Energy dependence on observer framework

  1. Jun 13, 2014 #1
    Does mechanical energy of a system depend on the framework of an observer (neglecting a constant)?
  2. jcsd
  3. Jun 13, 2014 #2


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    Staff: Mentor

    Yes. The kinetic energy of a bullet is zero in the frame of an observer who is at rest relative to the bullet, non-zero for an observer who is at rest relative to the target of the bullet.
  4. Jun 13, 2014 #3


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    Staff Emeritus
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    I agree with Nugatory but I can't help but wonder what you mean by "neglecting a constant".
  5. Jun 15, 2014 #4
    Ok, Right. The statement "neglecting a constant" is my mistake.
    I clarify my purpose of the question:
    Newton's laws are only valid in inertial framework. I like to know whether energy formalism is valid in non-inertial framework or not? In other words, can one solve the problems exactly, using conservation of energy in non-inertial framework?
  6. Jun 15, 2014 #5

    [itex]\int_{t_0}^{t_1}\vec{F}(t)\cdot\vec{v}(t)dt = \frac{1}{2}m v^2(t_1) - \frac{1}{2}m v^2(t_0)[/itex] is valid in frames where [itex]\vec{F}(t) = m \frac{d\vec{v}(t)}{dt}[/itex]

    That is, in inertial frames.

    You still can use it in non-inertial frames IF you add "inertial forces".

    [itex]\int_{t_0}^{t_1}\vec{F}(t)\cdot\vec{v}(t)dt = U(x(t_0),y(t_0),z(t_0))- U(x(t_1),y(t_1),z(t_1))[/itex] is valid in any frame where [itex]\vec{F}(x,y,z) = -\nabla U(x,y,z)[/itex]

    where [itex]U(x,y,z)[/itex] does not vary with time in this frame.
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