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Working out an equation for forces upon an object

  1. Apr 23, 2012 #1
    I am a very inquisitive person and the other day I started to think about perpetual motion. Since then I have been trying to name the forces that act upon an object and place them into a formula. The whole idea of the formula is really just so I can see what laws of physics need to be broken to allow perpetual motion to work.

    All I have been able to come up with so far is some forces that act upon an object and I know I will have missed a lot. These are:-
    • Gravity
    • Air Resistance
    • Momentum
    • Friction

    What I am asking is for help listing the forces that act upon an object and a formula which includes all of the named forces.

    Thanks in advance.
  2. jcsd
  3. Apr 23, 2012 #2

    Stephen Tashi

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    Science Advisor

    Verbal lists are not the basis for the formulas of classical mechanics. An object in a mechanical system is often acted upon by forces exerted by other objects. Adding "forces exerted by other objects" to a list doesn't tell you how to incorporate such forces in a formula.

    I suppose there is nothing good or bad about being an inquisitive person. Being inquisitive can be practiced as a form of amusement, rather like watching TV. Or you can be both inquisitive and determined to find answers. If you want answers to the the formulae of phyics, you'll have to study real mathematics and phyics, not simply make lists.
  4. Apr 23, 2012 #3
    In terms of classical physics, perpetual motion is entirely possible, it does not violate any of those laws of physics. In fact perpetual motion is basically an axiom of classical physics: Newton's first law of motion, inertia, the assumption that an object in motion will keep moving in a straight line (or keep spinning) forever unless it is acted upon by an external force.

    The reason why perpetual motion is 'impossible' realistically is exactly because of that 'external force', like the ones you've listed, which eventually cause the object to stop (in our perspective). Technically, the motion is still occurring though, the motion has just shifted to density waves in the air (i.e. made a sound), or produced vibrations in the object and the surroundings (i.e. warmed stuff up), so, from this perspective, perpetual motion is impossible to avoid (simply because of the axioms of classical physics).

    If you want to complete your list, you really only need to list the two fundamental forces of classical physics:
    - Gravity
    - Electromagnetism
    Everything else will just be some form of the previous two forces. Friction and air resistance, for example, is just a manifestation of the overall electromagnetic interactions between the object with the external system.

    Another more elaborate example would be a machine that uses electricity to move an object which then uses the movement to generate more electricity to move the object some more. This system eventually stops because the the electrons interact electromagnetically with the atoms in the wires to produce vibrations (i.e. increased temperature), which in turn causes the electrons to lose some of their motion.

    I hope you don't feel too unsatisfied that you haven't received any formulae. Personally I don't think they would have been that useful because they'd just give you the theoretical motion of the system, not explain what seems to be the core issue, that is, why perpetual motion is 'impossible'. Not to mention the fact that you'd have to get very specific about the system, and the equations would probably get very uselessly complicated (to be honest I'd have a very hard time calculating the exact motion of something like a hammer attached to a spinning wheel).

    Momentum isn't a force by the way X).
  5. Apr 24, 2012 #4
    Thanks, that has really helped me in my understanding in perpetual motion. I can put my mind at rest now. :smile:
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