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Post by Progenitor A on Sept 19, 2012 8:25:43 GMT 1
 Here is food for thought In A a weight is suspended from a spring balance. It weighs 10 kg However the weight is the force exerted upon a mass by gravity, and if we have a force opposing the gravitational force then the weight is reduced (isn't it?) So here's a puzzle. The force of gravity in A is opposed by the pulling force of the spring balance - they are equal and opposite. So the mass of 10kg has no weight. Does it? You can test this as you could easily move the 10kg mass up with your little finger! So we are measuring a weight of 10kg that has no weight! In B we pull down on the 10kg weight with a force that apparently increases the measured weight. In C we let go of the weight and the spring exerts its upward force and the weight ascends upward But it overshoots the 10kg weight line and reaches , say, the 2kg weight line, so that its apparent weight is 2kg. Gravity then asserts itself (it has been doing so all the time of course) and pulls the weight back down, and again the weight overshoots the 10kg line and registers a weight of say 12kg. This process continues - the spring oscillates until it reaches a steady state where it stops oscillating and our mass weighs 10kg once more (except that it is weightless!) Now, when the spring is at its top point (where our mass 'weighs' 2kg) the mass changes direction and starts to come back downward Classical mechanics say that at this turning point the mass is weightless. (You can try it in an aeroplane) But the spring does not register weightlessness - it measure 2kg! So what is the 'weight' at the spring top point? Is it 0kg, 2kg or 10kg? |
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Post by striker16 on Sept 19, 2012 10:26:10 GMT 1
Well, all this is covered by Newton's laws which says that a body under the influence of a force experiences *inertia*, i.e. it wants to remain in the same state until some other force changes that state. In your examples, we have a) gravity and b), the attractive force of the spring acting together so that the mass is not totally under the influence of either force. To say the mass becomes weightless is not correct in the strict sense because it is still attracted by gravity. A truly weightless object would not be subject to gravitational force and, therefore, be massless. An airplane in flight is not actually weightless but is able to defy gravity by using its engines and aerodynamic design to enable it to produce counter-forces which oppose gravity, allowing it to fly. A helicopter is a good example of the use of angular momentum in the way it maneuvers by applying torque to its rotors.
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Post by buckleymanor1 on Sept 28, 2012 0:46:55 GMT 1
Not strictly true, photons are massless yet are still subject to gravitational forces.
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Post by mrsonde on Sept 28, 2012 1:59:04 GMT 1
Food for thought indeed. 2 kg, I'd say. I still don't really buy your "weightless" theory at its turning point. Though if the spring ceased its action when it was at 0 kg, I think I might!
That isn't "the strict sense" of "weight".
That is.
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Post by Progenitor A on Sept 28, 2012 12:39:58 GMT 1
Food for thought indeed. 2 kg, I'd say. I still don't really buy your "weightless" theory at its turning point. Though if the spring ceased its action when it was at 0 kg, I think I might! Well in engineering the weight of an object is defined as the gravitational Force in Newtons acting upon a mass. In that strict sense, the weight in a gravitational field is never 'weightless', but conversationally we use weightless to mean the [apparent?]absence of 'heaviness' (I know, how do we define 'heavy'). This is weight as a subjective phenomenon. Thus when an aeroplane reaches the top of a parabolic climb the passengers experience 'weightlessnes'. Similarly if someone assists you to lift a load so that he takes all of the load we speak of the load being 'weightless' |
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Post by mrsonde on Sept 28, 2012 13:09:12 GMT 1
Food for thought indeed. 2 kg, I'd say. I still don't really buy your "weightless" theory at its turning point. Though if the spring ceased its action when it was at 0 kg, I think I might! Well in engineering the weight of an object is defined as the gravitational Force in Newtons acting upon a mass. Quite so. The gravtitational force acting in Newtons is no different from an inertial acceleration in the opposite (or any other) direction though, is it. So, as you say, if they're equal and opposite, the mass will be weightless - there will be no gravitational force "acting" at all. Ermmmm...I'd say I think, this weightlessness is experienced immediately after, and only if the aeroplane then enters freefall. That is, when there is no inertial resistance to gravity - that's "weight" in its "strict sense", as far as I know. Yes, accurately. Did you do that illustration yourself, Nay? I'm impressed. |
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Post by Progenitor A on Sept 28, 2012 13:31:10 GMT 1
The gravtitational force acting in Newtons is no different from an inertial acceleration in the opposite (or any other) direction though, is it. So, as you say, if they're equal and opposite, the mass will be weightless - there will be no gravitational force "acting" at all. Is this quite correct Nick? There willl certainly be no resultant force acting in the direction of the gravitational field, but there is always the gravitational force actingh upon the weight but it is neutralised be an equal and opposite force. . Did you do that illustration yourself, Nay? I'm impressed. Yes, just 5 minutes work with CoralDraw - I used it all the time in my telecoms consultancy role |
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Post by mrsonde on Sept 28, 2012 17:04:39 GMT 1
The gravtitational force acting in Newtons is no different from an inertial acceleration in the opposite (or any other) direction though, is it. So, as you say, if they're equal and opposite, the mass will be weightless - there will be no gravitational force "acting" at all. No - I spelt gravatitional wrong. Yeah - so no resultant force, as you say. From a Relativistic point of view - you'd agree that this is the "correct" one, as near as we know so far anyway, rather than Newton's conceptual framework? - the mass is simply moving in the "straightest" - least effortful - line in the space-time metric. Right? There's no difference between the "force of gravity" and any other acceleration is subjected to, is there? They're both merely distortions of that metric. I'm not trying to complicate matters any further, believe me! It's just I do believe this is the right way to look at motion and inertia and "forces" - it's certainly helpful, I find. Well - good for you. I'm a complete and utter numbskull where that sort of artistic milarkey is concerned. |
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Post by mrsonde on Sept 28, 2012 17:05:38 GMT 1
See? Don't know what happened with the boxes there!
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Post by mrsonde on Sept 29, 2012 17:48:46 GMT 1
Here's more food for thought along the same lines then. Bearing in mind this relationship between inertia, motion, and gravitation.
Why do comets have tails? (bear in mind that Venus, and probably all the planets, also have cometary tails.)
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Post by Progenitor A on Sept 30, 2012 9:28:15 GMT 1
Here's more food for thought along the same lines then. Bearing in mind this relationship between inertia, motion, and gravitation. Why do comets have tails? (bear in mind that Venus, and probably all the planets, also have cometary tails.) Well my first guess is the solar wind. This stream of partcles emitted from the sun exerts a force on any mass that it meets. If that mass is composed of a loose aglommeration of particles, then the solar wind will cause the lighter particles to be accelerated more than the larger particles, causing a 'tail' to develop. This might be supported by the fact that the 'tail' is sometimes ahead of the main body of mass, dependant upon its orientation wrt the sun |
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Post by mrsonde on Sept 30, 2012 18:27:20 GMT 1
Well my first guess is the solar wind. This stream of partcles emitted from the sun exerts a force on any mass that it meets. If that mass is composed of a loose aglommeration of particles, then the solar wind will cause the lighter particles to be accelerated more than the larger particles, causing a 'tail' to develop. This might be supported by the fact that the 'tail' is sometimes ahead of the main body of mass, dependant upon its orientation wrt the sun Correct. So the comet tail has been given a momentum in the opposite direction to the gravitational. Maybe that's what Striker means by his "displacement of mass"? It's not as simple as it seems in this case though. For a start, if this was all there was to it, the tail of a comet should be curvilinear, shaped rather like the arm of a spiral galaxy, as the particles given a momentum away from the sun over time lag further and further behind the course of the comet on its tangential orbit. Right? This isn't what we see. Secondly, the shape of that tail would be, more or less, a teardrop shape, an aerofoil if you like, stretching away from the head, in the main body's umbra, or like a jet's contrails, or the twin streams around a rigid body in a wind tunnel. This isn't what we see either. They're a chaotically bulging twisting filamented mess, more like the blotchiness you get when you knock a bottle of wine over, on your mother's favourite white woolen hearthrug. I remember it well. Thirdly, and most intriguing of all - the "wind" part is a somewhat misleading metaphor. Unless you're talking about a wind you experience starting at the eye of a hurricane, moving further and further out to its fastest spinning arms. The particles in the solar wind accelerate away from the Sun. By the time they interact with our magnetosphere - the aurorae, say - they can be moving at many thousands times the velocity they left with. There's no way to predict what this acceleration will be either. Sometimes a coronal emission or flare will take over a week to get here - sometimes only three days. Sometimes to everyone's complete bafflement the effect seems close to instantaneous, with the charged particles apparently being accelerated close to relativistic velocities. Where's the "force" - ma - acting here? Another interesting question, relating to Striker's puzzlement about how Laithwaite could possibly have anything interesting to say about Newtonian mechanics if the rest of the Physics community disagreed with him, or as in this case had never noticed what he was pointing out. Before Eugene Parker gave a good scientific description of this hypothetical "solar wind", which until then no physicist or astroscientist gave any credence to whatever - as recently as the end of the 1950s (apologies to Eddington, and Birkeland, and a few other "cranks") - how was the characteristic behaviour and appearance of comets explained? No account could possibly fit with Newtonian mechanics, could it? Well, they didn't explain it - they just ignored it. That's what happens in "normal" science. |
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Post by mrsonde on Sept 30, 2012 19:54:32 GMT 1
And: why do we "see" anything at all? The current consensus is that comets are merely remnants from the solar system consolidation leftovers from the collapse of dust that made up the planets, deriving from a hypothetical ring of dust called the Oort Cloud. They orbit the Sun at wider distances than planets and asteroids, that's all - that's the difference. Sometimes their orbits wander into the plaentary "solar system". So why the pyrotechnic display? We see Mars clearly enough, and Jupiter; but not the billions of asteroids between. Yet comets are usually much smaller masses than thee asteroids. Halley's for example is tiny compared to Ceres or Vesta - detectable through a powerful telescope, sure. But no blazing light or tail. You know what Newton has to say about this? Nothing.
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