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Definition of The first law of motion

  1. Mar 20, 2012 #1
    Text book definition is "In the absence of forces, ("body") at rest will stay at rest, and a body moving at a constant velocity in a straight line continues doing so indefinitely".

    My thinking.
    Moon is orbiting earth in a circular path, and not in a straight line. Still that motion follows the first law

    So shouldn't the definition be

    "In the absence of forces, ("body") at rest will stay at rest, and a body in (any state of) motion continues doing so indefinitely".

    Non-linear motion also can be subject to 1st law. So why is the bolded terms used in text book?

    I am missing something, Please help me get it.
    Last edited: Mar 20, 2012
  2. jcsd
  3. Mar 20, 2012 #2
    "In the absence of forces". In case of the moon, the earth exerts a force( gravity) to the moon. Thus the velocity changes, orbiting the earth, due to gravity.
    It is the same force that pull Newton's apple to earth and "bend" the moon motion from going in straight line.
    Last edited: Mar 20, 2012
  4. Mar 20, 2012 #3
    I get thet gravity makes moon orbit earth, else it would have gone in a straight line away from earth.

    Gravity has acted and moon is now orbiting earth. Even in this state of circular motion, the first law will be applicable( though motion is NOT in a straight line)

    1)In the absence of forces, " a body in circular motion " also will continue its circular motion .

    2) In the absence of forces "A body in spinning motion" also will continue it spinning motion,

    not just the standard test book definition of

    In the absence of motion, "a body moving at a constant velocity in a straight line" will coninue its motion.

    So isn't the text book definition about only stright line incomplete?
  5. Mar 20, 2012 #4


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    welcome to pf!

    hi johsun123! hiazizlwl! welcome to pf! :smile:
    correct :smile:
    rubbish! :rolleyes:

    where did you get this from?​

    azizlwl's analysis is correct :wink:
  6. Mar 20, 2012 #5
    Re: welcome to pf!

    I used circular to mean orbiting motion of planets around sun or moon around earth.

    Moon is in a circular motion w r to Earth( due to gravity). Now unless it is acted upon by an asteroid or similar object, will it not continue it's circular(orbiting) motion?
    Last edited: Mar 20, 2012
  7. Mar 20, 2012 #6


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    but it's not "in the absence of forces" …

    it's in the presence of forces!! :smile:
  8. Mar 20, 2012 #7
    As tiny tim said azizlwl,s analysis is correct.You seem to have contradicted yourself johsun in that you state that "in the absence of forces a body in circular motion also will continue its circular motion" but you also indicate that there is no absence of forces "due to gravity"
  9. Mar 20, 2012 #8
    So, you cannot apply first law to a satellite oribiting earth, and which is already under influence of gravity?
  10. Mar 20, 2012 #9
    The second law describes satellite motion.For circular motion there is a resultant force acting towards the centre of the circle(the gravitational force for the moon,planets and other satellites) and the satellite accelerates in the direction of the resultant force.This acceleration is due to the satellite continually changing direction ie at all instants moving tangentially to the circle.
  11. Mar 20, 2012 #10
    Second law describes satellite motion. Got it.

    So can we say, satellite motion cannot be described by first law. If not, why not? Please explain.
  12. Mar 20, 2012 #11
    The first law describes what happens if there is no resultant force.The object continues doing whatever it happens to be doing.If at rest it will remain at rest.If moving it will continue moving in the same way,in a straight line and at steady speed.

    The second law describes what happens if there is a resultant force.It does not remain at rest or move in a straight line at steady speed and therefore it must change speed and/or direction.In other words it accelerates.
  13. Mar 20, 2012 #12
    Let assume your hypothesis is right. The moon is orbiting the earth due to 1st. law with added state of orbiting.

    Now let take a bigger picture, the solar system.
    Why the planets are orbiting at different rate, shown in Keppler's Law.
    Maybe they are also obeying "your" first law. How about if the earth as far as Jupiter from the sun. What speed it will take?
    But why there's correlations between orbiting period and mass of the planet.
    What they got to do with "your" first law that doesn't say anything about mass?

    Newton's 1st law does not mention mass. How big or small the object is, without resultant force it will never move or change direction.
    Last edited: Mar 20, 2012
  14. Mar 20, 2012 #13

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