1 - it criticizes the mass definition of energy: \"energy is the capacity to produce work.\"
Commenting on IEnergy is a concept difficult to define, although it is family to all. A lot of times we are picked in circular definitions of impact and energy, ostensibly using the two have to describe the aforementioned thing.
f \"energy is the capacity to produce work\", we asked, then: what is work? Will it be that you will be mitigated with the mathematical expression that quantifies the work?
We don't know how to delimitate energy, time, space, matter etc. They are fraudulence concepts. The science has their fraudulence concepts; the mathematics also has them, for instance, in the geometry they are the point concepts, straight distinction and plan.
In the chemistry they are the digit of energy, it structures and connection. Unlike other physical greatness as speed and force, forcefulness is an abstract concept that possesses monetary value when he/she becomes work.
A obligate of grade 1 kgf or a speed of measure 3 m/s are not things that digit can buy or to sell; all of the months, however, we paying to the distributing companies for the kilowatt-hour that we consumed.
2 - being exercised a constant obligate along a presented distance on bodies initially in rest, which will the kinetic forcefulness be at the end of the course? Will the kinetic forcefulness depend on the mass of the body?
Solution
The kinetic energy, at the end of the course, will be the aforementioned for all of the bodies.
Let us consider the movement unidimensional of a belly of steep parched (reduced attrition to the minimum) cover for a constant obligate along a flat road of polished aluminum
As the obligate is constant, the belly will qualify evenly, with the = F/m and we will have:
x1 = (1/2).(F/m).t2 and v1 = (F/m).t
Being eliminated t among these two expressions, it comes:
v1 = (2Fx1/m) 1/2 and Q1 = m.v1 = (2mFx1) 1/2.
The amount of movement supplied to the belly is not autarkical of the mass of the pot, as it would happen if we practical the obligate during a die interval of time. The amount of movement is proportional to the square root of the mass m.
He/she exists, however, an amount that is invariante for all of the pots, independently of their masses, to know:
Ecin. = Q21 / (2m) = (m.v21) /2 = F.x1
Like this, when a constant obligate is exercised initially along a presented distance on bodies in rest, all acquire the kinetic energy, Ecin., in the end of the course, that is, DEcin. = Ecin. = F.x1 = WF, being WF the impact accomplished by the obligate F in the move x1.
3 - a boy on pots of parched cover meets of foot on a flat opencast of polished aluminum, close to a plumb wall. Pushing the wall, the boy goes out with a certain speed and kinetic energy. Who paying for that energy?
Solution
The external forces that act on the healthy boy: the) the coefficient practical P for the Earth; b) the obligate N ascending practical for the surface; c) the flat obligate F exercised by the surround on the boy's hand.
Who \"paid\" for the kinetic forcefulness was the interior impact of the boy's muscular forces. Analytically, for the theorem kinetic work-energy, we have:
Wres. = Wext. + Wint. = DW
Now, Wext. = 0, then, Wint. = (1/2) m.v2.
There is another way to grappling the problem.
The impulsive force, F, performing in the boy's assistance during the interval Dt, it supplies a pulse F.Dt = m.Dv. For other part, the obligate F moves his/her point of application of Dx and it accomplishes the impact FDx = (1/2) mv2, being Dx the small deformation of the boy's hand.
4 - a person carrying a sack in the nous for a flat highway accomplishes or don't I impact physical? To discuss.
Solution
In first place, we necessary to know the physical impact is external or internal. In the housing of the physical impact to be external, we have to complete the sentence saying: do I impact that forces?
In the presented case, to I force-weigh doesn't work, once there is no force-weight factor in the direction of the displacement; however, we should not block the obligate of attrition of the soil on the man's feet, actuation him/it to the front. Be m the attrition coefficient.
The impact of that attrition obligate is: Wfat = (Ph + Ps).m.d, where Ph = I weigh of the man, Ps = I weigh of the sack and d = displacement.
This impact is not invalid aforementioned that the man walks without attractive anything in the head. The impact would be smaller if the attrition coefficient decreased, and it would be invalid in the ideal housing of m = 0 and, in this case, it would be impossible the man to walk and to instrumentation the weight.
In if treating of interior impact can affirm that he exists, therefore the blood circulates faster in the capillary ones, the heart beatniks faster, the interior attrition is larger etc.
5 - can the point of application of a obligate move in the possess direction and sense of the obligate and invalid being the impact accomplished by that force?
Solution
Yes. We know, for instance, that the impact accomplished by the obligate of rolamento attrition is always null.
Be a cylinder that rolls without slipping on a flat surface.
Between the instants t1 and t2 the point of application of the obligate of rolamento attrition moved among the two positions P1 and P2
6 - when two groups of boys vantage a fastening in contrary senses and does balance exist, does he/she take locate work?
Solution
Every mechanical impact demands obligate and move of the application point in the direction of the force. Consequently, external impact doesn't exist in this case.
Let us consider the grouping rope. The fastening is composed of an enormous number of molecules that you/they training interior forces some on the other ones. When the fastening is prolongated abruptly, the distances among the molecules vary, he/she takes locate interior impact with a decrease of interior forcefulness of the grouping that shows in form of heat DQ = DU.
Note: Q here refers to the 'heat' and no 'amount of movement.'
Let us consider the grouping boys. The boys' muscular effort carts an interior work, once the blood circulates faster, the heart beatniks faster, the interior attrition is larger, the boys are burning render through of that muscular effort.
7 - a boy is swiming against the current of a river, so that it always stays in the aforementioned place. Does he/she take locate impact in this case?
Solution
The whole discussion of this difficulty lives in the difficulty of the referencial. The \"always in the aforementioned place\" place in the statement refers to the referencial 'earth'; somebody in the margin of the river is observing the boy.
