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!

B Definition of Force

  1. Sep 16, 2017 #1
    Can we say that the First law of Newton defines the force whereas the Second law gives the magnitude of force?
  2. jcsd
  3. Sep 16, 2017 #2
    No, Newton's I law only states that In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
    If you accept mass and acceleration as primitive notion, then Newton's second law (In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma.) defines force.
  4. Sep 16, 2017 #3


    User Avatar
    2017 Award

    Staff: Mentor

    I'd say the first law defines the absence of force and the second has not only magnitude but also direction in it. Turned into the positive direction, the first law says that the change in motion requires a force. It doesn't say what force is.
  5. Sep 16, 2017 #4
    All three laws together define force.
  6. Sep 16, 2017 #5
    Are you sure? First Newton's law concludes from the second Newton's law (you also have to assume that if force is not acting then F=0). Therefor the first Newton's law is not needed to define force.

    \text{Newton's II law}
    \\\text{force does not act} \to F=0
    \ a=\frac{F}{m}
    \\\text{force does not act} \to F=0
    \end{cases}\Rightarrow \text{force does not act} \to a=0 \Rightarrow \text{force does not act} \to _\Delta v=0 \Rightarrow \text{Newtons I law}##
    Last edited: Sep 16, 2017
  7. Sep 16, 2017 #6
    According to the first law force is the reason for changes of motion. This causality doesn't follow from the second law. You can omit the first law if you have no problem with an identity of force and the change of motion. But that's not what Newton had in mind.
  8. Sep 16, 2017 #7
    Newton's laws involve the effect of an unbalanced (net) force on rest or motion. I don't think they define force. If I have a spring attached to a wall at one end and some kind of gauge at the other end and I pull on the spring, I can measure the force.
  9. Sep 16, 2017 #8
    ##a=\frac{F}{m} \to \frac{\partial v}{\partial t}=\frac{F}{m}##⇒change of velocity(motion) is proportional to force.
  10. Sep 17, 2017 #9
    That just means that force is a measure for the change of motion. It doesn't mean that force is the reason for the change of motion as the first law says.

    PS: Motion means momentum and not velocity. Newtons term for momentum is "motus" and his term for velocity is "velocitate". In the second law he used the term "motus". That means that force is proportional to the change of momentum but not necessarily to the change of velocity.
  11. Sep 17, 2017 #10


    User Avatar
    Gold Member

    ∫ F dt = Δ mv.
    ∫ F dx = Δ K D' Alembert
  12. Sep 17, 2017 #11
    I agree. Newton's II law only defines netforce.
  13. Sep 17, 2017 #12
    Force is basically the change of momentum per second.
  14. Sep 17, 2017 #13
    Netforce is change of momentum per timeunit.
  15. Sep 17, 2017 #14
    Or to be more exact, that.
  16. Sep 17, 2017 #15
    Force is equal to the corresponding change of momentum and netforce is equal to the total change of momentum per time unit.
  17. Sep 18, 2017 #16


    User Avatar
    Gold Member

    It seems that force is proportional to velocity with : ∫ F dx = ∫xxo mv dv/dx (dx) = ∫vv0 mvdv = 1/2mv2 - 1/2mv02 , Δ KE ≅ v - v0
    As opposed to ∫tt0 F dt = tt0 m dv/dt = ∫t0t d/dt mv = dp/dt
    F ≅ t - t0
    Last edited: Sep 19, 2017
  18. Sep 19, 2017 #17
    I don't know what you are talking about.
  19. Sep 19, 2017 #18
    Does this statement contain any information, that equation ##\frac{\partial^2 x}{\partial t^2}=\frac{F_x}{m}## doesn't?
  20. Sep 19, 2017 #19

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Law I establishes the equivalence of inertial reference frames. It is not, as many textbooks report in error, a special case of Law II. In modern physics it's the Principle of Relativity

    Law II defines force. For a particle acted upon by a single force, it equals by definition the rate of change of the particle's momentum. A definition that remains valid in modern physics.

    Law III, taken together with Law II implies conservation of momentum. In modern physics Law III is not valid, but that conservation law is.
  21. Sep 20, 2017 #20


    User Avatar
    Gold Member

    And also with ∫ F dt
    Edit: ∫ F dt = ∫tt0 dp/dt dt = Δp = mv2 - mv1
  22. Sep 20, 2017 #21
    Yes, it contains information about causality: According to the first law F is the cause for the effect d²x/dt². The second law just tells you that F and d²x/dt² are proportional if m is constant.
  23. Sep 20, 2017 #22
    I doubt that this statement is even meaningful. Could any experiment prove that? Newton's 1st law as worded in Wikipedia ("In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.") does not say anything about causality.
    Are you using uncommon definition of causality
  24. Sep 21, 2017 #23
    It can't be proved and it doesn't need to be proved because it is a definition.

    That's a modern replacement for the first law. The original wording can also be find in Wikipedia:

    "Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare."

    "Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed."

    That means that force (and only force) causes the change of the state motion of a body (which can be read as "closed system" in this case) - no less, no more. As this has no practical relevance, the original first law is often replaced by some kind of definition for inertial frames (like in the wording you posted above). The original version doesn't even mention inertial frames or frames of reference at all. Newton already introduced them in a previous chapter of the Principia.
  25. Sep 21, 2017 #24

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Yeah, think of Law I as providing a qualitative definition of force, it's the thing that causes an object to depart from a state of uniform motion or (equivalently) a state of rest.

    Law II provides a quantitative definition.

    This is by no means the only approach to understanding, but it is in my opinion the best.

    Once you go beyond that and refine your knowledge you appreciate the claim that a state of rest is equivalent to a state of uniform motion. This is the Principle of Relativity. There's no experiment that can distinguish between the two states so any distinction is purely artificial.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted