The meaning of the term "Energy"

[krish]

Thank you Sir Craig .
I am sorry if my impatience has perturbed u .I was terribly confused With the term potential energy .To be honest Sir ,this is the thought that was cripping in my mind ."If a body is supposed to be at a "particular position" and "still" "remain" there, then "something" must be doing some amount of work on it such that the body remains at that "position" only.
I couldn't convince myself .I kept beating round the bush asking myself as to whether my thinking is genuinely acceptable .
Since there were no replies i thought i had asked something annoying or silly .

[deleted-2131]

krish,

Just because it takes a while to get a response to your question doesn't mean that it is silly or annoying. Because of the way the Forums work, you can expect to wait up to 48 hours for a response to your question; you have been unusually lucky in getting responses fast.

[deleted-71588]

I am sorry if my impatience has perturbed u

It takes a LOT more than that to perturb me. Part of the problem with not getting responses faster is that in the United States, this is the summer break from school and most of the volunteers here also take a break. I personally am only responsible for answering things that appear or are unanswered on Wednesdays. Not all of the volunteers on the physics forum have enough recent theoretical physics background to be able to adequately address your more abstract theoretical questions.

You can either accept the physics definitions and work with them and be a bit puzzled by why they don't make intuitive sense or you can dig really deep over many months/years into what if the definition were different so that it would match your intuition and what would change. If you do enough of this, you likely will come to the realization that changing it to make one thing make more sense would also change it to make something else counter-intuitive in either a worse way or an equally obnoxious way. The physicists community has honed the basic physics definitions over centuries and it would take some major discovery of a new phenomenon and many years to cause an adjustment in the definitions.

This is why I attempted earlier to guide you into looking at some of the online physics course materials with examples to see things that make more sense as a first step in attempting to understand Potential and Kinetic Energy. Essentially, avoid the counter intuitive until you understand the more intuitive cases. The Physics definitions for Energy and Work aren't very useful when dealing with biology. A given human resting flat on a bed will requires some number calories per hour. The same human sitting up will require more calories per hour. If you apply the basic physics definitiions at a gross macro level, you find that there is no difference in the work being done so the calories required should be the same. The basic Physics definitions probably would apply if could break down all of the things going on in the human body and apply them to each piece; however, the human body is so complex and the thermodynamics involved is so difficult to comprehend that you won't likely come close to explaining the differences between laying and sitting. Since our intuition is based on our human body experiences, the physics definitions are often counter-intuitive; however, they were developed using much simpler models and problems and have been found satisfactory for most basic physics problems.

[deleted-71712]

krish,

One thing about potential energy that you might not have grasped yet is that it only makes sense when defined with reference to the potential energy at another point. For this reason, physicists talk about "delta-U" (gravitational potential), "delta-V" (electrical potential, also called voltage), etc (written with an uppercase Greek letter delta). (As you learn more physics, you might find it interesting to pay attention to the many analogies between mechanical and electrical systems.) Technically, any time an author does not define the reference point for a potential, he or she is being sloppy with notation -- but in many circumstances, there is a standard reference point that those familiar with the scenario being discussed would assume.

In most textbook physics problems, gravitational potential energy is defined as zero at the surface of the Earth, or the floor, or wherever the lowest relevant point is. So if, for example, a 1-kg weight is at the edge of a table or in your hand a meter above the ground, then it has 1 kg * 1 m * 9.81 m/s^2 more of potential energy than it would have on the ground. This can become kinetic energy if it falls or is dropped, so it would be moving with a velocity v such that 1/2 * 1 kg * v^2 = 1 kg * 1 m * 9.81 m/s^2 by the time it hit the ground -- *assuming the ideal case* in which none of the potential energy is converted into heat, sound, or other forms by air resistance or some other mechanism. If you wanted, you could define the potential energy at the Earth's surface to be 3.1415926... J or -10,000 J, etc, and it wouldn't change the answer to the problem, because only the difference matters. Similarly, voltage only has meaning when defined with respect to a reference point, which if not otherwise stated is the "ground" in the circuit (often the Earth, because it's such a large reservoir for charge that some current flow in or out barely changes its potential).

Something else you might be missing is that according to the definitions you'll find in physics books, even if you are applying a force to an object, you aren't doing any *work* unless you're moving it through a distance. Work = force * distance (or, integral of F dot dl, but don't worry about that if you haven't studied calculus yet). Yes, you are expending energy, but that is due to the complicated biological systems that Craig alludes to and that you probably don't want to worry about as you are just grasping the concepts. A table holding up a weight is not expending energy, if that is more comfortable to think about. The table is exerting a force on the weight (and the weight is exerting an equal and opposite force on the table), but gravity is exerting an equal force in the opposite direction (force is a vector quantity), so the weight does not accelerate.

I second Craig's suggestion that you find a good introductory general physics text (high-school or first-year college level would be appropriate) either online or in hard-copy format, and I would add that you should just do lots of problems related to these concepts. As he indicated, it will gradually become clear to you why the definitions are the way they are, and they will begin to make sense intuitively -- you won't have the urge to reword them, or if you do, you will understand what about them is important so that you can do so accurately. Although some of the definitions you have proposed are technically correct IF you qualify them to refer only to ideal cases, the definitions that you'll find in texts are much more useful for analyzing situations, determining how things will move, calculating unknown quantities, etc. The problems you'll be given in a textbook are 'simple' in the sense that they are close to the ideal cases that I keep harping about and/or it is explicitly stated that you should ignore effects that make the problem non-ideal. As you are doing them, just keep reiterating the definitions to yourself as they are written without trying to reword them, and eventually you will "get it".

Again, if you tell us about the science experiment you have in mind, we can give you much more specific information.

Good luck,
Amanda

[deleted-71712]

krish,

I did a quick search for "online physics text", and these look like they might be useful:

http://en.wikibooks.org/wiki/FHSST_Physics:Index
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
http://www.motionmountain.net/welcome.html
http://www.archive.org/details/ap_physics_b (pick a lesson, then click "click here to begin lesson" on the left)
http://www.lightandmatter.com/area1book1.html
http://www.glenbrook.k12.il.us/gbssci/P ... u5l1b.html

There are also lots of problems (and problem help) online, if those texts don't include them or if you run out:

http://zebu.uoregon.edu/~probs/
http://www.physics247.com/physics-homew ... ndex.shtml
http://www.oberlin.edu/physics/dstyer/S ... blems.html

Amanda