A steel mass hangs by a spring from the inside of an upside-down teflon beaker, as seen in the photographs below. The photograph at the right is looking up into the beaker to see how the spring is attached.

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The beaker will be held in the air and released from rest. Of course, the entire system will fall vertically downward. But the question this week involves what the steel weight will do relative to the beaker immediately after it is released.


The beaker might act like a parachute and catch the air, so the weight would then extend the spring a bit more after it is released. On the other hand, perhaps because the teflon beaker is less dense than the steel weight, the beaker will accelerate faster and the weight will move up into the beaker. Or perhaps the entire system will simply fall downward in the same configuration - that is, with the weight extending out the same distance that is shown in the still photograph. Or maybe the logic here is entirely dorked up.


When the beaker system is released, the weight will:


  • (a) extend the spring further.
  • (b) move into the beaker.
  • (c) remain in the same configuration as it falls.


After May 16, 2014, click Read More for the answer

The answer is (b): the mass will move up into the beaker immediately upon its release, as seen in an mpeg video by clicking your mouse on the photograph below. The video has been edited to move at one fifth speed to facilitate observation of the effect. Here is a video at full speed but a bit blurry. You will have to view this one frame-by-frame.



When the beaker is released it begins to fall downward with the acceleration of gravity, a state described by physicists as "weightlessness in free fall," or "apparent weightlessness in free fall." The beaker is not actually weightless; its weight is the force that accelerates it downward. However, this means that for an observer in the frame of reference of the falling beaker everything appears weightless, so there is no longer any force pulling on the spring to extend it. The spring therefore pulls the mass into the beaker.

This is the reason why things "appear" weightless in satellites moving in earth orbit. Using a rocket they are given a velocity appropriate for a stable orbit at the desired height above the earth and parallel to the surface of the earth. The gravitational force then supplies the centripetal force necessary to cause the satellite to move in circles around the earth. This is equivalent to saying that the satellite is falling toward the center of the earth, so that in the frame of reference of the satellite everything is "weightless."