Talk:Work (physics)/Archives/2013/February

I hope Prof McCarthy will revise "Work by a spring" section

I shall not intervene in his edits as he did in mine; instead, I only offer a few arguments.

The most elementary approach would be to consider the work done on the spring while the spring is extended from the origin to some point x. In order to do that, some body (or somebody) acts on the spring by the force F=(kx, 0, 0) and does work

$W=\int _{C}\mathbf {F} \cdot d\mathbf {x} =\int _{0}^{x}kxdx={\frac {1}{2}}kx^{2}.$

Actually, the first integral - the line integral with dot product - typically is not used in this derivation in physics courses. This is an example where the application point moves in the direction of the force, and many teachers like to present it before dot product must be introduced into the work calculations. But an encyclopedia article may, understandably, have a different approach.

However, if it does, its purpose and assumptions should be clearly described, and the results derived correctly - which is not the case in this article. Here are some observations/comments:

1. Why is the spring "horizontal" and how is that related to the proposition that it acts only in the x-direction? Why is that "illustrated" by vertical springs, with some irrelevant drawings of forces (action-reaction)?

2. For any deflection x, the spring will exert the force F=(-kx, 0, 0). Note the minus sign. (The spring rate k is positive by any standard definition; only hypothetically one may choose to define it differently, but not in the context presented and without clear explanation).

3. Specific result for the work done by the spring given by Prof McCarthy can be true only if the body moves (has a component of motion) in the direction of the spring force (so that this force does postive work). That may happen only when that end of the spring (in the contact with the body) moves from deflected position towards equilibrium.

4. For the assumed general curvilinear motion of the body, most readers will certainly wonder how the contact with the spring is maintained and how the spring remains "horizontal".

5. Regardless of such "techincal" details, in order to obtain the specific result given in the article, the spring end must move from deflection x (positive or negative) to deflection zero. This should be clearly described in the text, and specified by the appropriate limits of integration, of which the author seems to be entirely unaware. So, of course, he does not explain why this particular motion of the spring is described in his contribution. --Ilevanat (talk) 01:22, 5 February 2013 (UTC)

Waves and energy transfer

Can a section be added to discuss energy transfer in mechanical and electrical waves. For example, how is energy manifest in a wave while that wave is traveling but before it encounters a barrier or is absorbed? Since waves are often able to propagate over long distances, it seems they lose very little energy during propagation. Yet the energy is there, waiting to be transferred upon termination of the wave. (One particularly dramatic manifestation of this effect is a tsunami. While the wave is traveling from the earthquake toward shore, it seems tame, I believe it can travel past boats without incident, yet it is ready to transfer substantial energy when it reaches shore.) If examples are given for ocean waves, sound waves, various mechanical vibrations, standing waves (no energy transfer) radio waves (in presence and absence of a receiver) etc. it will be very helpful. Thanks! --Lbeaumont (talk) 16:10, 21 February 2013 (UTC)