
#19
Mar214, 09:38 PM

Mentor
P: 16,484





#20
Mar214, 09:47 PM

Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
P: 11,070

Where you do have forces: the force is the negative gradient of the potential energy.
If you know how PE varies in space, you don't need to know the forces. A lot of complicated systems are easier to handle that way  and you can end up, as Dalespam says, with situations where you have energy and no forces: so you cannot use "forces" to analyse them. It's OK to use forces where they make the math simpler  but you should get used to using energy directly, as it's much more useful in the long run. 



#21
Mar214, 09:55 PM

P: 997

Claude 



#22
Mar214, 10:40 PM

Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
P: 11,070

I was just concerned about the emphasis on forces in OPs thinking. I suggest further discussion about force vs energy descriptions to be continued in another thread. 



#23
Mar314, 08:56 AM

Mentor
P: 16,484





#24
Mar414, 03:03 AM

P: 47

This maybe a bit off topic but, I struggle to define energy without relating it to force.
So by stating this: How can Force = 0, while Energy = x. I'm troubled here... how can a field have energy in general, like when a magnetic field or electric field store energy, while forces are zero? 



#25
Mar514, 01:30 AM

Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
P: 11,070

I'm thinking of Gravity in GR and the Fields of QFT.
In both cases  the "force" is an emergent phenomenon. In GR the "force" of gravity is a pseudoforce  a product of geometry rather than a Newtonian inertial force. In QFT the appearance of "force" is the result of lots of interactions via gauge bosons. Have a look at: http://www.hep.manchester.ac.uk/u/da...trotoQFT.pdf http://math.ucr.edu/home/baez/physic...particles.html But DaleSpam may have other things in mind or a better illustration. 



#26
Mar514, 01:31 AM

Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
P: 11,070

I'm thinking of Gravity in GR and the Fields of QFT.
In both cases  the "force" is an emergent phenomenon. In GR the "force" of gravity is a pseudoforce  a product of geometry rather than a Newtonian inertial force. In QFT the appearance of "force" is the result of lots of interactions via gauge bosons. Have a look at: http://www.hep.manchester.ac.uk/u/da...trotoQFT.pdf http://math.ucr.edu/home/baez/physic...particles.html But DaleSpam may have other things in mind or a better illustration. 



#27
Mar514, 07:42 AM

Mentor
P: 16,484

In short, the electric field has an energy density proportional to EČ, and the magnetic field has an energy density proportional to BČ. So, if you consider a closed region of vacuum there is, by definition, no matter in there and so nothing on which to exert a force. Nevertheless, despite there being no forces, if there is a magnetic or electric field inside the region then there is energy there, as given above. Energy can be transferred to or from the region by increasing or decreasing the fields inside the region. 


Register to reply 
Related Discussions  
Does robotics follow the laws of physics?  General Physics  3  
electric motors  General Engineering  9  
Just curious how electric motors work, specifically DC motors.  General Physics  17  
Why do both Coulomb and Gravity follow 1/r^2 laws?  Classical Physics  8  
Electric Motors  Electrical Engineering  3 