completely separate effect and does not affect whether the night time tidal force of the sun decreases your weight.
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Re: Are we lighter in the daytime?

#71  Postby Thommo » Jan 01, 2015 3:51 pm

Veida wrote:
DavidMcC wrote:If you accept that "weight" = "contact force with the earth" when in steady contact with it, then you must include dynamic effects on it.

Even if I did, the rotation of the Earth around its own axis does not produce any dynamic effect on weight (as you define it). It's irrelevant to the question at hand.


:this:

Rotation only matters if you're moving on the surface of the Earth. Even then it's still unconnected with tidal force.
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Re: Are we lighter in the daytime?

#72  Postby DavidMcC » Jan 01, 2015 4:02 pm

Thommo wrote:...

Rotation only matters if you're moving on the surface of the Earth.
Nonsense. Learn some Newtonian physics.
Even then it's still unconnected with tidal force.

Not directly, but it is still rel;evant to weight. I leave it to jamest to decide if he regards it as relevant to the broader topic of weight than the very specific one he raised.
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Re: Are we lighter in the daytime?

#73  Postby Veida » Jan 01, 2015 4:03 pm

DavidMcC wrote:
Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.

No. This is simply not correct.

I don't know where you went wrong. If you couid show the math leading to this conclusion perhaps we can sort it out for you?
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Re: Are we lighter in the daytime?

#74  Postby Thommo » Jan 01, 2015 4:07 pm

DavidMcC wrote:
Thommo wrote:...

Rotation only matters if you're moving on the surface of the Earth.
Nonsense. Learn some Newtonian physics.
Wow, that's a quality quotemine. :lol:

Rotation only matters as a dynamic effect on weight if you're moving on the surface of the Earth. This is why that text appears under "this" referring to Veida saying exactly that. Please exercise some primary school level reading comprehension.
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Re: Are we lighter in the daytime?

#75  Postby Thommo » Jan 01, 2015 4:07 pm

Veida wrote:
DavidMcC wrote:
Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.

No. This is simply not correct.

I don't know where you went wrong. If you couid show the math leading to this conclusion perhaps we can sort it out for you?


:this:

Good suggestion, let's see the maths.
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Re: Are we lighter in the daytime?

#76  Postby DavidMcC » Jan 01, 2015 4:09 pm

Veida wrote:
DavidMcC wrote:
Veida wrote:That's completely irrelevant.

No, it isn't irrelevant, because without it, we would, indeed, be lighter in the daytime, with the sun's pull subtracting from the earth's.

No. This is simply not correct.

I don't know where you went wrong. If you couid show the math leading to this conclusion perhaps we can sort it out for you?

You will not be able to sort it out for me, because I am not wrong. All you need to realise is that the force of gravity is a vector, and that we are not usually in free-fall around the earth (in which case we would be weightless).

EDIT: Oh, and you also need to realise that there is such a thing as centrifugal force - the apparent force generated by changing to a rotating frame of reference.
Last edited by DavidMcC on Jan 01, 2015 4:17 pm, edited 1 time in total.
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Re: Are we lighter in the daytime?

#77  Postby Veida » Jan 01, 2015 4:17 pm

DavidMcC wrote:
Veida wrote:I don't know where you went wrong. If you couid show the math leading to this conclusion perhaps we can sort it out for you?

You will not be able to sort it out for me, because I am not wrong. All you need to realise is that the force of gravity is a vector, and that we are not usually in free-fall around the earth (in which case we would be weightless).

Yes, you are wrong.

I'm afraid I can't help you figure out where you went wrong unless you explain yourself a bit more.

Do you accept that the Earth and everything on it is in free fall in relation to the sun?
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Re: Are we lighter in the daytime?

#78  Postby DavidMcC » Jan 01, 2015 4:21 pm

Veida wrote:
DavidMcC wrote:
Veida wrote:I don't know where you went wrong. If you couid show the math leading to this conclusion perhaps we can sort it out for you?

You will not be able to sort it out for me, because I am not wrong. All you need to realise is that the force of gravity is a vector, and that we are not usually in free-fall around the earth (in which case we would be weightless).

Yes, you are wrong.

I'm afraid I can't help you figure out where you went wrong unless you explain yourself a bit more.

Do you accept that the Earth and everything on it is in free fall in relation to the sun?

The earth is certainly in free fall around the sun, but dealing with everything ON IT, requires a more complicated model than you seem to realise, because of the finite size of the earth, and its rotation about its own axis.
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Re: Are we lighter in the daytime?

#79  Postby Veida » Jan 01, 2015 4:29 pm

You have no idea what I realize or not.

Next question: do you think that the discus is in free fall with regards to the discus trower, before he lets go of the discus?
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Re: Are we lighter in the daytime?

#80  Postby DavidMcC » Jan 01, 2015 4:34 pm

Veida wrote:You have no idea what I realize or not.

I was trying to infer what you realize from what you said. It seemed a reasonable inference.
Next question: do you think that the discus is in free fall with regards to the discus trower, before he lets go of the discus?

Of course not. It isn't in a gravitational orbit around him! Likewise, we are not in free fall around the earth, or we would be weightless.
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