1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newtons first law should not be termed an expression of inertia

  1. Jan 19, 2012 #1
    Newtons first law should not be termed an expression of inertia

    LAW I.

    In case inertia could be removed from a body (e.g by a relative of Maxwell`s demon), the following situation is presumably true;

    PART I
    1. A body B has a state of uniform motion in a right line
    2. Inertia is removed from the bodies mass
    3. The bodies state of motion is unchanged*

    4. Another (inertial)body A comes in contact with Body B
    5. This contact causes B to change in its state of motion wich is described as mass over a distance per second squared. Also known as force.

    I conclude the following about LAW I;

    PART I
    Every body perseveres in its state of uniform motion in a right line regardless of inertia.

    Any body can be compelled to a change in its state of motion by forces regardless of inertia.

    Newtons first law is therefore independent of inertia and should not be termed an expression of inertia or «the law of inertia».

    Some examples where LAW I is termed an expression of inertia;
    http://www.khanacademy.org/video/newton-s-first-law-of-motion?playlist=New and Noteworthy
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jan 19, 2012 #2
    Your post is hardly coherent... but it seems you just don't understand what 'inertia' is.

    'Inertia' (as you can find from a simple google search), is not a physical thing---its a property, or a 'tendency'. In particular, 'intertia' IS exactly newton's first law. In other words, Newton's first law defines 'inertia'.

    [tex]F = 0[/tex]
    [tex]ma \sim \Delta (mv) = 0[/tex]
    Edited equation for clarity

    That is inertia.

    What? Maxwell's demon removed certain types of particles from an ensemble of many... how do you remove inertia from an object without exerting a force upon it?

    You don't 'remove inertia from mass'.
    You put an '*' on part '3' without expanding... '3' is also completely false. Inertia is an objects state of motion.

    This isn't really related to the overall point, but force is in units of mass * distance per second squared.

    For the record; random videos aren't especially pertinent sources or references...
    Last edited by a moderator: Sep 25, 2014
  4. Jan 19, 2012 #3
    What does
    stand for?

    The mark(*) is there because i don't know if the statement is true.
    I figure you do;
    Last edited: Jan 19, 2012
  5. Jan 19, 2012 #4
    The 'Delta' ([itex]\Delta[/itex]) refers to a 'change in' a parameter. A more exact statement would have been [tex] \frac{d}{dt} (mv) = 0 [/tex] which means, the derivative* of the mass times the velocity. If the mass is constant, this simplifies to
    [tex]\textrm{if} \hspace{0.2in} \frac{d}{dt} m = 0 \hspace{0.2in} \textrm{then} \hspace{0.2in} \frac{d}{dt} (mv) = m a[/tex]
    Where the acceleration is the derivative of the velocity.

    * If you are unfamiliar with derivatives, its basically the instantaneous rate of change of something.
    Last edited: Jan 19, 2012
  6. Jan 19, 2012 #5
    ΔZHermes: Force has units based on mass, distance and time squared. Not what you said.
    Your equation: ma = Δ(mv) is incorrect.
    It should read ma = m(Δv/Δt).

    johann1301: Δ(mv) is more commonly and correctly written as mΔv and means change in momentum.

    To start changing mass by breaking off bits or whatever changes the object in discussion so that it is neither Object A nor Object B any more.
  7. Jan 19, 2012 #6
    Thanks @daqddyo, fixed and clarified these respectively.
  8. Jan 19, 2012 #7
    I understand why you disagree and find my demon silly. Its not possibly to remove inertia from matter.

    If i said;

    Lets say that some day we find a rock from space which is made of matter/mass but has no inertia. I know, its a stupid thought. (but imagine that! wow!)

    Why should or would we think that it would be an exception from Newtons first law of motion?

    My answer is; it wouldn't be an exception from the law, it would follow the law exactly. Is there any reason to think anything else?
  9. Jan 19, 2012 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    This is one of the most meaningless waste-of-time discussion that I've ever seen on this forum.

    1. You have not shown ANY physics that decouples mass from inertia.

    2. Yet, without showing #1, you THEN somehow make this outlandish assertion of "what if".

    This thread is in clear violation of the PF Rules on over-speculative post.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook