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B Units of Inertia and Moment of Inertia

  1. Jul 26, 2016 #1
    Unit of 'moment of inertia' is kg.m^2, Which I know. But about Inertia itself, is it a quantity phenomenon or quality phenomenon? What is it's unit?
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  3. Jul 26, 2016 #2


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    Inertia is the resistance of an object to be affected by a force. It can be caused by mass, but e.g. EM energy can have this effect too. See here!("Feynman lectures on physics" vol.2 has a chapter about this too.)
    Moment of inertia is the resistance of an object to a torque. Its called moment of inertia because of the way it is calculated. See here and here!
  4. Jul 26, 2016 #3

    David Lewis

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    Inertia is a property defined both as a general quality (what it is), and also in the particular sense (how much you have of it), in which case -- i.e. when you want to put a number to it -- kilogram (kg) is the SI unit.
  5. Jul 28, 2016 #4
    Let me explain what I understood from your explanation and references mentioned. Please correct if you find mistake:
    1- Inertia is a quantitative and it's unit is 'Kg'. Am I right? Do Inertia and Mass have same unit? Does it mean that both are the same?
    2- Moment is a CONCEPT. And it refers to DISTANCE of a subject (like force) and the point which that subject (e.g. force) is applied to that subject. Yes?
    3- Torque is 'moment of Force' which is shortened to 'moment'.
  6. Jul 28, 2016 #5


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    'Moment of inertia' is for rotational motion what 'mass' is for linear motion.
  7. Jul 28, 2016 #6

    David Lewis

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    Correct. The SI unit of measure for inertia is kilogram (symbol kg -- both k and g are lower case). Kilogram is one of the base units in the SI.

    Moment is both an abstract concept and a physical reality. I would substitute the term physical quantity for subject -- and, incidentally, this physical quantity may be a scalar or a vector (force, mass, area, electric charge, etc).

    Yes, moment does refer to the distance from the quantity to the reference point (or the perpendicular distance to the reference axis). I call this distance the moment arm. You have to multiply the quantity in question by the moment arm in order to get the moment. (It's a cross product but ordinary algebraic multiplication works in simple cases.) Moments are classified according to the power to which the moment arm is raised (usually the first or second power).

    Right. Torque is the first moment of force, and it's a vector.
    Last edited: Jul 28, 2016
  8. Aug 4, 2016 #7
  9. Aug 4, 2016 #8

    David Lewis

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    In that thread you appear to be considering a machine that can function as a pump (power is absorbed) or a turbine (power is produced), correct?

    They're closely related but technically not the same thing. Mass in all cases is the quantity of matter. But mass can mean the hypothetical ability to be pulled by gravity, or it can refer to resistance to acceleration (inertia).
  10. Aug 5, 2016 #9


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    There is a similarity between the equations of linear and rotational motion...eg...

    Force = mass * acceleration
    Torque = moment of inertia * angular acceleration

    The linear equations of motion (SUVAT) can be similarly modified.
  11. Aug 5, 2016 #10
  12. Aug 6, 2016 #11

    David Lewis

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    When the direction of rotation of the impeller reverses then the flow is also reversed. That is, fluid enters the pump's discharge outlet and comes out through the suction inlet. In this situation, mechanical power is available at the impeller shaft.

    The advantages of operating a pump as a turbine are convenience, simplicity and low cost. The disadvantages are low efficiency and lack of versatility.
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