The Freefall Slinky: Misconceptions

1
The Freefall Slinky Misconceptions in
extended object motion
Keith Andrew Physics and Astronomy Western
Kentucky University
Do the experiment!
Take Physics Months of buildup as a Viral
Video and a staggering rush on sales as students
become transfixed watching an object fall to the
ground.
Reporter Derek Muller Producer Adam
Collins Researcher Derek Muller Camera Daniel
Shaw, Jeff Malouf Sound Grant RobertsEditor
Andrew GloverStory Contacts Assoc Prof Rod Cross
Physicist, University of Sydney
2
PHYS.ORG Viral Video Takes Over PHYSICS
Secrets of the 'Levitating' Slinky Viral web
videos trigger physicists to explore a striking
phenomenon
Physicists needs alter national Holiday Shopping
trends in Dec. 2012
While holiday shoppers search frantically for the
Moshi Monsters, LeapPad Explorers, or Lalaloopsy
Silly Hair dolls atop their children's wish
lists, many physicists remain engrossed in the
properties of a simple 1940s-era toy -- the
Slinky.
Costs less than an iPad!!
3
The Experiment
Release an extended slinky from rest so that it
is in freefall towards the ground Of course
everything near the surface of the
Earth accelerates at a constant rate of g9.81
m/s2
Dr. Derek Mueller and Professor Rod Cross
Asking the question How will the extended
slinky fall towards the ground? Predict
the overall motion of the Slinky. In
particular how does the bottom ring move?
4
What is a spring?
1. M suspended mass 2. m spring mass 3. Spring
has uniform constant mass density 4. Velocity of
each location on the spring is proportional to
the length 5. v- velocity of M located at L 6. u-
velocity of dm located at y 7. Length dy contains
mass dm
Physlet from Colorado Demos
Effective Mass of Spring
Empirical studies show that the factor of 1/3 is
only valid up to about 7m (AJP 1970, 38. pp98)
5
Some Data on Answers
Unofficial survey of 118 web based responses at
13 blog sites- i.e. Protons for Breakfast No
correction or direct knowledge of science
background of responders but a number, 17, were
known to be science students and teachers.
Two Categories of answers I did not look at
closely
1. Students worried about being tricked yet
again by their teacher
Physics Student to teacher well if you are
asking then it is surely not what I was
thinking..
2. Several asked to not anthropomorphize the
slinky .the Slinky knows what to
do. .the Slinky is smart enough to.. .the
Slinky coils are clever .
In some sense, many layers down, I worry a little
about what these responses mean about science
etc. but not today.
6
Answers from a Group in the Video
LadyOhhhh!Lady It wasnt what I
expected.Assoc Professor Rod CrossAnd then
Im going to drop the slinky. But I want you to
predict whats going to happen. Will the top end
fall first? Will the bottom end fall first? Or
both ends fall together? Or will the two ends
approach each other in the middle?Dr Derek
MullerThat is a tough question. When I let go
what does the bottom bit?BoyShoot
up.ManIts going to fall. Its gravity.Dr
Derek MullerIts actually going to
fall.LadyBottom goes up, top goes down.Girl
It might come up together. ManYoure going
see the top come down to the middle and the
bottom come up to the top.Girl Come to meet it
and then drop. Lady No, its the other
way ManThe top will accelerate faster than the
bottom.Boy I reckon that bottom will stay
there, this will come down to there and then
theyll both fall
7
Answers Collected in Bins
1. 41 Most Common answer the bottom of the
spring will move upward Especially for
students told to watch the bottom of the
slinky. 2. 18 It will all fall 3. It will
oscillate and fall and tumble and 4. Physics
fails in this case like the bumble bee. 5.
Well the spring constant is related to the
fall 6. The problem is once I see the video
it is right out of the cartoons, like Wylie E.
Coyote going over the cliff, he goes straight
across and then straight down.
8
Classroom Center of Mass
Discrete Masses Sum Continuous
Integrate Usually uniform and rigid
9
Classroom Extended Object Motion- for a rigid
object
CM Red Dot

1. Motion of CM and
2. Motion about CM

10
A Simplified Physics Model 6 Identical Masses
Connected by Identical Springs
Note springs are stretched by different amounts-
not uniform and not a rigid solid
11
Tracking Each Mass
Mathematica code is available at several sites
12
Longitudinal Wave Traveling along the Slinky
13
Slinky with linear density function
Is this what happens.
14
Conclusion
Do the Experiment- get a Slinky- bring it to
class.

1. Measure k of slinky
2. Did you hear it? Inelastic sound energy is a
   loss..
3. Measure longitudinal wave speed on Slinky AJP
4. How long can you make the delay?
5. Wait- there is a much faster torsional wave 6 x
   faster
6. Use a stiff spring and compare
7. The collisional wave is a nonlinear shock wave-
8. the top ring would actually Pass the bottom
   ring. Unruh.

1M. G. Calkin Motion of a falling spring Am.
J. Phys. 63 261 (1993) 2 See for example
http//en.wikipedia.org/wiki/Shock wave physics
and detonation physics (retrieved Oct 8,2010) 3
Unruh arXiv1110.4368v1
15
(No Transcript)