what is work ??????????
Answers
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The quantity force multiplied by time is called impulse. “How long” need not always mean time. It can mean distance also. When we consider the quantity force multiplied by distance, this concept is called work. We do work when we lift a load against Earth’s gravity. The heavier the load or the higher we lift it, the more work we do. Two things enter into every case where work is done: 1.) the application of a force 2.) the movement of something by that force. The simplest case is the force is constant and the motion takes place in a straight line in the direction of the force. Then the work done on an object by an applied force is the product of the force and the distance through which the object is moved.
work = force × distance
In equation form,
W = Fd
If we lift two loads up one story, we do twice as much work as we would in lifting one load the same distance, because the force needed to lift twice the weight is twice as great. Similarly, if we lift one load two stories instead of one story, we do twice as much work because the distance is twice as great. The definition of work involves both a force and a distance. A weight lifter holding a barbell weighing 1000 N over his head does no work on the barbell. He may get really tired holding it, but if the barbell is not moved by the force he exerts, he does no work on the barbell. Work may be done on the muscles by stretching and squeezing them, which is force times distance on a biological scale, but this work is not done on the barbell. Lifting the barbell is another story. When the weight lifter raises the barbell from the floor, he is doing work on it.
Another category of work:
Work done against another force. When an archer stretches her bowstring, she is doing work against the elastic forces of the bow. When you do push-ups, you do work against your own weight. You do work on something when you force it to move against the influence of an opposing force - called friction.
Another category is work done to change the speed of an object. This kind of work is done in bringing an automobile up to speed or in slowing it down. The unit of measurement for work combines a unit of force, N, with a unit of distance, m. Resulting unit is N (Newton) & m (meter) = N.m also called Joule in honor of James Joule. One joule of work is done when a force of 1 N is exerted over a distance of 1 m, as in lifting an apple over your head. For larger values we speak of kilojoules (kl)-thousands of joules or megajoules (MJ)-millions of joules. The weightlifter does work on the order of kilojoules. To stop a loaded truck going at 100 km/h takes megajoules of work.
We are taught a rather circular definition of work. Work is energy transferred by force; and energy is capacity to do work. I am okay with this circular definition provided the equations that describe the same are explained correctly.
The first definition of work we learn is : work is force times distance. Note that we are given this as a definition without any fair justification into why that is true.
Force is defined as F = ma. I will not explain this equation in this post, but let us take if for granted. So work is W = FS = maS. Where S is the distance (or displacement).
It is easy to understand why work is proportional to mass. If we had 10 Kg to carry to a particular distance, and we split into two 5 Kg each. We would still end up doing the same amount of work. Similarly if we have to carry a 10 Kg twice to the same location it is same as carrying 20 Kg once. So we are very clear about the dependence of mass. Twice the mass, twice the work. Half the mass, half the work. So work is proportional to mass.
It is also easy to understand why work is proportional to distance. Carrying 5Kg twice is same as carrying 5Kg twice the distance. Similarly if we have to carry something twice the distance, we have to spend twice the amount of work. So again the relationship is clear, half the distance it is half the work, double the distance it is double the work.
But we know it should be intrinsically a property of the box we are pushing. (We are living in a “frictionless” world, aka friction is nothing but electromagnetic forces at microscopic scales, so everything is still consistent). The energy of a box cannot depend on where it is located, it will be same wherever it is. Energy cannot depend on acceleration, because it is then acquiring or losing energy. If nothing is acting on the box it will stay at uniform motion. So it is reasonable to expect energy is a function of velocity E = f(v) given everything else of the box is a constant (like mass, ideally it should increase mass when you push, but let us ignore that for a moment, we are in Newtonian world right now).