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!

Physics Newbie - Newtons 1st Law - Frames of References & Observers

  1. May 6, 2014 #1
    Hi All
    Let me start by saying that I am new to this forum and I am new to physics. I've always had an interest in getting an entry level understanding of Newton's Laws of Motion. I also apologize if these questions are a bit stupid or overly covered in other posts.

    I watched a Yale video lecture on Newton's Laws and as expected, I'm a little stumped.

    1. Inertia has to do with an object's resistance to stopping and it's desire to continue moving forward at the same speed, correct?

    2. What does a reference frame actually mean? In an example of a car accelerating, is the car the frame of reference? Does it matter whether or not the car is accelerating or just coasting?

    3. If I am watching the car go past me, I am the observer?

    4. Am I correct in that an object obeying the 1st law is an inertial frame of reference and an object not obeying it is a noninertial frame of reference?

    5. When a car moves after being at rest, the dice hanging from the rear-view mirror swings backwards, this is noninertial, correct? Is the dice it's own frame of reference? Does the same apply when the car takes a bend and the dice swings to the side?

    I'm struggling to understand the relationship between inertial/noninertial references and observers.

    Any help would be great

    Please remember that I am a noob to all of this.

  2. jcsd
  3. May 6, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    Inertia is the name used for a particular property of matter: matter in motion tends to stay in motion, in a straight line, unless an external force is applied. Galileo stated an early form of this, which Newton generalized.

    A reference frame is where the observer is standing and carrying out measurements; the lab frame is your laboratory, with a set of coordinates defined by you, the experimenter.

    Clearly you can define many different reference frames; Newton tells us that they are all equally good if they are "inertial reference frames": all of the "laboratories" are undergoing inertial motion, with no external forces.

    Thus when you watch the car go by - and take some measurements, perhaps with a stop watch and some markers - then you are the observer, and the local ground is your laboratory. But you could just as easily be on a boat sailing by, floating with the current down a river: then you are moving wrt the ground, but your reference frame is still valid!

    Thus your statement (4) is correct; as is statement (5).
  4. May 6, 2014 #3


    User Avatar
    Science Advisor

    To a reasonable approximation and speaking classically, a frame of reference is a cartesian coordinate system with three dimensions of space and one of time. It allows you to specify where any object is with the three spatial coordinates and when any event occurred with the one time coordinate. [More formal definitions of frame of reference abstract away some of the details that go into defining a coordinate system]

    If you have such a coordinate system already set up then it is "inertial" if objects that are not subject to external forces obey Newton's first law.

    So if you have an object that obeys the 1st law _as judged using a particular frame of reference_ then that frame of reference may plausibly be inertial.

    If you have an object that is not subject to any external forces, that object can be used to define the origin of a coordinate system. As long as the coordinate system does not rotate, it will be inertial.

    If you take the car as defining your frame of reference then it defines a non-inertial frame. The die is subject to no net external force and yet it accelerates. In both cases -- the car moving after being at rest or following a bend at speed the resulting defined frame of reference is non-inertial.

    Saying that a car "is" a frame of reference or that the die "is" a frame of reference is incorrect. It is also incorrect to say that an object "is in" a frame of reference.

    It is more correct to say that you are "using" or "adopting" a particular frame of reference to describe a particular set of observations. You do this by expressing your measurements using coordinates from the chosen frame of reference. The same set of measurements could be expressed differently depending on what coordinate system/frame of reference is used. For instance, you could say that the die accelerates (using the car-centered frame of reference) or that it does not accelerate (using a ground-centered frame of reference).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook