### Kinetic & Mechanical Energy

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Could someone explain the difference between mechanical and kinetic energy? Please provide examples of mechanical energy that do not qualify as kinetic energy and examples of kinetic energy that do not qualify as mechanical energy.

-- Michael Gatton (mg143@aol.com), June 27, 2000

I've been consulting with Frank Hicks from the New York Academy of Science who has been extremely helpful in formulating a more useful answer to my question about mechanical/kinetic energy. I think this is the definitive answer on the subject as far as a Q&A forum goes, but I begin with some words of wisdom from Thomas Kuhn about the limits of "textbook definitions" in bringing about understanding of fundamental science concepts:

If…a student of Newtonian dynamics ever discovers the meaning of terms like “force,” “mass,” “space,” and “time,” he does so less from the incomplete definitions in his text than by observing and participating in the application of these concepts to problem- solution.”
Thomas Kuhn, The Structure of Scientific Revolutions

The concept of mechanical energy can seem rather complicated and involves several layers of prior knowledge in order to really understand.

I. In Short
In short, the answer is this: Kinetic energy, the energy of motion, is one particular kind of mechanical energy. Another kind of mechanical energy is potential energy. If you want to know the total mechanical energy of an object, you add up the different kinds of mechanical energy it has, kinetic and potential.

Here are the different types of mechanical energy:
Kinetic energy – the energy associated with moving along a path (“kinetic energy of translation”) or spinning (“kinetic energy of rotation”)
Gravitational potential energy – energy that is stored or released as an object moves up or down in a gravitational field
Elastic potential energy – energy that is stored or released as an elastic object (like a spring or a rubber band) is deformed or relaxes

II. Teaching strategies
Mechanical energy itself is easy to define. The first thing that you should do is make sure that you understand the definition and can give examples. (There is an example in Section III if you would like to help clarify the definition further for yourself.)

To understand mechanical energy, a student really needs to understand potential and kinetic energy -- those are the two important concepts, whose understanding requires more than just memorizing a definition. Once a student understands kinetic and potential energy, mechanical energy really is just a "construct"; a student just learns that "mechanical energy = kinetic energy + potential energy".

So if it is just a shorthand way of talking about kinetic and potential energy, why is the term "mechanical energy" important? For one thing, in high school or college physics courses, the first conservation law students learn about is usually the conservation of mechanical energy. It is a stepping stone toward the big conservation law that all energy is conserved. That doesn't mean that your students have to know what mechanical energy is right now, but it at least is a clue as to why all those textbooks seem to use the term so regularly.

Another, perhaps more relevant reason for why mechanical energy is important is that some of your fellow teachers, teaching, say, biology or earth science, may talk about "mechanical energy" when they are discussing energy transformations. For example, in photosynthesis, light energy is changed into chemical potential energy, and then when we humans eat plants, we can transform the chemical energy into mechanical energy as we sprint down the hall or stretch a rubber band and aim it at the teacher's back. (This is the sense in which the NY State core curriculum uses the term).

Some of your colleagues may not be used to hearing students calling this "kinetic" or "potential" energy instead of "mechanical". Likewise, some standardized tests may use the term "mechanical energy" in general for kinetic and potential energy. Again, this does not mean that you must teach your students the formal definition of "mechanical energy," but it is something you should be aware of.

Suggestions:
1. Talk to the other teachers at your school about how they cover energy in their classes. At least let each other know whether you use the terms mechanical, potential and kinetic energy.

2. When you discuss energy, use lots and lots of examples. This builds a context and a real understanding not based just on definitions. (See section III below for one example.)

3. When you talk about mechanical energy (whether you choose to use that term or not), make sure your students understand that other kinds of energy are chemical, sound, light, and thermal (heat).

III. Longer Science Explanation
Mechanical energy is the combined energy of motion and position of an object (specifically, its kinetic, gravitational potential, and elastic potential energies). On a practical level, the term is generally applied to relatively large objects and so it is not used for molecular, atomic, or sub-atomic particles. On that level gravitational and elastic potential energies are negligible, and there are other potential energies that are much more relevant to the total energy of the particle, like chemical or electrical potentials, which are not included in the calculation of mechanical energy.

The "energy of motion" is called kinetic energy; it is the energy that an object has because it's moving, like a baseball flying through the air, a bee whizzing by, an ice skater twirling, or a model train barreling down its track.

The "energy of position" is often a little harder to understand. This energy is usually called potential energy. Two examples of potential energy are gravitational potential energy, like the energy stored in your coffee cup that makes it rush to the floor when you knock it off the edge of your desk by accident, and elastic potential energy, like the energy stored in a compressed spring or in the rubber of a blown up balloon.

An example:

Imagine you see an uninflated red balloon sitting on the ground. It's not moving, so it has no kinetic energy. It's not inflated, so the rubber isn't stretched at all -- no potential energy has been stored in stretching the rubber. And it's sitting at ground level, so it's gravitational potential energy is zero as well. You are looking at a red balloon with no mechanical energy (remember the mechanical energy is the kinetic and potential energies all added together).

You're feeling winsome today, so you pick up the balloon and blow it up and hold the opening pinched between your fingers. Is its mechanical energy still zero? Well, the kinetic energy is still zero, it's sitting right there in your hand. How about potential energy? Aha, you have stretched the rubber and stored up energy that way. Yes, and you also picked it up off the ground and moved it higher -- it now has some gravitational potential energy, too.

Now you let the balloon go, and it flies up and away from you. Now, finally, there is some kinetic energy. There's a little more gravitational potential energy than before you let it go (because it is flying higher). And what about the elastic potential energy? It's getting smaller because the balloon is shrinking (becoming less stretched), and that potential energy is being transformed into the kinetic energy and gravitational potential energy of the now-moving balloon. When the balloon is completely deflated, its elastic potential energy will be zero, its gravitational potential energy will peak, and then begin to decrease. Its kinetic energy will increase proportionally as it falls (ignoring air resistance). Just before the balloon reaches the ground, its gravitational potential energy will be zero, and all its energy will be kinetic. After it hits the ground, the sum of its kinetic and potential energies, i.e. its mechanical energy, will again be zero.

I'm leaving the original discussion below in case anyone wants to see how or why this final answer came about...

-- Michael Gatton (mgatton@vzavenue.net), November 03, 2002.

I am a life science teacher at Mott Hall (IS223). It is clear from these two e-mails about energy that there is a lot of confusion about the vocabulary used to describe forms of energy and conversions of energy from one kind to another - both on the part of teachers and students.

I was just looking at the NYState 8th grade science test sampler and there is a question about energy conversion that required students to state what kind of energy is being converted into what other kind of energy - so it is clear that we need to know just what vocabulary in this area forms the basis of their evaluation. For example, on the test sampler, they show a candle burning and ask what kind of energy is being converted to what kind of energy. Are they expected chemical energy to heat and light? I did not see an answer key to the test sampler - is it there and I just didn't look carefully enough?

In life science I always review the different forms of energy and how energy is converted from one kind to another so that my students can understand photosynthesis. I find my students will often say that the two kinds of energy are potential and kinetic, when I am looking for heat, light, chemical (bond), motion energy, etc.

Can you point me toward a clear description of types of energy and energy conversion for classroom use? Interesting worksheets or activities?

Best Regards, Susan Herzog

-- Susan Herzog (sherzog@hotmail.com), October 08, 2000.

Hi Susan: I posted the questions as you can see a long time ago as sample questions, but they are also authentic questions in the sense that I really don't know the answers. I have searched through many Q&A forums on the web as well as physics textbooks with no satisfactory answer. I recently submitted the "definition of forces' question to Scientific American's "Ask the Experts," but it may take a while to get an answer from them if at all. As I now see I am not the only one who is confused, I will forward the questions to a scientist at the Max Planck Institute in Germany who recently offered to be of assistance. His specialty is astrophysics, still he may have an answer. I am also taking the survey cource at CCNY where I plan to address the questions when appropriate.

The answer to the candle burning question is
"Chemical energy is converted to light energy; chemical to light
or Chemical energy is converted to heat energy; chemical to heat.

The bell ringing is:
Mechanical energy is converted to sound energy; mechanical to sound.

Thanks for your questions and stay tuned...

-- Michael Gatton (Science Facilitator) (mwgatton@aol.com), October 08, 2000.

This is not a distinction I'm very familiar with. However, the definition I've seen most often is that mechanical energy is the sum of kinetic and potential energy -- ie the quantity that is conserved in the absence of dissipation etc. For an example see eg:

http://theory.uwinnipeg.ca/physics/work/node9.html

Cheers, Phil

-- Phil Armitage Max-Planck Institute for Astrophysics

-- Phil Armitage (armitage@mpa-garching.mpg.de), October 09, 2000.

Let me re-state the problem. Our Prentice Hall textbook defines BOTH kinetic energy and mechanical energy in different sections of the book as "the energy of motion." The book offers no distinction between the terms but does further confuse things by stating that mechanical energy can be either potential or kinetic. Since running across this problem I have consulted countless websites and textbooks, including the text used currently in the CCNY survey course (which doesn't even use the term "mechanical energy!") but have found little to clear up this muddy terminology. Generally I understand the concepts, but I am unable to STATE the distinction between these two terms. If the answer is too lengthy to cover in this forum, could someone point me to a text that does answer the question clearly?

-- Michael Gatton (mwgatton@aol.com), October 12, 2000.

I posted this question on the Rockefeller University Classroom Assistant Listserv Mailing List and here are the responses I got:

Mechanical Energy is Energy which can do (macroscopic) Work. It is kinetic and potential energy which is capable of providing a force which can act upon an object to cause a displacement. The following web page gives some excellent descriptions of physical concepts:
http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1d.html Some excerpts:
"The energy acquired by the objects upon which work is done is known as mechanical energy."
"An object which possesses mechanical energy is able to do work."
"A moving baseball possesses mechanical energy due to both its high speed (kinetic energy) and its vertical position above the ground (gravitationalpotential energy)."

From: George D Sukenick

Another:

Interesting question,
At the macroscopic level, I cannot think of the difference between kinetic and mechanical energy. However, at the microscopic level, one includes the kinetic energy resulting from thermal motion of the atoms. In water, the hydrogen and oxygen atoms in a given molecule are in motion relative to each other, then the h2o molecules are moving around, etc. - this is all kinetic energy. However, the thermal motions do not contribute to the mechanical energy of the system.

Dr. Kalman Migler
Polymers Division
National Institute of Standards and Technology
100 Bureau Drive
Building 224, Room A207
Gaithersburg, MD 20899-8544
USA

tel: 301 975 4876
FAX: 301 975 4924
kalman.migler@nist.gov

Hope that helps!

Mike Gatton

-- Michael Gatton (mg143@aol.com), January 03, 2001.

As for mechanical and kinetic energy: I have discussed this with a couple of people and the problem seems to be one of fuzzy terminology. Again our anonymous retired physics teacher writes:

Mechanical energy is a vague term that should probably be avoided. Among the mechanical forms of energy are kinetic energy -- the energy an object with mass has because it is moving, like a flying baseball; (gravitational) potential energy -- the energy an object has because it is up high in place where there is gravity so it can fall and do work -- like a brick about to fall on someone; and also elastic energy -- the energy stored in a stretched rubber band.

I don't disagree in principle that the term might be better avoided on the middle school level. As I mentioned earlier, the textbook used for the survey course at CCNY avoids the term completely. Problem is, NY State uses the term in its MST standards:

4.1c Most activities in everyday life involve one form of energy being transformed into another. For example, the chemical energy in gasoline is transformed into mechanical energy (emphasis added) in an automobile engine. Energy, in the form of heat, is almost always one of the products of energy transformations.

It seems the best approach would be to use the term in context and give copious examples, avoiding strict definitions. In terms of energy transformations, I would use the terms kinetic & potential in pairs. Thus, I would discuss potential energy converting into kinetic energy or vice versa. Avoid the terms potential & kinetic when discussing OTHER energy transformations such as chemical to mechanical, light to heat, etc. I realize that this isn't strictly true, but how else to alleviate some of the confusion?

Any one see a problem with this?

Mike

-- Michael Gatton (mg143@aol.com), March 01, 2001.

Mechanical energy equals kinetic energy (energy moving, at work) plus potential energy(energy at rest).

Kinetic energy equals 1/2 the mass of a mass, times the speed of which a mass is moving.

To quanitize the kinetic energy of a system does in no way mean to mention any of its potential energy which anywhere on this planet has a value in relation to gravity as energy that can become a force.

Mechanical energy involves also quantizing the value of its potential energy as well as other energies, so that the total energy of the system is in some relation to is volume, thus much better control of variables relating to its function.

Mechanical Energy: mechanical energy is the amount of energy a mass is is utilizing because of the availabilty of potential and any conversion of other energies plus the value of any energies already acting upon it to produce force or transformation of energy, including its own mass.

Traveling in space is mechanical energy because the lack of air resistance thus also the help of other forces. No resistance from earth's gravity makes it not just kinetic energy. Only taking into accounts its kinetic energy would make this a very nervy experience.

Kinetic Energy: kinetic energy is the energy which is in the process of being utilized not including the mass of an object and having a value of force with or without motion. A leave being blown up a hill doesn't show any mechanical energy inclinations because it will always come down in ralation to gravity.

-- Adam Luis Hugo (sportbank@hotmail.com), July 22, 2002.