# Mechanical Energy Basics

#### Greg Bernhardt

These are the topic areas covered in the notes for this article: The Concept of Energy, Work and Power, Potential Energy, Kinetic Energy, Change in Energy, Conservation of Energy, Elastic Collisions

http://physicspost.com/articles.php?articleId=178 [Broken]

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#### zoobyshoe

Just looked at the first page and I think "the ablity to cause change" is an improvement over "the ability to do work".
The latter suffered from being misconstrued by some to only mean work that is useful to people. They found "the ability to do work" objectionable because it seemed to exclude energy that we couldn't put to some practical use. "The ability to cause change" takes care of that objection while saying essentially the same thing. I wonder if Gale 17 will see this and find it more informative.

#### Chi Meson

Homework Helper
I've been teaching this definitaion ("ability to cause change") for several years now. I've asked students to try to think of any sort of change and I'd explain what energy transfer had occurred to make that change. The best response to date was: "The change in color of the sky." That was hard to explain in words, and I still don't have a stisfactory short answer.

Anyone?

#### russ_watters

Mentor
Originally posted by Chi Meson
I've been teaching this definitaion ("ability to cause change") for several years now. I've asked students to try to think of any sort of change and I'd explain what energy transfer had occurred to make that change. The best response to date was: "The change in color of the sky." That was hard to explain in words, and I still don't have a stisfactory short answer.

Anyone?
Any change? I think the question is a little too broad. Not all changes require energy in the thermodynamic sense. (how would you change Pi?) Unless you mean literally the energy required to replace all of the nitrogen in the sky with another gas, its an irrelevant example.

#### Phantym

I don't like this definition for many of the same reasons I don't like the definition "The ability to do work."
First of all "the ability to cause change" is rather vague. Secondly, it is not even accurate!
Consider a book lying on an inclined plane. If the inclined plane has friction such that the book does not move, you have a situation where the book has potential energy, but has NO ability to cause change or do work.
A similar situation occurs for a book lying on a table above the ground. It has no ability to cause change or do work because the table is pushing up on it, not allowing it to move.
Using this definition, you cannot redefine the "zero point" of potential energy to be the height of the book in each case, because then a similar book at the same height but without support would drop and have "negative potential energy." What does it mean to have "negative ability to cause change"?

I prefer the following definition for Energy:
Energy is the ability to exert a force. The amount of energy an object has equals the the work the object would do in the absence of other forces.

Note that in my two examples the book is still exerting a force.

The article has other difficulties as well.
1. It classifies forces as either potential or kinetic without indicating that this definition really depends on how closely you look. Chemical bonds look like they are potential energy at the molecular level, but that same energy is derived from both position and movement at the subatomic level.
2. It gives an incorrect definition of elasticity for collisions. The elasticity of a collision is the ratio of the separation velocity to the approach velocity in the direction of contact. In the case of a two-dimensional system, this will not equal the ratio of kinetic energies.

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