AT last a clear explanation of how we get 2 tides per day !

[pmagowan]

I just watched this on iplayer and in fact he states that 'fictitious' forces are involved so he has made it clear he is not talking about fundamental forces.

 

[JumbleDuck]

I just watched this on iplayer and in fact he states that 'fictitious' forces are involved so he has made it clear he is not talking about fundamental forces.

I think the problem is that people don't realise that "fictitious" has a specific meaning in this context which is not the same as "unreal". A little knowledge, and all that.

 

[VicS]

I think the problem is that people don't realise that "fictitious" has a specific meaning which is not the same as "unreal". A little knowledge, and all that.

There are 6 meanings in the OED ! q.v.

 

[grumpy_o_g]

I think it's a crappy explanation, if not bollocks. The inverse square law is enough to explain the tides.

You could argue that the two explanations are equivalent. If gravity is the centripetal force and diminishes as the square of the distance, then it's less on the far side of the moon. If you are willing to regard diminution of centripetal force as an increase in (non-existent) centrifugal force then you might give some credit to his half-assed explanation.

However, why would you invoke two concepts to explain something that is simple to explain with one? Bad science.

I think you meant the far side of the Earth (from the Moon)? The main problem I have with his explanation is that the primary reason is added after the secondary reason and he doesn't actually quantify the effects of either.

 

[grumpy_o_g]

If interested see the link below

http://www.bbc.co.uk/programmes/p04...ture&intc_linkname=vidclip_tides_contentcard7

I still prefer this explanation for simplicity.

 

[grumpy_o_g]

I think it's a crappy explanation, if not bollocks. The inverse square law is enough to explain the tides.

You could argue that the two explanations are equivalent. If gravity is the centripetal force and diminishes as the square of the distance, then it's less on the far side of the moon. If you are willing to regard diminution of centripetal force as an increase in (non-existent) centrifugal force then you might give some credit to his half-assed explanation.

However, why would you invoke two concepts to explain something that is simple to explain with one? Bad science.

You are (obviously IMO only) absolutely correct that the centrifugal force part of his explanation is bollocks but not because this doesn't cause the local sea level to vary. Tides are a very specific and clearly defined phenomenon and are due solely to the distance of one part of a body of water (or anything else) from a significant mass to another part of the body water. Other things may cause the water level to vary and gravity may be key component of the mechanism that cause the water level to vary but they won't be tides unless it's due to the difference in distance from another mass (and therefore gravitational force).

To put it very simplistically tides are caused by the fact that Pacific Ocean is about 240,000 miles from the moon at one point in time and 12 hours later it's only about 236,000 miles. The gravitational force of the moon acting on the Pacific Ocean is calculated by

and that, to all intents and purposes, G, m1 and m2 are constant. Changing r, even by such a small amount, changes the force acting upon the Pacific Ocean - basically it's pulled up towards the Moon less when it's on the opposite side of the Earth to the Moon.

 

[guernseyman]

While most explanations of the tides concentrate upon the the regions of the earth closest to the moon and farthest from the moon, what is actually going on is that the sea from the poles around to those regions are subjected to a nett force pulling them towards those regions. Thus there is a piling up of water there.
At the side of the earth closer to the moon the moon's gravitational effect is larger than the opposing centrifugal force so that the resultantant force is towards the moon, while at the other side of the earth the centrifugal force is larger than the moon's gravitational force so that the resultant force is away from the moon.

 

[paulburn]

I still prefer this explanation for simplicity.

I still don't understand why the water bulges out on the far side of the earth. I don't get their explanation of vector forces causing this - what vector forces are these ? If gravity from the moon and the centre of the earth, then they would act to minimise a bulge ??

 

[bedouin]

At the side of the earth closer to the moon the moon's gravitational effect is larger than the opposing centrifugal force so that the resultantant force is towards the moon, while at the other side of the earth the centrifugal force is larger than the moon's gravitational force so that the resultant force is away from the moon.

That is what Brian Cox appears to be saying - but is it wrong and that is why you have to treat all his explanation with a lot of caution.

Do the maths - you will see on the side of the earth closest to the moon the "centrifugal" force and the moon's gravitational attraction are operating in the same direction - so any explanation the relies on them working in opposite directions is wrong.

 

[macd]

If gravity from the moon and the centre of the earth, then they would act to minimise a bulge ??

But surely they do act to minimise the bulge (as opposed to removing it altogether). If they didn't exist, the oceans would be in space. (Of course they wouldn't, since the planets wouldn't have formed in the first place. And we wouldn't be having this conversation.)

 

[paulburn]

The Nottingham Uni video seem to be saying that gravity vector forces alone cause the water bulge on the far side of the earth and I just don't understand how ?!

 

[JumbleDuck]

Do the maths - you will see on the side of the earth closest to the moon the "centrifugal" force and the moon's gravitational attraction are operating in the same direction - so any explanation the relies on them working in opposite directions is wrong.

It's the gravitational attraction of the earth-moon system which matters, and that is in the opposite direction to the centrifugal force, since the earth-moon centre of gravity - the barycentre - is inside the earth.

 

[bedouin]

It's the gravitational attraction of the earth-moon system which matters, and that is in the opposite direction to the centrifugal force, since the earth-moon centre of gravity - the barycentre - is inside the earth.

That is true - but you can look at gravity either of the system as a whole or as the individual components and sum the effects. Since the earth itself exerts pretty much an equal gravitational pull on all the water it is perhaps easier just to look at the impact of the moon on its own as that is the variable quantity - but I am sure if you do the maths you get the same answer both ways.

The so called "centrifugal force" always acts in the opposite direction to the gravity. Where Brian Cox is wrong is in asserting that in one case they act together and the other they act opposed.

That really is the problem with centrifugal force - it doesn't exist and people tend to invoke the concept when they are too lazy to resolve the forces involved correctly.

 

[guernseyman]

Do the maths - you will see on the side of the earth closest to the moon the "centrifugal" force and the moon's gravitational attraction are operating in the same direction - so any explanation the relies on them working in opposite directions is wrong.

I disagree, the two forces are always in opposition.
I wonder if you are confusing the centrifugal force from the Earth's rotation with that due to the revolution about the common centre of gravity of the Earth and Moon. The former plays no part in tidal activity.

 

[bedouin]

I disagree, the two forces are always in opposition.
I wonder if you are confusing the centrifugal force from the Earth's rotation with that due to the revolution about the common centre of gravity of the Earth and Moon. The former plays no part in tidal activity.

See #53 if yo consider the system as a whole the forces have to be in opposition because all centrifugal is is a reaction to the affect of the gravitational pull.

But Brian Cox appears to be excluding the gravitational force of the earth (also reasonable as it is effectively constant) and then saying that on one side the forces are working in the same direction and on the other in opposition. Whichever way you choose to look at it Brian Cox is wrong.

 

[guernseyman]

See #53 if yo consider the system as a whole the forces have to be in opposition because all centrifugal is is a reaction to the affect of the gravitational pull.

But Brian Cox appears to be excluding the gravitational force of the earth (also reasonable as it is effectively constant) and then saying that on one side the forces are working in the same direction and on the other in opposition. Whichever way you choose to look at it Brian Cox is wrong.

I'm not defending Brian Cox, just trying to get it right.

 

[JumbleDuck]

The so called "centrifugal force" always acts in the opposite direction to the gravity. Where Brian Cox is wrong is in asserting that in one case they act together and the other they act opposed.

As you say, it's variation from the uniform (ish) gravity of earth which matters, and that does change direction: under the moon the variation is upwards and opposite the moon the variation is downwards.

That really is the problem with centrifugal force - it doesn't exist and people tend to invoke the concept when they are too lazy to resolve the forces involved correctly.

It most certainly does exist, except for people who are incapable of resolving forces in rotating axis frames.

 

[Deleted member 36384]

Can someone give a numpty friendly explanation that explains why the barrycentre and gravity cause two bulges without using the term centrifugal force. Then, if the we had a second moon would we have four bulges or three or still just two.

 

[guernseyman]

Can someone give a numpty friendly explanation that explains why the barrycentre and gravity cause two bulges without using the term centrifugal force. Then, if the we had a second moon would we have four bulges or three or still just two.

When your car goes round a corner your upper torso, obeying Newton's first law of motion, wants to continue to move in a straight line, and so it moves outwards relative to the car.

In the same way, as the earth revolves around the barycentre, the water on the far side from the moon moves outwards, the moon's gravitational attraction not being strong enough to prevent it.

On the other side of the earth, the moon's gravitational attraction overcomes the tendency of the water to move outwards.

So you have a bulge on both sides of the earth.

The case of a second moon is complicated by the fact that if it was big enough to have a significant tidal effect, the three body system would be chaotic and so the tides would be chaotic as well.

 

[Deleted member 36384]

When your car goes round a corner your upper torso, obeying Newton's first law of motion, wants to continue to move in a straight line, and so it moves outwards relative to the car.

In the same way, as the earth revolves around the barycentre, the water on the far side from the moon moves outwards, the moon's gravitational attraction not being strong enough to prevent it.

On the other side of the earth, the moon's gravitational attraction overcomes the tendency of the water to move outwards.

So you have a bulge on both sides of the earth. ....

Thanks, the dots have all joined up now, that is a good explanation guernseyman.