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Post by mrsonde on Sept 10, 2012 17:21:44 GMT 1
Hi Nick Yes I was an engineer and no, I do not have a clue about this phenomenon that Laithwaite demonstrated That there is an 'anti-gravity' force at work is undoubted - pity he called it that as it invokes mad images of science fiction and Dan Dare. But an ant-gravity force is simply some force that acts against gravity, and we are all familiar with such forces Hi Nay. I'm not aware that Laithwaite did call it an anti-gravity force. He didn't in his fascinating book, Engineer Through the Looking Glass, anyway. I agree. I'm still eagerly awaiting Mak's calculation.
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Post by mrsonde on Sept 10, 2012 17:36:15 GMT 1
Well, no, nor me - sort of. "Weight" is a phenomenal quantity, of course. That's why I've been careful to put "apparent" change in weight, on the whole. But as Nay says, there's a force acting against the gravitational attraction - we don't have to call this anti-gravity, but it is reducing the experienced weight of the wheel. I could lift 40 lb above my head with the ease of a ballerina, if the cameras were on me - but I don't think Prince Charles could; not without his trousers falling down. Hmmm. I think that's like Marchesa, a misdescription of what's going on. The two forces aren't opposing. To calculate the ballistic path of the stone, or a discus, you merely add the two vectors together. What I mean is that in the angular momentum equations, derived from newton's Laws of Motion, there is no means at all to calculate this apparent loss of weight. Where does this phenomenon come from, why? It's not in the equations - no term relates to it. No means to predict or calculate it. Therefore, it's not like the dicus or the stone or the missile at all. My feeling is you're on the right lines - it's something to do with torque. But I don't understand why. Not that I matter - no physicist was able to persuade Laithwaite that his failure to understand why was due to ignorance, either: and in that case, I suspect that was because no one understands why. But I'm eager to be corrected, and enlightened. Incidentally, Laithwaite did something quite entertaining with this principle he'd demonstrated. He built a rowing boat and propelled himself across the Thames, using roughly the same spinning wheels on the end of his paddles. Not by dipping them into or touching the water, but by merely moving them up and down in the rollocks - the resulting torque was then translated into forward motion.
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Post by striker16 on Sept 10, 2012 17:46:30 GMT 1
All Laithwaite had to contend with in his demonstration was the precession of the wheel, that is, its axial rotation, which was relatively easy to alter.
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Post by Progenitor A on Sept 10, 2012 18:19:44 GMT 1
A stone thrown does not lose weight .... I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects)
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Post by striker16 on Sept 11, 2012 9:12:40 GMT 1
A stone thrown does not lose weight .... I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects) Weight is a measure of how strongly gravity pulls downwards on something due to its mass, so does not change, but if another force, such as angular momentum, is applied, then this will oppose gravity's pull. An object does not intrinsically become less massive and, therefore, lighter just because it is moving, so as soon as no other forces are being applied to it retains the same weight (on earth). You could argue, by your reckoning, that when we walk we lose weight, since our forward momentum is opposing the pull of gravity, which would be a bit silly. Does an aeroplane lose weight because it can fly?
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Post by Progenitor A on Sept 11, 2012 9:41:31 GMT 1
I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects) Weight is a measure of how strongly gravity pulls downwards on something due to its mass, so does not change, but if another force, such as angular momentum, is applied, then this will oppose gravity's pull. An object does not intrinsically become less massive and, therefore, lighter just because it is moving, so as soon as no other forces are being applied to it retains the same weight (on earth). You could argue, by your reckoning, that when we walk we lose weight, since our forward momentum is opposing the pull of gravity, which would be a bit silly. Does an aeroplane lose weight because it can fly? Hmm.. a couple of confused sentences. Weight is the effect (aceleration) of gravity on a mass. If the gravitational force changes the effect (acceleration) on a mass changes and the weight changes Any other force that changes the accelerating effect of gravity causes the weight to change. Weightlessness in an aeroplane at the top of a parabolic climbing curve is not a pretend effect it is very real - when the acceleration due to an external force is equal and opposite to the gravitational force, the weight of a mass is zero. Its mass remains constant Try this simple experiment Try picking up a hundredweight sack of potatoes. (muesli will do if you are dieting) Now strap a little rocket (not an ICBM- although if that is all you have it will just have to do) undeneath it and when it is thrusting upward, try lifting the potatoes again. Does the sack weigh the same to you? Would it still weigh one hundrdweigt if it was suspended from a spring weighing machine?
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Post by striker16 on Sept 11, 2012 10:49:02 GMT 1
Weight is a measure of how strongly gravity pulls downwards on something due to its mass, so does not change, but if another force, such as angular momentum, is applied, then this will oppose gravity's pull. An object does not intrinsically become less massive and, therefore, lighter just because it is moving, so as soon as no other forces are being applied to it retains the same weight (on earth). You could argue, by your reckoning, that when we walk we lose weight, since our forward momentum is opposing the pull of gravity, which would be a bit silly. Does an aeroplane lose weight because it can fly? Hmm.. a couple of confused sentences. Weight is the effect (aceleration) of gravity on a mass. If the gravitational force changes the effect (acceleration) on a mass changes and the weight changes Any other force that changes the accelerating effect of gravity causes the weight to change. Weightlessness in an aeroplane at the top of a parabolic climbing curve is not a pretend effect it is very real - when the acceleration due to an external force is equal and opposite to the gravitational force, the weight of a mass is zero. Its mass remains constant Try this simple experiment Try picking up a hundredweight sack of potatoes. (muesli will do if you are dieting) Now strap a little rocket (not an ICBM- although if that is all you have it will just have to do) undeneath it and when it is thrusting upward, try lifting the potatoes again. Does the sack weigh the same to you? Would it still weigh one hundrdweigt if it was suspended from a spring weighing machine? I think the problem here is how you are defining weight. In the strict scientific sense, weight is that force the earth exerts on a mass, so, the more mass the heavier it will be. This will be different on the moon or in space, of course. Although a rock travelling through the air, for example, in a sense is lighter than a rock sitting on the ground, it is not intrinsically lighter in terms of the relationship of its mass to the pull of Earth's gravity in the absence of any other force. In the absence of any other force is the key point here because this is one of the Laws of Motion Newton was able to identify and isolate. If we start defining how light something is in relation to other parameters it would lead to confusion, therefore, we have to have basic laws about physical objects on which to base our calculations. It's a bit like maths, where we define some basic axioms, or rules, and apply these to various mathematical theorems.
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Post by marchesarosa on Sept 11, 2012 13:12:09 GMT 1
Everything is "relative" isn't it?
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Post by striker16 on Sept 11, 2012 14:28:20 GMT 1
Everything is "relative" isn't it? Of course, but we have to have clear definitions about how physical objects behave that everyone agrees with, otherwise it would just lead to confusion. It would be like using different concepts for the same definition. To take a familiar example, weight and mass are often used interchangeably, however, these are not the same because weight refers to the pull of the earth, due to gravity, on a given mass, but the same mass would not be the same weight on the moon or in free space.
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Post by mrsonde on Sept 11, 2012 19:45:46 GMT 1
All Laithwaite had to contend with in his demonstration was the precession of the wheel, that is, its axial rotation, which was relatively easy to alter. Umm...I don't follow what you're saying here, sorry. If you have to lift a 40lb weight above your head, surely you're contending with the gravitational attraction exerted on that mass? The issue is: why is it easier to do so when that mass is spinning?
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Post by mrsonde on Sept 11, 2012 19:47:54 GMT 1
A stone thrown does not lose weight .... I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects) I would contest your contestation, sir! At what point is "the turning point" of a parabola?
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Post by mrsonde on Sept 11, 2012 19:53:19 GMT 1
I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects) Weight is a measure of how strongly gravity pulls downwards on something due to its mass, so does not change, but if another force, such as angular momentum, is applied, then this will oppose gravity's pull. Angular momentum is not a force. Not as classically understood, anyway. It can't oppose anything. Neither is torque, strictly speaking. Quite so. So why is it easier to lift a weight when it's spinning?
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Post by mrsonde on Sept 11, 2012 20:02:03 GMT 1
"Weight" is not a force either. It's the phenomenal resistance of mass to being moved from its path (determined by the forces it's influenced by: primarily, gravitation.) That is, we experience it when we try to deflect a mass from its response to gravitation: we measure its inertia. Nay's case of a free-falling object being weightless illustrates this. But how is Laithwaite's wheel in free-fall when he's lifting it into the air?
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Post by Progenitor A on Sept 11, 2012 20:10:05 GMT 1
I would contest this statement. Its weight does indeed vary as it travels the parabola of its trajectory, falling to zero weight at the turning point of that parabola What does not vary is its mass (ignoring relativistic effects) I would contest your contestation, sir! At what point is "the turning point" of a parabola? Long time since I have done this stuff, but ignoring drag air resistance, when a stone is thrown up and forward against gravity it will follow an arc that approximates to a parabola. The 'turning point' of this parabola is when dh/dt = 0, where h= height of the stone, t= time. In plain English, the 'turning point' is where it is neither going up nor going down and that is the point of zero weight.
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Post by marchesarosa on Sept 11, 2012 20:54:42 GMT 1
Because some energy has been imparted to the object by an electric drill! Another form of WORK is happening that is reducing the work which otherwise would have to be done by the unaided muscle power of the human being.
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