Forces and Energy

[RichIsnang]

Where do the four fundamental forces get their energy from?

it takes energy to move things, and the forces do it constantly seemingly without an energy source?

Edited May 24, 2012 by RichIsnang

[timo]

The "four fundamental forces" are better thought of as "four fundamental interactions". The term "force" is misleading in the sense that none of them is commonly described in a framework footing on the concept of "force" that you are familiar with from classical physics. It is not unambiguously true that "it takes energy to move things". For example, if you drop an object from some height the speed it has upon impact on the floor is usually evaluated from assuming that the total energy is constant.

[RichIsnang]

Hmm, I don't really know what you mean with that first bit there.

 

Hmm, I don't really know what you mean with that first bit there. I got the bit about the constant total energy

[timo]

What I meant by the first sentence is that the term "force" can be considered to have two meanings:

1. On the one side there is the colloquial term rather vaguely meaning "strength", "source of an effect" , "compulsion", ... and not physically precise (think "air force", "brute force", "forcing something", ...).
   
2. On the other hand, classical mechanics offers a very precise meaning of a physical force, the one you presumably know from the famous relation "F=ma".
   

Type #2 is arguably one of the most fundamental concepts in school physics. And there comes the problem: Particle physics does not work the same way as school physics. The term "forces" in "four fundamental forces" is better to be understood in the vague 1st interpretation (the colloquial usage of the term) as a source of an effect, not as a force in the sense "F=ma". That's why I believe it's better to call it "four fundamental interactions" in the first place. Not because it has any different meaning, but merely because it avoids a potential pitfall for laymen interested in modern physics.

[swansont]

Where do the four fundamental forces get their energy from?

it takes energy to move things, and the forces do it constantly seemingly without an energy source?

Force and energy aren't disconnected, so a force "needing" energy is a misnomer. A force is the gradient of potential energy.

 

[math]F = -\nabla{U}[/math]

 

So if you have a non-uniform potential energy, there will be a force. It's a different way of looking at the same thing.

[RichIsnang]

thats really cool, never thought of it that way.

how do the interactions differ from classical physics?

or is that just too long to explain lol

[timo]

Depends what you mean exactly. The formulations used to describe modern physics are Lagrangian Mechanics and Hamiltonian Mechanic, which both do not (bother to) explicitly contain forces. Explaining those would be a bit beyond the scope of a forum thread, though - it's more on like the scope of a 2nd or 3rd semester university lecture. However, if you believe the relation Swansont gave, then it's straightforward to formally eliminate forces in the physics you presumably know:

 

1) an object at position [math]\bf x[/math] experiences a force [math]{\bf F}({\bf x})[/math] that can be expressed by a suitable derivative of a potential energy U via [math] {\bf F}({\bf x}) = - \frac{\partial}{\partial {\bf x}} U({\bf x})[/math] (where [math]\partial/ \partial {\bf x}[/math] is to be understood as the gradient).

 

2) The change of the object's motion as a function of force is given by the relation [math]\frac{d^2}{dt^2}{\bf x} = {\bf F}({\bf x}) / m[/math], where m is the mass of the object, and [math]{d^2}/{dt^2}[/math] denotes the 2nd derivative with respect to time t (note that the 2nd derivative of the location with respect to time is the acceleration).

 

3) Combining 2) and 1) the law describing the motion of an object can be expressed as a relation that depends only on the structure of the potential energy in space, i.e. [math]\frac{d^2}{dt^2}{\bf x} = - \frac{\partial}{\partial {\bf x}} U({\bf x})/m[/math]. Given an initial position and an initial velocity (and the value of the potential energy for all times and all locations), this yields a unique result for the motion of the object, just as a=F/m does (if you know the force F).

 

That may merely look like a bookkeeping trick. In fact, it merely is a bookkeeping trick. But it is the way that most modern physics is formulated in: Rather than thinking in terms of "a given charge configuration generates a force fields for some charged particle flying through it" one usually thinks and works in terms of "a given charge configuration generates a potential energy/voltage field for ...". Explaining what "force" in the sense of "four fundamental forces" means is a bit beyond what I can explain at the moment. Simply put, it is the mechanism that dictates what the [math]U({\bf x})[/math] looks like.

 

Note that formally eliminating the "F=ma"-force is of course not sufficient to understand modern physics. To go all the way down to particle physics, there are other and more severe deviation from school physics that one encounters, namely the huge fields of relativity and quantum mechanics and the rather abstract issue of representations of symmetry groups.

[RichIsnang]

Ok that seems to make sense thanks

[swansont]

That may merely look like a bookkeeping trick. In fact, it merely is a bookkeeping trick.

 

That's the secret of physics (and perhaps all of science). It's not a description that makes it sound sexy, but it's the application of bookkeeping tricks, and learning which ones best apply to your particular situation.

[Aethelwulf]

Where do the four fundamental forces get their energy from?

it takes energy to move things, and the forces do it constantly seemingly without an energy source?

 

The forces get their energy from the fields which describe them. Gravitational fields for instance, is due to a gravitational energy. Electromagnetic fields store energy as well, as much as the weak and strong nuclear forces. The energy source then is the fields, in which quanta move in.

 

 

[swansont]

!

Moderator Note

Gravity followup split into new topic
http://www.scienceforums.net/topic/67072-gravity-and-gravitons/