The goals of this experiment were:
- To use physics to analyze and explain the motion of a falling slinky
- To learn how to obtain motion data using video and video analysis software
As of March 16, 1999, videoclips of falling plastic and metal slinkies were taken, observations were drawn, and qualitative attempts to explain these observations, using physics, were made. There was not enough time by this date to determine the acceleration of the top part of the slinky in various drops. However, this project is not final. Accelerations will be determined from position data gained from the video analysis program VideoPoint, and high-resolution clips focusing on only the most important features of slinky motion will be made.
Sources of Error
Since this experiment was not quantitative, it is unlikely that parallax error, or having slinky motion not take place in the same plane as the distance scale, affected the quality of each videoclip enough for erroneous observations to be made. Wave formation, bounces, and motion of the top and bottom appeared to be easily resolvable in each videoclip. However, these are sources of error, and are included below amongst others:
- Manually dropping the slinky.
Sudden motions of the hand impart an initial velocity to the portion of the slinky being held, and releasing the slinky unequally can affect the way it falls. This was minimized in the best available manner by trying to keep the hand steady.
- Motion of the bottom of the slinky in vertical relaxed drops.
Because of the short drop time (less than one second), any strong, visible motion of the bottom may appear as if it is falling up or down, when it is actually supposed to be stationary. This was minimized by placing a hand at the bottom to quieten the motion as much as possible.
- In some of the vertical relaxed plastic slinky drops, there is a noticeable shift in view when going through the clips frame by frame.
This small shift is invisible when playing the clip at normal speed. Since observations were made by going through each clip frame by frame, a sudden shift in view of perhaps five centimeters (step through
, and notice the distance scale, which is marked at intervals of ten centimeters) will alter the accuracy of observations after the shift. This shift could have been caused by sudden movement of the camera, and because it could not be anticipated, it could not be minimized.
The distance scale not being exactly vertical. This would affect the accuracy of the scaling factor developed from this scale in VideoPoint. This was minimized by running a string from the bottom of the distance scale, after it has reached equilibrium, to the floor and securing it there.
Parallax distortion. The amount of error in determining distances accurately increases the further from the center of view one measures. Also, distortion of the distance scale will make the scaling factor developed in VideoPoint inaccurate. This was minimized by placing the camera near the opposite side of the room and using the telephoto lens to zoom in on the motion.
Slinky motion not occurring in the same plane as the distance scale. If the slinky falls in front of the distance scale, the slinky will appear larger, and if it falls behind the scale, it will appear smaller. If the slinky appears differently, positions will be measured differently, and this error will make acceleration calculations inaccurate. To minimize this error, the person dropping the slinky stood perpendicular to the plane of motion, making sure that the front of the slinky was in the same plane as the distance scale upon release.
Manual focus error. If the camera is focused on planes other than that of the distance scale, then slinky motion will be unclear, and the different parts of the slinky will be difficult to resolve. A forming longitudinal wave could be seen as just glare on the slinky from the overhead fluorescent lights. This error was minimized by zooming in on the distance scale at the camera's highest magnification, 16X, focusing on the scale, and zooming back out. However, since the scale is not at the center of view, the plane the camera is focused on will lie behind the plane of motion. This is because the distance between the scale and camera is greater than the distance between the plane of motion and the camera, if the scale is positioned away from the center of view.
Questions proposed before this experiment, and their answers, as found from the results of these experiments:
- When the slinky is hung freely at rest, then dropped, the bottom seems not to move until the entire spring collapses. Would this be so for all weights and elasticities of spring?
- It appears so. The metal slinky has 100 grams more mass than the plastic slinky (see Introduction, Slinky Measurements), but when held by one coil and dropped, the bottom remains motionless until the entire slinky collapses. Elasticity was not measured directly in these experiments, but qualitatively, the plastic slinky has more elasticity than the metal slinky. Differences in elasticity do not seem to matter for these two types of slinky; upon release, the bottom remains motionless until full collapse.
- Which parts of the slinky move first during a vertical relaxed drop?
- The top falls first, and the bottom last. This result holds true for both types of slinkies.
- What is the acceleration of the top in both the metal and plastic slinky drops?
- As of 3/16/99, accelerations have not been determined. However, the top always falls faster than g in the vertical relaxed drops.
A summary of observations:
Holding coils causes the location of the center of mass to shift along the slinky's length.
Holding more coils causes the center of mass to shift farther up the slinky, since more of the slinky's mass becomes concentrated at the top.
As more coils are held, the time available for longitudinal waves to be produced and to travel towards the bottom shortens.
Also as more coils are held, the acceleration of the top decreases towards g.
All of these things have been found by observation. This list of important observations brings about more questions than have been solved by this experiment, and further experiments will allow for better explanations to be made.
