Kind: captions
Language: en
in 2014 the qualifying exam for the U.S
physics team had this as question 19. a
helicopter is flying horizontally at
constant speed a perfectly flexible
uniform cable is suspended beneath the
helicopter air friction on the cable is
not negligible so which of the following
diagrams best shows the shape of the
cable as the helicopter flies through
the air to the right is it a hanging
straight down
B hanging diagonally to the left C this
hook shape D the inverted hook shape or
E kind of s Bend now apparently there's
been a certain amount of controversy
about the correct answer to this
question so today we're going to go up
in this helicopter and put this question
to rest once and for all
let's go
a portion of this video was sponsored by
Simply Safe which allowed us to rent
this helicopter more about them at the
end of the show
I've got several thousand hours of
carrying sling locks firebuck it's
concrete buckets doing Powerline
construction actual power poles you you
name it anything needs to be moved
around the mountains I've done biggest
concern that I have is that the rope's
going to interact with the rotor wash as
it's interacting with the ambient air
around the aircraft and we'll get a
whipping action that falls down the rope
and flips around on the end to a point
that we worry if it will have a tendency
to work its way back up towards the
aircraft obviously we don't want it
getting in the rotors or the tail rotor
all right we're all set
up
[Music]
here I'm deploying a battle rope like
you'd see at the gym this one is about
15 meters long and it weighs 20
kilograms
happy and win
now the setup looks pretty simple but
few people agree on what the right
answer should be
when I pulled YouTube the most common
answer was C
well done on making that beautiful bell
curve by the way
we got in touch with the questions
author Professor Paul Stanley there were
some creative students who actually
constructed their own homemade scenarios
one had a fan to the side and another
fan blowing downwards so that they could
mimic the motion of the helicopter and
suspended a string and said oh it's this
design and the faculty members looked at
it then said oh obviously I can prove
that the answer is this answer
they just didn't agree with each other
if we approximated as a Chinese digit
link I don't think you can just guess
what do you think I think more you're
more likely to be safe I think it is
D my guess is B what do you think B I'm
gonna go away interestingly no one chose
e
locked in your prediction
to make sure the Rope doesn't come up
into the rotors of the helicopter pilot
Craig wanted to keep the Rope on our
right side so he could keep an eye on it
pretty good I'm trying to make it look a
bit I mean I'm really working on it
you're working hard
so we're not going straight forward
we're actually going diagonally forward
and to the left
but you can clearly see the Rope is
hanging straight diagonally to the left
so the correct answer to the question is
B
we're going to try a few more
experiments adding a weight to the end
of the rope and then a parachute but
first I want to discuss why the answer
is B there are two external forces
acting on the Rope gravity pulling it
down and air resistance to the left when
flying along at constant speed these
forces must be perfectly balanced by the
tension in the Rope now when I set out
to do this experiment I wondered if the
Rope would be affected by the air pushed
down by the helicopter's rotor but
judging by our results this was not the
case the rotor wash doesn't extend down
below the helicopter all that far it
dissipates pretty quickly so you can
consider the air resistance on the Rope
as entirely due to its motion through
still air
imagine dividing the Rope up into many
short sections each section has the same
weight and experiences the same amount
of air resistance because it has the
same cross-sectional area and it's
moving at the same speed now the tension
in any section of the Rope must balance
the sum of the air resistance and weight
of all the sections beneath it so the
tension is zero at the bottom of the
rope and it increases linearly up to a
maximum at the top you can see the
tension is small at the bottom of the
Rope that's why it Wiggles around while
the top is much steadier now although
the magnitude of tension changes
throughout the Rope its direction
doesn't and that's because the ratio of
air resistance to weight is the same at
every point along the rope that is why a
uniform flexible cable hangs in a
straight diagonal line when pulled at
constant speed by a helicopter if the
helicopter flies faster the angle of the
Rope changes but it still makes a
diagonal straight line because the ratio
of air resistance to weight is still
constant along the entire length of the
Rope
but this got me wondering what would
happen if we added a weight to the end
of the Rope here I have a 20 pound
that's a eight or nine kilo kettlebell
and I want to ask you to make a
prediction what shape do you think the
Rope will make now will it still be that
diagonal or will it be one of the other
five options one more question for you
if we checked a weight at the bottom
which shape would it be probably the
View
or possibly does that change it then I
think it would be B
itch easy to do on the ground
[Music]
all right we're dropping the Rope over
the side this one has a 20 pound
s how's that feel when it's dangling no
lateral gets flexible at all
all right how's your forearms doing I'm
feeling good you know I feel like this
is the easy way obviously
it's gonna feel different when I gotta
pull it up
okay let's give it a shot
here the helicopter is flying at nearly
a hundred kilometers per hour
and the Rope makes a different shape
[Music]
you can clearly see it looks like an
inverted J this is option D in fact this
is how the question on the qualifying
exam originated so I taught for a
semester in Hong Kong and would go
hiking in the new territories one of the
times I was hiking I saw a helicopter
fly with a cable beneath it it was
carrying something to one of the remote
parts of the Hong Kong new territories
and I saw the shape of the cable and
thought to myself that looks a little
counter-intuitive and very neat this
might make a good multiple choice
question for the selection exam for the
students the cable that I saw had a
weight hanging on the end which gave it
a certain curved shape when I shared
this with the other coaches another
Coach Andrew Lynn looked at it and said
I think the question might even be more
fun if there is no weight on the cable
because that is even harder to imagine
to understand why the Rope makes this
shape we can use the same analysis as
before but now we need to add a large
weight to the end of the rope and this
means at the bottom the tension needs to
be almost vertical to support the weight
of the Kettlebell which has a lot of
weight but not much air resistance
as you go up the Rope the ratio of total
air resistance to weight for everything
beneath increases so the Rope turns more
horizontal in order for the tension to
balance out that increasing air
resistance
for the final test I wanted to add
something to the bottom of the rope that
added almost no weight but significant
air resistance
so I picked a veritasium flag of course
so what do you think about hanging this
at the end of the Rope well you know
it's all science experiment for me
because these are all the things you
never do with a helicopter in a rope
what would be the real risk like tail
rotor or this rotor or both
either any kind of this in the main
Hotel rotary would be a destruction of
the aircraft
[Music]
[Applause]
[Music]
now the Rope still seems to be pretty
straight I think it's because the flag
isn't actually adding much drag
they look pretty straight interestingly
so we decided to add a small parachute
to the end of the Rope all right let's
try it
the concern here was that the parachute
could get flipped up into the rotors
during deployment so we bundled the
shoot up into a backpack
very nicely done
does it deploy more than that no not
once we start living it's gonna drive a
lot of hair
now with the parachute at the end of the
Rope it makes a j shape which is answer
C
with extra air resistance at the end of
the Rope but not much weight the tension
has to be virtually horizontal and then
as you go up the Rope the ratio of total
air resistance to weight for everything
beneath decreases so the Rope becomes
more vertical to balance out the
increasing weight so depending on what's
on the end of the Rope you could get
answers B C or D
now if you enjoyed this video I bet you
would also like my bullet block
experiment Series so check it out
now hanging out the side of a helicopter
was not my safest moment but I do always
feel safe here at home thanks to
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I want to thank you for watching