science with bobert's video: does a falling slinky break the laws of motion a science with bobert video short

Now this is really strange. Have you ever seen a spring with weights dropped in slow motion? Check this out! The bottom of the spring doesn’t move! It appears to defy gravity! “voice: That’s impossible! everything is supposed to fall at the same rate! What’s going on here?” Check out the slinky: it’s the same thing! the bottom of a spring remains in place and doesn’t fall until the top of the spring reaches it! Is this breaking Newton’s laws? (voice: better not be!) Nope! it’s not. We really should not say “all objects with the same air resistance fall at the same rate”, what is better and closer to reality is to say “all centers of mass of objects with the same air resistance fall at the same rate!” It is the center of mass of an object that is accelerating toward the ground. Paying attention to the center of mass in a free falling object is especially important when dropping a slinky. The center of mass of an unstretched slinky is indicated by the black dot. When an unstretched slinky is dropped it is very easy to see the bottom of of the slinky falling immediately from the initial drop as this slow motion video illustrates…like all objects the center of mass of an unstretched slinky accelerates as it falls toward the ground as indicated by the accelerting black dot in the video. (show video again but with black dot as many times as necessary to cover words…..) Now, how about the dropped stretched slinky? This is where the center of mass is for a hanging stretched slinky. When this slinky is dropped, notice that as the center of mass accelerates toward the ground ONLY the top springs decompress, the bottom springs remain unchanged! the center of mass falls due to gravity until it reaches the bottom of the slinky and then the entire slinky falls just as any uncompressed slinky falls! It is really interesting to synchronize the slow motion video for dropped stretched and unstretched slinkies. Should they both strike the ground at the same time? In the absence of air, they would however air resistance is a lot higher for the stretched slinky because it has a much larger surface area than an unstretched slinky! The air resistance of stretched and unstretched springs is more similar so when their slow motion videos are synchronized we do observe the effect of air resistance as much as we do with the dropped stretched and unstretched slinkies. (spring sound….) review let’s review. - the center of mass of an object is the “balance point” and is always located directly in line with the support - an understanding of center of mass helps us balance objects in peculiar positions - in light of free falling springs and slinky’s, the statement “when air resistance is the same, everything falls at the same rate” does not seem to fit what we are seeing! - regarding ALL free falling objects, it is closer to the truth to say “when air resistance is the same, all objects’ centers of mass fall at the same rate” and then no matter how peculiar or complex the free-falling object is,…. we have correctly described the motion!