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Energy and force

  1. Mar 11, 2013 #1
    Hello all .
    I have two questions
    Energy can create force ? for apply a force we need energy ?
    Think about one electron , if you want to apply a force on it , you must have a particle in moving or a filed . and fields and particles in moving have energy .
    So is that correct for create force we require energy ?

    If answer is negative can give me some examples that can create force without object in moving or fields .
    Last edited: Mar 11, 2013
  2. jcsd
  3. Mar 11, 2013 #2


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    Without knowing what level you can comprehend, I have to simply guess at what you are able to understand.

    Force is related to the gradient of the potential energy.

    http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/pegrav.html [Broken]

    Last edited by a moderator: May 6, 2017
  4. Mar 11, 2013 #3
    This is a consequence of teachers using the word "energy" so liberally.
    It takes energy to do work.
  5. Mar 11, 2013 #4


    Staff: Mentor

    No. While fields do have an associated energy density, it is not necessary for any of that energy to be expended in order to produce a force.
  6. Mar 11, 2013 #5
    But i think it is necessary .

    Imagine two freedom electrons , one of them is at rest and another one is in moving .
    When the moving electron comes closer and closer to rest electron , the electric filed of it apply force to another one and electron at rest starts to moving and get energy .
    Also the rest electron apply reaction force to another electron and cause reduce it's speed .
    Total energy is conserved but moving electron do work on another by electric field .

    So i think there is no apply force in the universe without transfer or expend energy .
    Last edited: Mar 11, 2013
  7. Mar 11, 2013 #6
    not so simple a question as may be inferred from the different perspectives already posted.

    Work involves a change in energy,an expendture of energy, and work involves a force over a distance.....W = Fd......but if no distance is covered, no work is done and no energy expended.....

    examples of no energy expended:
    It takes force to hold you on the earth, but no energy...F = GMm/22...
    [You lose or gain potential energy by moving up or down in the gravitational potential and such movement does involved work.] Nor is energy expended holding a nucleus together,in a stationary configuration, the binding energy remains fixed and steady; but you must spend or gain energy by reconfiguring the nuclear potential......moving the nuclear pieces....say fission or fusion
  8. Mar 11, 2013 #7


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    Constraint forces do no work e.g. the normal force.
  9. Mar 11, 2013 #8
    big bounce: I see you posted while I was composing.

    no, not in general......in the usual language of physics.....I gave two contradictory examples above....
  10. Mar 11, 2013 #9
    in case you are not familiar with the details of the math notation [and I am not] an example of this is the magnetic force....F = qv x B.....where the magnetic force is normal to the direction of particle motion so the work done on the particle by the magnetic force is zero....no change in magnetic field energy, nor the KE energy of the particle, but a force IS again exerted....
  11. Mar 11, 2013 #10


    Staff: Mentor

    No, it isn't necessary.

    What about electrons which are not free and which are exerting forces on each other without moving?

    Again, this is simply wrong. Let me give a brief (non-exhaustive) list:

    A book on a table
    Your butt on your chair
    A screw in the wall
    A tool leaning against a wall
    A magnet on a refrigerator
    A balloon charged with static electricity and stuck on a wall
    The EM force between an electron and a nucleus while the electron remains in its orbital
    The gravitational force for a circular orbit
  12. Mar 11, 2013 #11
    Good examples thanks . i got about expend energy and force .

    But my original perspective was if you want to move a object you must create force through distance and you must do work .
    If you want increase speed of a object you must give it accelerator and for give accelerator you must produce a force . this force must apply through distance to able increase speed of object so you need energy for this .
    So energy cause to motion and moving object not force .
    Is it correct ?
  13. Mar 11, 2013 #12


    Staff: Mentor

    Not necessarily. If an object is already in motion then no work is required to make it continue to move. And if you simply want to turn an object that doesn't require work either.

    Yes, it takes energy to increase speed.

    No, not all motion involves acceleration, and not all acceleration involves a change in speed.

    You shouldn't try to hammer concepts into places where they don't fit. Force is the gradient of energy (for a conservative system), so they are closely related. But they aren't the same thing. I wouldn't say that force causes motion since you can have velocity without acceleration, and energy has even less of a claim since you can have acceleration without a change in energy.
    Last edited: Mar 11, 2013
  14. Mar 12, 2013 #13
    I think and i have a idea for expend energy and force please say what's the problem .

    You said for holding a nucleus or quarks together there is no require expend energy .
    I agree but for create these forces already expended energy in early universe and after that There is no require expend energy for holding them together .
    Or you said "It takes force to hold you on the earth, but no energy"
    I agree but when you stay in earth force of your weight (mg ) exactly cancel out with normal force (N) and "net force" is zero newtons .
    Last edited: Mar 12, 2013
  15. Mar 12, 2013 #14
    Yes i know but for a object in motion you already expended energy and gave it energy .

    How ? can give some examples for both of them .
    Thanks a lot .
  16. Mar 12, 2013 #15


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    Centripetal acceleration. Velocity changes(the direction of the vector), but the speed and energy remain constant.
    Another example for constant energy despite acceleration is any body in free fall in a gravitational field - it merely exchanges potential energy for kinetic energy, but the total remains the same.
  17. Mar 12, 2013 #16


    Staff: Mentor

    No, not necessarily. The object in motion could have been created already moving at high speed. It will then continue to move at high speed without ever having work done on it. This happens all the time when muons are created in the upper atmosphere. They can travel at relativistic speeds for their entire brief lifetime without ever having work done on them.

    Uniform circular motion.

    Look, your idea is simply wrong on multiple levels:
    1) energy does not need to be expended to create a force
    2) force is not required for velocity
    3) energy is not required for acceleration

    What is correct is that energy and force are related. Force is proportional to acceleration. Energy is a constant of motion and therefore a very useful quantity.

    Now, if you have new questions about how energy and force work in general, we will be glad to answer. In particular, you may be interested in the Lagrangian formulation of classical mechanics where energy concepts are the focus and forces are secondary. But do not circle back to your refuted personal theories.

  18. Mar 12, 2013 #17
    big bounce:
    I was going to suggest this sooner and did not, but should have:
    if you are really interested in understanding the relationships between work, force, energy,acceleration and so forth, you should really consider going back to basics...
    the definitions of these in physical science.

    Unless you know how each of these are defined and understand their application, and how they relate, you'll just go on and on and on among different examples....
    an advantage of using a textbook, likesay, Halliday and Resnick, is that it is not only well edited
    but has been revised,updated, and utilized for probably 50 years....
  19. Mar 12, 2013 #18
    This is all an unfortunate result of the popular media and high school teachers using the word energy where it does not belong, people get the idea that it's some galactic currency that gets things going.

    Energy is just a convenient quantity that is time independent in closed systems.
  20. Mar 12, 2013 #19
    There is a problem .
    For free falling in gravitational field we have a system not a object .
    When you moving up a box from earth you must expended energy and because of this total mass of system increase . but total mass of earth and box remains constant .

    This expended energy stored in gravitational field and after that the gravitational potential energy applies force to object and give it kinetic energy .
    So please give a example that we have accelerator but object don't get energy .
    Your example was a system involves two objects and gravitational filed between them .
    Last edited: Mar 12, 2013
  21. Mar 12, 2013 #20

    Yes . when we give energy to photon it never changing it's speed because C is limit speed in universe and when a black hole apply force to photon it never get accelerator because it's moving at C . but it's not true for object's that they move slower than light .
    It never violate What I said .

    Let me give another example :
    Consider a man wants to pull a box along a concrete floor . the box has a mass of 500 kg and the coefficient of friction between the box and the floor is 0.2 and g = 10 m/s2 .
    Now if he applies 700 N on box , box produce 700 N in opposite direction
    if he applies 800 N on box , box produce 800 N in opposite direction
    if he applies 900 N on box , box produce 900 N in opposite direction
    if he applies 1000 N on box , box produce 1000 N in opposite direction
    in this case "net force" is zero and man doesn't do work .

    According to what you said If we have a engine that applying 600 N on box in a day , the engine doesn't need energy for do this during this time .
    Is it correct ?

    But when he apply 1001 N on box , box starts to moving and gets accelerator and energy and now we expend energy .

    So i say when we have non-zero "net force" that "act" on a object the force always do work and for produce a non-zero "net force" you must expend energy .
    I know when theta be 90 degree work doesn't done but in reality there is example that "net force" be vertical and object moves in horizontal ?

    Last edited: Mar 12, 2013
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