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

[Blue Sunray]

Newtons Law is a good approximation and allows interpretation of gravity as a force.

Really, and what are the units of this 'force'? As a hint, the normal approximation for g at the Earth's surface in 9.81 N/Kg or m/s^2 you may (or may not) be aware that the unit of force is the Newton.

 

[bedouin]

Really, and what are the units of this 'force'? As a hint, the normal approximation for g at the Earth's surface in 9.81 N/Kg or m/s^2 you may (or may not) be aware that the unit of force is the Newton.

But 'g' does not stand for gravity.

 

[Deleted member 36384]

So, who amongst his detractors and rejectors can explain the tide. That would be interesting to read. Go on, give it a go, everyone these days knows about the reality of Centrifugal force from Wikipedia. So how are tides created, expalined in a simple way.

bedouin, Elecglitch, Daydreamer - fancy explaining in manner that allows those not familiar with forces to understand?

 

[Deleted member 36384]

Really, and what are the units of this 'force'? As a hint, the normal approximation for g at the Earth's surface in 9.81 N/Kg or m/s^2 you may (or may not) be aware that the unit of force is the Newton.

But 'g' does not stand for gravity.

Very interesting, a quick google suggests that everyone calls it a force, but look beyond the quick search and it becomes obvious that no one knows wtf it really is. Fascinating stuff.

Gravity is a force which tries to pull two objects toward each other. Anything which has mass also has a gravitational pull

Gravity or gravitation is a natural phenomenon by which all things with energy are brought toward (or gravitate toward) one another .....

We often speak of gravity as a force. More accurately it is a feature of spacetime. Even more accurately, we don't know what it is.

 

[bedouin]

Very interesting, a quick google suggests that everyone calls it a force, but look beyond the quick search and it becomes obvious that no one knows wtf it really is. Fascinating stuff.

Gravity is a force, 'g' is a "constant" known as the "acceleration due to gravity" - or the force exerted by gravity per unit mass. Which I think was the original point.

It is much more esoteric to discuss whether gravity itself is a force but within the context of this discussion of a so-called scientist talking about "centrifugal force" gravity most certainly is.

 

[Deleted member 36384]

Gravity is a force, 'g' is a "constant" known as the "acceleration due to gravity" - or the force exerted by gravity per unit mass. Which I think was the original point.

It is much more esoteric to discuss whether gravity itself is a force but within the context of this discussion of a so-called scientist talking about "centrifugal force" gravity most certainly is.

Well, in that case, Cox can perhaps use the term centrifugal force. However, can you describe tides simply such that the explanation is not so dumbed down as to be meaningless, apparently folks think that Brian Cox has failed miserably?

 

[ex-Gladys]

Well, in that case, Cox can perhaps use the term centrifugal force. However, can you describe tides simply such that the explanation is not so dumbed down as to be meaningless, apparently folks think that Brian Cox has failed miserably?

Probably the standard Day Skipper is enough, but you can use that as an "approximation". For sailing purposes you need to take it further, by introducing that there are additional lower order cyclical elements, and that by the time you add up a large number of "sine wave" type, you can end up with any shape of wave form including a square wave...

 

[Daydreamer]

No - you are missing my point. It is not just about terminology it is about the genuine forces involved.

I think Cox is wrong in his explanation and his invocation of a non-existent force is hiding the error in his theory. He needs to be more explicit about what is rotating about what and how fast - there are too many different rotations involved

No point missed.

Although centrifugal force is described as fictitious the effect is real and can be experienced.

The common example is a playground roundabout, as it speeds up you will feel a force throwing you off. This is centrifugal force. If you reach the stage where you are hanging on by your arms you will feel tension in your arms. This tension is pulling you into the circle of rotation. This tension is a real force and is called the centripetal force and is equal and opposite to the fictitious centrifugal force.
The reason this force exists is because velocity is a vector quantity which has Magnitude and Direction. and because you are travelling in a circle at constant velocity the direction is constantly changing and therefore you are accelerating. Since you have mass you are generating a force according to the equation F=ma. If you let go the roundabout you will carry on in a straight line at constant velocity until another force intervenes ( Newtons first law).

In front of a different audience I expect Brian Cox would describe all this mathematically as would any competent physicist.

The real point that is being missed here is the fact that the earth and moon rotate around each other round a point below the earths surface called the barycentre which effectively means that the gravitational pull on the earths surface opposite to the moon is constantly changing and the sea reacts to this.

To add bit more, the mass of the sun also gets involved so when earth moon and sun line up in that order you get spring tides and( sun earth moon arrangements give neaps.) Incorrect, should be sun earth and moon at 90degrees to that axis for neaps.

http://clivebest.com/blog/?p=5572

This link by a so-called scientist uses quotation marks round the word centrifugal to avoid time wasters. Do read it, Its good.

 

[John the kiwi]

To go back to the root of this thread, i have found that the clearest simple explanation of tides is in the NZ Almanac.
The section is online here http://www.linz.govt.nz/sea/tides/introduction-tides/cause-and-nature-tides

Oh no - they mention centrifugal force! I would write and tell them the error of their ways but i suspect the government hydrographers actually do know what centripetal force is and choose to use this term to make the explanation simpler.
Educated people know that the term centrifugal force while incorrect technically is simply a description of the subjective effect of centripetal force. As such it does simplify some explanations. Poorly educated people will either not care or argue about this till bluie in the face...

 

[bedouin]

http://clivebest.com/blog/?p=5572

This link by a so-called scientist uses quotation marks round the word centrifugal to avoid time wasters. Do read it, Its good.

As far as I can tell - that makes sense - and contradicts what Brian Cox was saying

 

[Deleted member 36384]

As far as I can tell - that makes sense - and contradicts what Brian Cox was saying

If my understanding of clivebest is correct, the moon's gravity effect on earth is far too small to lift any water but it is sufficient to cause drag on the planet, which on a surface of liquid slows the water, opposite the moon (on both sides of earth), from rotating with the planet, thus causing it to bunch up. A bit like blowing across a saucer of water at the correct speed causes a standing wave.

 

[Daydreamer]

As far as I can tell - that makes sense - and contradicts what Brian Cox was saying

How contradicts ?

Now I have actually watched the clip in full I can see that it is simple, simplified, oversimplified and probably simplistic but what else could be done in that time in that style of programme. Your criticism seems to be based solely on the use of the word "centrifugal".
Why?

 

[bedouin]

How contradicts ?

Now I have actually watched the clip in full I can see that it is simple, simplified, oversimplified and probably simplistic but what else could be done in that time in that style of programme. Your criticism seems to be based solely on the use of the word "centrifugal".
Why?

If you listen again to Brian Cox's explanation he says that at the moonward side the gravitational force and "centrifugal" force are operating in different directions with the gravitational being stronger - but the maths shows that both forces are operating in the same direction.

BTW I am still not 100% happy that the Maths is correct - I would like someone to put the numbers in to prove it.

 

[pmagowan]

Gravity is a force, 'g' is a "constant" known as the "acceleration due to gravity" - or the force exerted by gravity per unit mass. Which I think was the original point.

It is much more esoteric to discuss whether gravity itself is a force but within the context of this discussion of a so-called scientist talking about "centrifugal force" gravity most certainly is.

I don't think so. Gravity is an apparent force, a bit like centrifugal force. We don't know enough about it to say it is a force equivalent to the other known forces. In Newtonian physics it is treated as a simple force whereas in the more accurate einsteinian physics it is simply an apparent force caused by the warping of space time.

Tides are actually quite complicated and to explain them they are generally simplified to the most important aspects. The effect of gravity and conservation of angular momentum being key determinants.

 

[JumbleDuck]

Oh dear - surely someone as clever as Brian Cox knows there is no such force as "centrifugal force".

In case you're not joking ... of course centrifugal force exists, as long as you're in a rotating reference frame. The non-existence of centrifugal force is one of the Great Misunderstandings of School Science, along with the notion that glass is a "supercooled liquid" which flows.

Edit: For what it's worth, I write as someone who taught three-dimensional kinematics at a Very Famous University for many years.

 

[JumbleDuck]

The reason the clip is brilliant is that it is simple and comprehensible to normal people. Whilst it would be more correct to refer to the reaction to centrepetal acceleration required to maintain a fixed radius from the centre of a rotating object I suspect this would not be nearly as comprehensible to most people.

Centripetal force is the inwards force required to keep something on a circular path when seen from an external and stationary reference frame. Centrifugal force is the outward force observed if the reference frame is rotating. They always appear together: action and reaction and all that.

 

[JumbleDuck]

But it's all tosh. There's no gravity: the Earth sucks.
But then there's really no such thing a suction...:ambivalence:

I worked in cryogenics for a few years and we used to refer to "coolth", as in "Better insulate that better or the coolth will escape". It drove the purists wild, as it was intended to to do.

 

[JumbleDuck]

The common example is a playground roundabout, as it speeds up you will feel a force throwing you off. This is centrifugal force. If you reach the stage where you are hanging on by your arms you will feel tension in your arms. This tension is pulling you into the circle of rotation. This tension is a real force and is called the centripetal force and is equal and opposite to the fictitious centrifugal force.
The reason this force exists is because velocity is a vector quantity which has Magnitude and Direction. ...

I agree, almost. Tension is not a scalar, because it has direction, and it is not a vector, because it has more than one direction. A rope with a tension of 10kN in it has equal forces (10kN) in opposite directions at each end. That's why stresses are alway specified with a face and a direction: sigma_xx is the force per unit area in the x-direction on a face whose outward normal is in the x-direction, tau_yz is the force per unit area in the y-direction on a face whose outward normal is in the z-direction and so on. Mathematical stress analysis calls them all sigma; engineers use tau for shear stresses.

But I digress. The arm of an operating centrifuge is under tension. At the outer end that tension is a centripetal force and pulls the bucket or whatever in. At the inner end it is a centrifugal force and pulls the arm out.

There are plenty of other examples of forces which depend on the axis system you are using. For example, if you are designing a car seat to make bumpy roads comfortable, you can use an axis system fixed to the road, in which case you see the car trying to force the passengers up when it hits a bump, or you can use an axis system fixed to the car in which case you see the passengers trying to force the car down. Either system can be used and the skill of the kinematicist is to choose the one which makes the solution, which will be the same, easiest to find.

 

[jonjo]

This is the best thread on the subject ever. Over the years I have been surprised by the number of people willing to accept a simplistic diagram depicting the moons gravity simultaneously creating a pull tidal bulge on the nearside of the earth while somehow magically pushing the water away against gravity on the opposite side.

Well done Brian Cox, for the first time I get it.

One remaining oddity troubles me. Given two different forces creating the two bulges is it a mathematical fluke that our two daily high waters are nearly the same?

 

[pmagowan]

Try this: