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Inertial Frame |
By definition, an Inertial Frame is a frame of reference in which
the law of inertia applies.
Therefore, at least in principle, it is easy to identify an inertial frame. All
you have to do is isolate an object from all gravitational, electromagnetic, and
nuclear forces (the only forces known to exist in nature) and then watch its
motion. If it remains at rest or continues moving in a straight line at
constant speed, you are in an inertial frame. If it accelerates, you are in a non-inertial frame.
Better yet, release several objects at rest and watch their motions. If all
remain at rest, you are in an inertial frame. If some remain at rest and others
accelerate, you are in a non-inertial frame.
Clearly a rest frame, should it exist, would be an inertial frame. And any
frame moving in a straight line at constant speed relative to such a rest frame
would be inertial. But any rotating frame, moving or not, would be
non-inertial.
Q1. Inertial frame.
Which of the following frames of reference most closely approximates an
inertial frame? (A) a non-spinning spaceship drifting in
interstellar space, (B) a spinning spaceship drifting
in interstellar space, (C) a non-spinning spaceship
orbiting the earth, (D) a spinning spaceship orbiting
the earth, (E) a spaceship sitting on the surface of
the earth.
Q2. Non-inertial frame.
Four non-coplaner objects are released at rest inside a spaceship. Two of the
objects remain at rest. The other two drift slowly away from the first two.
What conclusion can you draw? (A) The spaceship is located
in a gravitational field. (B) The spaceship is spinning.
(C) The spaceship's rocket engines are causing it to
accelerate. (D) The spaceship constitutes an inertial
frame. (E) The spaceship may be entering the atmosphere
of a planet and slowing down.
Q3. Free-falling frame.
Four objects are released at rest relative to a spaceship freely falling
vertically downward toward the earth. One object is above the spaceship,
another below, and the other two horizontally on opposite sides of the
spaceship. If the objects are located relatively close to the spaceship, they
will all fall with the spaceship. Therefore, the spaceship frame will be
locally inertial. However, if the objects are very far from the spaceship,
their motion will differ from that of the spaceship. In the spaceship frame,
how will the objects move? (A) All four objects will move
away from the spaceship. (B) All four objects will move
toward the spaceship. (C) The vertical objects will
move away and the horizontal objects will move toward the spaceship. (D) The vertical objects will move toward and the horizontal
objects will move away from the spaceship. (E) None of
these answers is correct. The motion is more complicated than that.
Q4. Which of the following is a
legitimate frame of reference for special relativity? (A) Any coordinate system
used by any observer. (B) An inertial frame. (C) A non-inertial or accelerating
frame. (D) Two of these. (E) Three of these.
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