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B Why do objects move?

  1. Jun 27, 2016 #1
    Hi all,

    Molecules have different degrees of freedom.

    So, when I push a wooden block, why doesn't all the energy that I provide via a push contribute to the degrees of freedom of the molecules of wood? ( my hand is also made of molecules, so it's basically molecules dealing with molecules )

    Why does the block move at all?
     
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  3. Jun 27, 2016 #2

    phinds

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    Your hand and the block repel each other due to electrical repulsion. It's the same reason you don't fall into your chair or the floor. So when you push the block, it wants to get away from your hand, so it moves.
     
  4. Jun 27, 2016 #3
    When you try to touch a wooden block - what happens at the molecular level?
    Try to imagine the outermost layer of wood and the surface molecules with nuclei and 'electron cloud'- so your fingers molecular layer interacts with it - you push harder the bounded molecules of the block provides reaction force .
    things remain static till your 'macro-push' aggregate of all the forces leads to a translation of the block- so as a lay person you say that the action of your 'pushing' force led to a displacement of the block.
    the motion gets generated by action of forces in nature.
    For classical bodies people have experimented with forces and 'motion' and have come to depict the laws of motion.
    If one goes to molecular level of action and reaction-deformation/stripping etc. one will go for many -body systems and to solve for motion will get pretty complicated and models ,idealized ones helps in understanding mechanical motions.
     
  5. Jun 28, 2016 #4
    Thanks for the answer.

    But, when I push the block some of the energy that my hand supplies surely creates vibrations in it.

    Why not all of the energy from my hand is used in increasing the vibrations of the molecules of the block rather than moving it. (if some why not all)
     
  6. Jun 28, 2016 #5

    phinds

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    I don't know what you are talking about with these "vibrations of the molecules" so I'll have to leave that to someone else.
     
  7. Jun 28, 2016 #6

    jbriggs444

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    Consider a 1 kg uniform rod floating in space. You exert a 1 Newton force perpendicular to the rod for a duration of 1 second. Does the work done by your hand on the rod depend on whether you push it at the end or in the middle?

    Answer this question and you should have the answer to yours.
     
  8. Jun 28, 2016 #7

    sophiecentaur

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    That is totally correct. The amount of energy that you can transfer to the motion of the block will be limited to how much of the work done on it is dissipated internally as the block is distorted. If you use a very large force for a brief time, then more energy will be used up by 'friction' between vibrating the particles as it distorts a lot. If you use a small force for a longer time then the amount of distortion will be much less and more of the work will turn up a KE of the whole block.
    I have deliberately used arm waving terms here but there are better, higher level arguments which say the same thing.
     
  9. Jun 28, 2016 #8
    I can reverse the question and ask why not all of the energy to go into macroscopic (translational/rotational) velocity and not microscopic (vibrational) velocity?

    Hard for me to give a satisfying answer to any of the two questions (the forward or the reversed :D). I guess the energy will be split not necessarily with equal percentages. When we observe macroscopic interaction of bodies like collisions, most of the energy goes into macroscopic velocity while a small percentage goes as heat or collision's sound which is both cases of microscopic velocity. On the other hand when we heat a body or we light it up a body with photons of high enough frequency most if not all of the energy goes into microscopic velocity.
     
    Last edited: Jun 28, 2016
  10. Jun 28, 2016 #9

    sophiecentaur

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    It all depends on the particular case. If the Energy Input is in the form of Mechanical Work (Force X Distance) and the material of the block is inelastic then the limiting final value of KE of the block will be equal to the Work done as Distance →∞ (and Force → 0) and the limiting value of KE will be zero as Distance → 0.
    If the block is perfectly elastic then the final KE will always equal the Work put in.
    I don't think it is necessary to consider the microscopic situation if you use Coefficient of Restitution or use the complex Modulus of the material (assuming linear behaviour).
     
  11. Jun 28, 2016 #10
    Thanks everyone for learned answers.

    But somehow I fail to understand this process.

    Let me put it in reverse.

    When my hand touches the block, millions and millions of electrons come very close by.

    When such a thing happens, instantaneous repulsion takes place.

    So, in such a scenario where is the scope for the energy provided by my hand to increase the internal energy of the block??

    Is there some loophole in this act of repulsion??

    Thanks for patience
     
  12. Jun 28, 2016 #11

    David Lewis

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    To be precise, when you push on the block, it accelerates. The block can move (at constant velocity) when no net force is being applied.
     
  13. Jun 28, 2016 #12

    phinds

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    So you have gone from asking why all of the energy goes into heat to asking why ANY of it goes into heat :smile:

    Your current question is answered in both post #7 and post#8
     
  14. Jun 28, 2016 #13

    russ_watters

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    It looks to me like you are confusing heat (thermal) energy with mechanical energy.

    If you lightly touch a cold object, the random motions of the molecules in your hand transfer energy individually to the molecules in the object, making them vibrate more.

    When you push on an object, all the force/motion is in one direction, so the entire object moves together. It can't just vibrate more in place because it isn't allowed to by your hand being in the way!
     
  15. Jun 29, 2016 #14
    If I can successfully rephrase the OP , maybe in a way that reflects my own thinking on this situation:

    " All forces between two bodies are of the A-type: that is, forces between the molecules/electron clouds of the two bodies. Why some forces of the A-type (like the force from a hand pushing a block) increase/change the macroscopic kinetic energy, while some other forces of the A-type (like the friction force) increase the microscopic kinetic energy?"

    Well the answer (maybe not satisfying but I am trying ) is that though all forces are of A-type, still each force might have some additional characteristics that make them differ. The force from our hand pushing a block is kind of an organized force, as Russ says all the force/motion is in one direction and this causes an organized-tuned microscopic movement which we perceive as a macroscopic movement. On the other hand a friction force is less organized, it is causing a random microscopic movement which we perceive as heat or sound wave energy.
     
  16. Jun 29, 2016 #15

    sophiecentaur

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    It is often best to take a simple model to get an understanding of a complex system. We can only take on board a limited number of variables in our search to understand a bit of Physics. There is absolutely no need to reach for the microscopic level for the first level of understanding of this, in fact it can just muddy the water. When the macroscopic situation is well understood then it may (or may not) be worth while looking at the smaller scale. It struck me that a motor car suspension system, treated in block diagram terms, could help.
    At its very simplest level, the whole system consists of a mass, a spring and a friction / damping element. When the car hits a speed bump, the spring will be compressed according to how fast you hit the bump. The amount of compression will depend on the mass, the stiffness of the springs and the rate that you hit the bump (the resonant frequency of the system and the time profile of the force. The faster (and hence more) the spring is compressed against the mass of the car, the more energy will be dissipated in the damper. Going very slowly over the bump will involve minimal spring compression and the car body will be raised by the full height of the bump. All the work goes into raising the car.
    If you hit the bump fast, there will be enough force on the spring to compress it and cause some dissipation in the damper. (That is your lost thermal energy.) Hitting the bump fast enough will displace the spring by the full height of the bump without raising the car at all - like my old 2CV, it will run over a sleeping policeman with no disturbance to the passengers. The damper will have dissipated all the energy of the wheels going up and down.
     
  17. Jun 29, 2016 #16
    I may be wrong but I feel that some answers are evasive. Sorry for this.

    Nevertheless, as Russ Waters said that thermal energy is different from mechanical energy... So, does it mean that thermal energy travels the space between the electrons, then reaches near all the atoms/nuclei in the molecule and thus makes the whole molecule vibrate?

    Or, as Sophiecentaur said... Should such questions be relegated to the background?

    Or, as per Delta... Are there other characteristics of forces that should be studied?

    I just wonder like a baby
     
  18. Jun 29, 2016 #17

    russ_watters

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    No, people are not being evasive, it's just that wrong questions sometimes don't have answers and sometimes it is even hard to say why. I think I may be better at that than average. For example:
    Thermal energy is not a "thing" like an object that moves around on its own. It is a statistical property of atoms. And it doesn't apply to subatomic particles. So for thermal energy, the concept of something happening inside the atom doesn't apply.
     
  19. Jun 29, 2016 #18
    @ Russ Waters
    " Sometimes it is even hard to say why"

    Is there an inkling that the question may be right?

    BTW, my question about thermal energy 'moving, was in an ironic sense.
     
  20. Jun 29, 2016 #19

    phinds

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    What do you mean? Questions are not right are wrong, they just ARE. Answers are right or wrong. Please clarify what you are asking.
     
  21. Jun 29, 2016 #20

    russ_watters

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    A question itself is "wrong" when it doesn't make sense, which is often because it is based on a flawed premise.
     
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