![]() Is a reference frame inside of a freely falling elevator inertial?.Now consider a reference frame attached to a person floating inside the International Space Station. By definition, a reference frame in deep space is an inertial reference frame. an object initially at rest stays at rest and an object in motion maintains its velocity as long as the net force acting on it it zero. Recall that an inertial reference frame is one in which Newton's First Law applies, i.e. In the free-fall situation, if the vessel is small and the time the experiment is carried out for is small, then there is no way to distinguish between orbit, free-fall, and deep space. The previous experiments can distinguish between free-fall and either orbit or deep space but can't distinguish between orbit and deep space. This is because the balls fall radially toward the center of the earth, which makes their paths converge. Alternatively, if you release two balls from rest from the same height but separated horizontally, the distance between the two balls will decrease with time (if you wait long enough or the initial horizontal separation is large enough) only in the free-fall situation. Since gravity is stronger the closer you are to earth, the distance between the two balls will increase with time (if you wait long enough or if the initial height difference is large enough) in the free-fall situation but not the other two. Consider releasing two balls from rest, one above the other. There are experiments you can do to differentiate between the three situations (orbit, free fall, and deep space). Without gravity, you would expect a pendulum to swing in a full circle since there is no restoring force, and that is exactly what happens when the box is dropped. ![]() The video below shows this effect using a pendulum inside a sealed box that drops 22 m. Furthermore, if you were in a freely falling elevator, you would no longer be pulled to the floor and would feel weightless, which is another situation in which you are only subject to gravity. In the second case, the people are only subject to gravity. In the first case, the people are not subject to gravity at all. You may also know that the same is true for people in the International Space Station, which orbits the earth. We know that people float inside of vessels in the absence of gravity. ![]()
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