Science Buddies: "Ask an Expert"

I am getting confused with the term kinetic energy .I seriously know that kinetic energy is the energy associated with the state of motion of a body.I would like to understand it in terms of work done because work done on a body is equivalent to energy
"Can i say that kinetic energy of a body is the work that has to be done on it in order to make it displace hence causing its motion and hence the term kinetic energy"

any kind of help would be appreciated ...
Thank you moderators
Krish
If there is any correction required in my question please let me know

why is it taking a long time to answer this question

please someone come forward and clear my doubt
ok now i understand the reason for the delay ..i am sorry i am going to give a presentation on energy and its transforms
but to be very honest with u, i myself havent understood the actual definition for energy .All that is running in my mind is just definitions with vague explanations.I am trying hard to understand it myself but still i feel someone's aid would be justifying.So please ,i request ur kind self to help me out with the question that is mentioned in the first post of mine

Sorry i didn't mention this,i was unaware .I apologise humbly
Thank you
Krish

Hi krish,

Welcome to the forum! Sorry that you've encountered some frustration. To help you understand how we work, someone is responsible for checking the boards and answering unanswered questions at least once a day, but the time varies depending on the schedule of that day's volunteer. In addition, it's possible for us to receive email alerts when a new message is posted, so many of us are aware of questions posted on other days and answer when our expertise is relevant. Unfortunately, your three posts occurred between 2 and 8 in the morning my time, which is the time of day when I am least likely to see an email alert. This is probably more or less true for most volunteers living in the US.

So yes, energy is the capacity to do work. In an ideal case in which the transfer of energy is 100% efficient and none is 'lost' due to friction, etc, then yes, an object's kinetic energy is equal to the work done on it to put it in motion. However imagine this: You push a box with a mass of 30 kg, initially at rest, 10 m across the floor with a force of 100 N, so you have done 1000 J of work. If all of this work had gone into the box's kinetic energy, it would be moving with a velocity such that (30 kg)(v^2)/2 = 1000 J -- but its velocity is zero. Why? The work was transformed other types of energy: maybe some squeaking or scraping noise (kinetic energy of air molecules), heat from friction against the floor, maybe some pieces broke off the bottom of the box (combination of mechanical work and breaking chemical bonds to the rest of the box material). At higher velocities, air resistance would become non-negligible.

So, even in cases where there is less friction, some of the work done on an object to accelerate it is lost as forms of energy other than kinetic, and the amount lost could depend on things like the surface it is pushed across, etc. So equating the kinetic energy of an object to the work done on it is inexact. Similarly, if the kinetic energy of an object is used to do work, it will (in real life) always be split into several forms of energy. It could exert a force over a distance, but things might break, there might be noise and heat, etc, so the force*distance that it moves something else will be a little less than the object's kinetic energy.

It is possible to have a nearly elastic collision in which almost all of the energy remains in kinetic form -- for example, billiards (aka pool) -- although even here, the balls eventually stop moving due to friction. If two objects are moving at the same speed but one has a larger mass, that one has more kinetic energy. If two objects of the same mass are moving at different speeds, the one going faster has more kinetic energy. And it's probably better to think of that kinetic energy as the capacity to do work rather than in terms of the history forces the object has experienced, for reasons explained above. That 'capacity' could be how fast a larger/smaller object would move if the object collided with it, how loud the noise would be if it hit a wall, how far it could penetrate into a big block of styrofoam, etc.

Hope that helps -- if you could tell us more about the science project/experiment you're planning, we might be able to give more specific advice and examples. If you haven't already, you might gain some clarification by poking around on Wikipedia or http://hyperphysics.phy-astr.gsu.edu/hbase/ke.html.

Good luck,
Amanda

Thank you Amanda .Sorry if i have been rude or hasty .Thank you once again ,your explanation was very convincing
but the actual trouble starts now once kinetic energy is known its counterpart potential energy needs more understanding
Now how do i understand Potential energy in terms of work done "Now i cannot say that potential energy of a body is the work that needs to be done on it in order to bring it to rest or to some position".If this is the case how to understand the definition of potential energy .But still all good books that i have read so far says potential energy is energy asssociated with a body due to its position .or potential energy is the energy of a system or some configuration .

I am sorry i am unable to understand the meaning of potential energy as said above.But still on the whole whatever i think must satisfy the definition of energy i,e "energy is the capacity to do work" or energy is equivalent to work done on the body

Try putting "Potential Energy" into a search engine. The wikipedia and several introductory college physics course material is online that you can use as a starting point. You should get a few hits on these with the "Potential Energy" search. The course materials often have examples that can be helpful when discussing your understanding. If you find one you don't understand, then posting a question with the URL to the material will help us help you.

Trying to help you in an abstract philosophical what is "Potential Energy" and how is it converted to and from other forms of energy can easily become post doctorate level physics in thermodynamics that even the experts can have some lively discussions about. There is a reason that PhD's in these fields are considered Doctor of Philosophy degrees - they involve a lot of abstract conceptual thinking about things e.g. philosophy of things.

sorry sir did i ask something wrong .Thank you anyway i will give it a try and make a search .

"Potential energy is energy which results from position or configuration. An object may have the capacity for doing work as a result of its position in a gravitational field".How does the position of a body tell me what work i can do on the body or what work it can do on some other body due to the work that i do on it ( generally a force instead of "i")

http://hyperphysics.phy-astr.gsu.edu/hb ... av.html#pe

This should help: http://en.wikipedia.org/wiki/Potential_energy

Amanda

the ability to do work

Thank you once again "Amanda".
Hence should i sum up everything by saying that potential energy of a body at some point "is the work that should should be done by a force in order to bring the body to that position by maintaining it at rest.......i,e by not displacing it further from that position."

Hence potential energy of a body in a gravitational field can never be zero ...unlike kinetic energy

Am i correct "Amanda"

Hence should i sum up everything by saying that potential energy of a body at some point "is the work that should should be done by a force in order to bring the body to that position by maintaining it at rest.......i,e by not displacing it further from that position."

potential energy of a body at some point "is the work that should be done by a force in order to bring the body to that position by maintaining it at rest.......i,e by not displacing it further from that position."

The definition of work and energy and how it relates to force times distance is a bit strange. If you were holding your hand out and somebody placed a one pound paper weight on it and you do not move your hand, then you will have to exert a force equal to the weight (gravitational force times the mass of the paper weight) to keep it there. From the definitions, you are not doing any work. Stand there for an hour and your muscles will get tired. Your body is actually expending energy in an isometric exercise (resisting a force); however, the physics definitions tell us that no work is being done wrt the paper weight.

If your hand is 3 feet above the floor, then the paper weight has 3 foot pounds of potential energy wrt the floor. If there was a hole in the floor that would allow the paper weight to fall down to a lower floor 15 feet below, then the paper weight would have 18 foot pounds of potential energy wrt the lower floor and still have 3 foot pounds of potential energy wrt the floor you are standing on. Potential energy is usually considered relative to something.

Back to your original statement, I would reword it to something like "the work that was done by a force to bring the body to a new position of rest is equal to the change in potential energy relative to the previous position".

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 .

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.

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.

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

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