BTW, I think weights were lifted on the descending side.
Bill are you sure they were not leveraged down ;)
You can put me in the camp the magnet is doing something by holding itself to the fridge. If you glued a rock to the fridge you would say the molecular bond of the glue is holding it up. Is the glue doing work? Yes and no... imo. The problem is we cross grey areas in between work and energy. If we remember all mass is made of energy then it not such a leap of faith to say that when the molecular bond of the glue breaks down the glue lost energy. How can you really define work... everything is moving around everything... it depends on your reference point.
Maybe because there is no measurement for it. Nothing moved to present us with a distance and a time to work with and apply a form of measurement. Now, if the magnet is holding a sheet of paper onto the refrigerator, there is something that can be measured....the weight of the paper being held up! I don't understand why the weight of the magnet being held up is not considered work.....maybe we just need a new unit of measure for this type of work.
In the scenario of the teacher pushing on the wall.....because nothing was accomplished....no work was done. Is this how it is viewed? If it is, I might have a few surprises in my paychecks......
Steve
Finding the right solution...is usually a function of asking the right questions. -A. Einstein
The method of calculating work done by gravity is useful only when there is measurable displacement. If there is no displacement then the calculation cannot be made. But just because there is no discernible displacement and we cannot therefore calculate the work done, does not necessarily mean no work was done. I know it is taught that without some displacement no work was done but it seems to me that that might be wrong.
A book on a shelf is held down by gravity and even though it isn't causing any displacement, may I suggest that perhaps it is doing work. If I place a powerful fan next to the book, it would take a mighty powerful fan to move that book. However if gravity wasn't pressing down on the book then there is nothing to stop the fan blowing the book away.
Lifting a book to put it on shelf to add potential energy to it is like moving it upstream against gravity. Position, vertically, is the key, but the book could be on the floor and it can still have potential energy - if the floor caved in, the book would fall again. So gravity is working, or doing work, all the time, whether or not anything is displaced.
The reason I believe this is because it is the only way to explain Bessler's wheel as requiring only gravity as a source of energy. I believe that no other source is required and that fits with everything Bessler said.
Think of gravity as a stream of energy, like wind or water flow.
Think of gravity as a stream of energy, like wind or water flow.
Some time ago I argued with you on your above statement. I mostly accepted the (laws) physics of prominent men as the facts (not withstanding, I still though/think PM is/was possible (contra-diction, if I ever heard one)). Time moves on, and facts and figures of yesterday are and should be challenged, as they were only facts on the research capability at that time. Now your statement makes perfect sense to me (it is possible I am wrong). A simple statement of, black is black and white is white is conceived by many as right and wrong at the same time, it all depends how stubborn we are.
'Work done' is a mechanical term - it is a measure of the amount of continuous effort required to displace an object after equilibrium is broken & against other forces that may resist that movement, such as windage or gravity etc - the equilibrium that must first be broken can be various e.g. frictional forces between objects, or molecular bonds or magnetic attraction - these equilibrium of forces e.g. between the weight of a book sitting on a table [weight = force due to gravity] & the table molecular structure holding it up by an equal force are common to all systems & therefore are not calculated as they are basal - all that is relevant is the force required to break these bonds & the force thereafter to move the object either horizontally or vertically or a component of each.
A man pushing on a wall which is effectively stuck to the ground has not caused any displacement of the wall so no work was done on the wall - lifting a mass vertically takes effort to overcome the gravitational attraction between the mass & the earth - once enough force is applied to cross the threshold & unbalance the opposing forces then the body can rise against the gravity field - if the mass weighs tonnes & you can't apply enough lifting force or enough leverage to lift the body then no work was done on the body, although you will be exhausted.
If something cannot be moved within a wheel [because forces are in equilibrium] then you cannot create overbalance & no work was done on the object.
Magnets on fridges are just like the force of gravity except the field can operate in any direction including horizontally - once the magnet is against the side of the fridge there are then also frictional contact forces to overcome to shift the magnet around but once in contact everything is in equilibrium again - if it can't move then no work is done thereafter.
If a block is sitting on a table & it takes a certain amount of force to shift it a certain distance then first we have to break the stalemate of frictional forces that gravity has helped cause - if the block is glued to the table top initially we will have to provide a far greater force to break the equilibrium of forces - once the bonds are broken we use far less force to move the object as in the unglued example.
Planetary systems elliptically orbiting a star are also in equilibrium where the inertia of the planet in its circular motion is matched by the gravitational attraction between the bodies, so no work is done unless a something physically impedes the motion.
Hovering helicopters are doing work because they are moving a mass of air towards the ground.
Bottom lining it, everything is in a state of equilibrium of forces which is also the position of least PE it can achieve - because this is common to all systems it is not accounted for because it is self evident & common - work done is a measure of the effort/force applied to a body that actually moves it a distance, not a measure of the equilibrium of forces always present in an undisturbed system.
I agree with all that you say Fletch - except that despite everything I still think gravity is doing work in just holding things down - yes, even though the force pushing against gravity creates equilibrium.
As you know I liken gravity to flowing water and when water flows it is doing work. If it meets resistance as in say a bridge pier, it pushes against it and if the pier doesn't move it flows around it, or in gravity's case through it. But I'm an eccentric myself so I'm off on a tangent I suppose ;-)
It will all be good with that explanation, because when I tie somebody's feet up then hang them from the roof, then tie their hands up and hang a 500lb weight to their arms, I can have peace of mind. Peace of mind in that amid the screams of agony there is no real work is being done.
If I leave a 100kg weight in the sand for 5 years and come back to find the weight has sunk 10cm... did it do any work? Of course it did, gravity is doing work at the microscopic level too, just because we can't see it doesn't mean it is not happening no matter how small it is.
One of us might be missing the point - it takes effort to provide the equalizing force to result in equilibrium i.e. no distance moved - in the case of the man hanging by his ankles it's his skeletal structure, ligaments, tendons & muscles working overtime [with a fair amount of pain, ask any torturer] - in the case of the 100 kg block the sand beneath it is compressed over time [moisture levels vary, air is displaced] & the sand either compacts under the load or moves to the side to let the block thru [just try walking over quicksand] - either way it cannot resist the force indefinitely until there is a solid base beneath it.
In a rotating wheel environment where torque is created by leverage, something must be lifted or shifted [moved] to create that leverage - just having forces in equilibrium e.g. gravity opposed by a structural resistance of some sort does not create leverage to turn a wheel - shifting something does.
John .. I don't want to dredge over old minings so I respectfully disagree with the analogy - water has mass & is a carrier medium [action & reaction forces come into play] - gravity has/is neither & that is why I would describe it as a field - in that sense it is also basal & applies equally to everything with mass & is not discriminatory at all - one of us will have to proved wrong by suitable experimentation I fear ;)
Sorry Flecther, I just disagree and playing the devil's advocate a little... in rereading it the tone is a lot harsher then I intended. But I still maintain that gravity is working at a microscopic level and we use work in numbers to the reference point to which we wish to relate to. I think with the man hanging none of his body is doing any work resisting the weight? His cell structure is just holding the weight up just as the table is holding up the book. I realize it is not what we are taught or allowed to think ;)
So with the sand analogy which law did I break. The constant force is the weight of the block and is constant, the force is down and the movement down?
No problem dax - I don't take anything personally - it's just discussion to get a better understanding of how things work.
Lets simplify the example - place a weight on a food scale - the weight does work as it compresses the spring balance until the forces are equalized - when they are equalized no more work is done according to the physics books - both the weight & the spring have PE though - this is analogous to your examples - no one disputes that until equilibrium is reached one body is applying an unequal force to the other & so one or both must move, even if it is microscopic movement - I just don't see how when things are in equilibrium this will help turn a wheel - to turn a wheel you need asymmetric torque & a force to produce it - the commonest way of achieving this is thru moving something until it has greater leverage on one side of a wheel - otherwise you have to produce a force that can directly move the wheel but that would require a force that didn't have any reaction force [or a reduced component of reaction force] & we know how difficult that is to achieve ;)
The problem I have is with the equilibrium part, well the conception, i know it does equalize in a way ... gravity force is constant at the weight of the object. So now lets say if the hanging man now uses his muscles to hold up the weight hanging from his arms. You can't deny he is using energy keeping the weight up. Where did the energy go? After 10 minutes the PE of the weight is exactly the same. That is a pile of heat energy, can't be just there? Why do you have to expend any energy if gravity is not using energy to pull it down? Once you equalize the forces doesn't mean gravity gives up. In reverse you lay the 500 lb block on your leg, after the forces equalize there is no pain or pressure? No.... You can feel the that gravity is always pulling down on the weight. Because we do not know how to extract the energy from gravity pulling down on the weight does not mean it is not there.
Fletcher Said:
Lets simplify the example - place a weight on a food scale - the weight does work as it compresses the spring balance until the forces are equalized
While in my mind the spring complicates things... oh the mind of a crank..lol You admit that the spring is now loaded and has energy in it? Where did all that energy come from? If I had a truck on a scale and loaded that spring, then tied down the spring storing the energy, then got out of the auto and pushed it on to the next scale one foot away ... where did it get the energy to load the springs? I did not pick up the auto, yet all the springs are loaded to the weight of the car? Yes, I know the height changed changing the PE. Gravity then must be energizing some of the spring, but according to science it is not because there is no energy in gravity.
Take an elevator going down, can two masses be leveraged in motion? Yes, but just to the point of freefall. After the masses are in freefall, the only way to move the lever is with a spring.
I know we've been here before Fletch and I agree there is no point in going over old ground, but there is one thing which I find perplexing. The suggestion that no work is done when I push with all my might against a wall. Even if the wall pushes back sufficiently to maintain equilibrium, subjectively I know I've done work. To me that is exactly the same as when gravity pushes down on an object which doesn't move because it is on the floor.
My point to all this is that I think that when the formula for calculating work was established it was obvious that measurable displacement of an object was required to provide a figure for calculation. With no displacement no calculation was possible, so they said, in that case no work was done. I maintain that it should read no measurable work was done.
This phrase 'no work was done' was merely used to show that no calculation could be done, but it has grown into a law which is applied regardless of what my common sense tells me.
No offence intended Fletch, but to me all the talk about static objects pushing back against gravity to provide equilibrium is just word play and meaningless. Gravity pushes things constantly and sometimes they give way and fall, but gravity is doing work all the time. Subjectively I don't need to do experiments to prove this, it is obvious to me.
Nice responses everyone. Enjoyable reads. Thanks for replies.
ovyyus wrote:
Randall wrote:Two people and a rope. At each end of the rope one person is pulling with equal force to the other at the other end. No net movement. No Fxd. No work.
Randall, flexing muscles do internal work even though the joints to which they are attached may not move. Two people pulling equally on a rope are doing work as their twitching muscle fibres constantly battle and strain (internal muscle fibre displacement) to maintain position.
Bill,
So we agree the people are expending energy.
Let me assume your explanation is exactly correct. If the people are expending "energy" to maintain a static arrangement of forces, the Fxd formula just can't apply at the macro level. Then shouldn't the same reasoning be applied to the magnet? Molecules/atoms spinning not unlike the muscle fibres twitching. Isn't the magent/fridge scenario also (by that logic) doing work?
On the other hand, if we say the two people are not expending energy, we know the two people will be taking a break for supper in a while to refuel, and perhaps they'll even have a nap or two.
How do the magnets do it?
...Maybe it's the molecular arrangement of the material.
...Maybe a wheel can be made to spin based on an arrangement of its internal parts.
