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Homework Help: Inertial Reference Frames and newtons laws

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data

    I am a bit confused about what are inertial reference frames and what is not. The text states:

    "We define an inertial reference frame as a reference frame in which Newton's laws are valid.... Accelerating reference frames are not inertial reference frames. Consequently, Newton's laws are not valid in a reference frame attached to an accelerating object. ...... In Chapter 4 we defined inertial reference frames to be those reference frames moving with constant velocity."

    Would someone mind explaining the reasoning to these answers a bit better, and make sure my reasoning is correct.

    3. The attempt at a solution

    Are the following inertial reference frames? Yes or No.

    A car driving at a stead speed on a straight and level road. ~ Yes I understand this one because the car is moving at a constant velocity so it is an inertial
    reference frame.

    A car driving at a steady speed up a 10 degree incline. ~ Yes
    The car is moving at a constant velocity so it is an inertial reference frame.

    A car speeding up after leaving a stop sign. ~ No
    The car is accelerating so it is not an inertial reference frame.

    A car driving at steady speed around a curve. ???
    I found the answer to be no, but I do not understand why. If it is moving at a
    constant speed shouldn't it be an inertial reference frame.

    A hot air balloon rising straight up at steady speed. ~ Yes
    Constant velocity so inertial reference frame.

    A skydiver just after leaping out of a plane. ~ No
    Not positive, but I think on this one its just because they are accelerating in free fall.

    The space shuttle orbiting the earth. ~ ???
    I know the answer is no, but I am not sure exactly why.
  2. jcsd
  3. Oct 30, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    NO. It's the velocity that has to be constant, not just the speed. If it's moving around a curve, it's accelerating. Accelerating reference frames are not inertial. Are Newton's laws obeyed in accelerating reference frames? No. When you're going around a turn, there seems to be a mysterious force that throws you towards the outside of the turn. Where the hell did it come from?

    Yes. Accelerating reference frames are not inertial.

    For the same reason as the car going around the curve
  4. Oct 30, 2008 #3
    Thank you so much!!
  5. Oct 30, 2008 #4
    This is a bit of an unfortunate question: if you know too much (namely general relativity), then you'll answer that a body in free fall is THE example of an inertial frame.

    But in Newtonian physics you'd indeed say you're accelerating so it's certainly not an inertial frame!
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