When two forces as same size and reverse direction act each other, there is keeping the balance, not being work, not varying the energy( not spending ) that is considered in general physics. For example, tug of war which doing same power, keeping a weight by hand, a weight which be put on desk, and so on.
Keeping the weight by hand, his arm feel tired. There might be necessary energy more than not keeping. By the physicist's interpretation, it is considered that energy is being thought necessity by biological muscle feature, some muscular kind isn't spend energy to keep it.
May we put it away by the explanation that it seems to bring biology into the discipline of physics? Anyway, let's examine a phenomenon about the force, and find out a problem.
Pushing an object on desk, there is usually friction (or resistance) between the object and the desk, the object can't be moved unless the force is enough. There are various kinds of the friction which are small like air friction, or large like destruction force of the steel frame.
A dramatic change will be observed at the moment when an object begins to move and before, because of work (necessary energy) turns zero to non-zero at the moment by the physicist's opinion.
It is considered that work is zero before the object moving in spite of acting force in general physics. If work equal with energy like general thought, added energy would be zero. if added energy was zero, the phenomenon would be the same as force not acting. Physical condition is evidently not same as pushing object or not. However, the mathematical expression would become to be the same in general physics. Such things is considered that the phenomenon could not be expressed correct.
Actually, it is clear that the condition is not same as the object which is acted force or not. If the power act to an iron stick, the stick will be bend by some force. Though the change doesn't happen suddenly, the stick would warp, and give off heat as it is hardly observed with the little power. If the stick was warped, everyone would think energy necessary. Why is it considered in general physics that not necessary energy until beginning bend of stick?
It is not rare not being in proportion of action to observed reaction in physics. A change is hardly seen even if the ice of -20 [C] degree is heated 10 [C] degree, but a dramatic change is observed when heated 10 [C] degree more. Such phenomenon is similar to the phenomenon which bends the iron stick. These phenomena are felt like a really different phenomenon, but ice melting and stick bending would be considered the same phenomena if it think that part of the stick was melted. Even in the case of which, before the dramatic change is observed, it seems that it isn't doing work, but it was necessary thing.
There are some ways of proving this thing. Let's decide the easiest experimental method to be used here which anyone must be understood.
The experiment of pushing an object with the stick is applied which had described at '2.1 Pushing an object'. Supposing that a cube with mass 'M' is put on little friction floor, and push it by stick.
If the just after velocity of when the cube was pushed is 'V', the energy of motion is in proportion to 'MV' which is also necessary energy for acting force 'F'. Supposing that the cube slide 10 [m] and stop by the little friction.
For the next, increasing friction between the cube and the floor, pushing the cube by the same force 'F'. Supposing that the cube slide 5 [m] because of increasing friction. It is clear that necessary energy to push the cube is also in proportion to 'MV '. Future event can not influence to past. Affirmation of such things should be told only in science fiction story.
If the friction is increased gradually like this, the gliding distance of the cube is inversely proportional, and decreases gradually. And then, the cube will be being rest macroscopic in spite of being pushed. Do you approve of the general physics conclusion that "energy is unnecessary to add a force against an object being rest"?
I do not intend to write science fiction story. Conclusion we obtain is;
Energy is necessary to add a force against an object whether the object move or not.This conclusion means that energy is always added to an object being rest on earth by the gravity. If energy is not added, we could construct a skyscraper by matches.
No one knows what the true character of this energy source by the gravity. We have known that it is not discover a phenomenon that gravity is made by exchanging something else, gravity is not attenuated/reflected, and keeping to be released.
If we receive the feature of gravity as the fact, it would be considered that energy of gravity is unlimited energy source, which is free energy.
However, it is not informed how to take out this energy efficiently for general people. Like the gravitational field of earth is being called with a conservative force field, it is considered in general physics that energy can not be taken out from the field. Is it possible that energy is taken out from the field?
Next chapter, we will consider the conservative force field.
2.5 Conservative force field
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