Kind: captions
Language: en
[Applause]
this video contains the answers to my
four revolutionary riddles so if you
haven't seen the riddles yet you should
probably watch them before you watch the
answers it's okay I'll wait just click
this card up
here now when I filmed the riddles I
also filmed the solutions at the same
time but that was before I received your
15,000 comments and dozens of video
responses so I'm re-shooting parts of
this solutions video to incorporate the
results I saw in the comments and to use
some of your video responses to help
explain the solutions let's get to it
okay by looking at the comments uh for
number one over 15% of you said a
cylinder containing sand now uh this
contains some powder see how it
rolls it uh it doesn't seem to roll very
far before it stops and then it won't
roll again because I think the sand just
kind of levels off in there uh maybe you
were thinking a bigger kind of sand like
this uh small grally stones let's try
that that actually rolls pretty well
nearly 25% of you said a a cylinder half
full of water so let's try
that this rolls very well um so it's not
water uh and nearly 45% of you said a
cylinder half full of a viscous liquid
here I have a half full container of
honey so let's see how it
rolls that's not bad it's rolling and
it's it's
stopping and it's rolling some
more this is a pretty good guess and I
think the behavior is not exactly the
same as the mystery cylinder but it
definitely is similar and that's no
coincidence uh the mystery cylinder
actually contains honey and pingpong
balls there are two pingpong balls
submerged in this honey so if I place
that on the ramp the center of gravity
is uh not above the point of contact
with the ramp and so it rolls forward
but now because those pingpong balls are
in the front uh they change the center
of gravity and so it's exactly under the
point of contact and so it stops briefly
but then as the viscosity of the honey
allows those ping pong balls to move up
the center of gravity shifts forwards
again allowing this little container to
roll so that is the trick of the mystery
cylinder pretty easy if you want to try
it out at home now I challenged you to
run two laps of this track where the
first lap you could go as slowly as you
like but the second lap you had to go
much faster such that your total average
speed was twice the speed of your first
lap now when I was first asked this
question by Simon Pampa it took me a
long time in scribbling on paper and
just something didn't seem to work out
and that's because you can't actually do
this it's impossible I mean you might
think uh I could run 3v1 for my second
lap and that would mean my total average
speed is 2V1 the problem is you can't
just add the two velocities together and
divide by two because you spent much
more time in your first lab so that
velocity is weighted more heavily into
the average so you'd have to run well
impossibly fast let me explain the
velocity of the first lap was the
distance around the track divided by T1
the time it took you now if you want
your total average time to be twice V1
well then it needs to be 2D over T1 you
need to run twice the distance in the
same amount of time as it took you to
run the first lap but you've already run
that first lap and so you have no time
remaining to run the second lap even if
you went the speed of light you would
not be able to increase your total
average speed up to twice the velocity
of your first lap it is just
mathematically impossible so this may
seem like a bit of a trick question but
the point to me is how doable it sounds
how it seems like something you should
be able to do but you can't it's it's
actually impossible riddle number four
the question about the train was
actually answered pretty well with most
people mentioning something to do with
the wheels but of course that makes
sense in a series of riddles which are
about rotation rotational motion uh some
people though did point out that maybe
it was the steam that was going
backwards or maybe air molecules in the
train and that is actually a pretty
clever point however I wouldn't really
consider the air in the train part of
the train uh so indeed the part of the
train that is moving backwards is the
flange part of the wheel which is below
the rail that is the part of the train
which is moving backwards to understand
why you just think about a spinning
wheel the top of the wheel is moving
forwards at speed 2v and the bottom of
the wheel is not moving forward at all
it is stationary with respect to the
track and that is what we call rolling
without slipping and that's how most
Wheels work at least that's how they're
designed to work now in the case of
trains they have to have flanges so the
train doesn't fall off its rails but of
course when these pieces come around
during the rotation of the wheel they
actually extend beyond the rail and
therefore they are going backwards with
respect to the ground so the part of the
train that's moving backwards is always
changing but it's always that part of
the flange that part that extends beyond
the wheel that is below the level of the
track so what happens when you pull the
bottom pedal of a bike backwards well
about 45% of you thought that the bike
would move backwards about a quarter
said it would move forwards and a
quarter said the bike wouldn't move move
at all and 5% said it depends on
something so let's give it a shot and
see what happens I'm going to pull
backwards on the bike pedal in three two
[Music]
one wa the bike did indeed move back and
for virtually all bikes this is what you
will find but the explanation is not
just as simple as well the net force on
the bike is back so therefore it has to
accelerate backwards and to prove that
that logic doesn't work we'll just have
a look at this video by George Hart
watch this again I pull the same pedal
backward but now the bike moves forward
I'll put a link to the full video here
and a link to his website in the
description so the reason the bike moves
backwards is because of the way these
gears are set up the diameter of the
tire and also the distance on this crank
to the pedal itself because as a bike
moves forwards the pedal even when
you're pushing back on it never actually
moves backwards with respect to the
ground it's always moving forwards so if
you drag a string behind a pedal of a
bike moving forward the string is always
moving forward now just play that movie
backward in your mind and it may be
clear how pulling the string backward
could make the bike move backward they
move forward together so they move
backward together another way to think
about this is to consider the path
traced out by the pedal as the bike
moves forward this is is called the
troid for all ordinary bikes the pedals
are moving much slower than the tires so
the pedal is always going forward with
respect to the ground but George
modified his bike so the ratio of the
pedal to the wheel radius was greater
than the ratio of the front sprocket to
the back sprocket and this ultra low
gear changes the troid so the pedal does
actually go backwards with respect to
the ground as the bike goes forwards and
that's why he could pull back on the
pedal and make the bike go forwards this
is the same reason why if you were to
pull backwards on the flange of a train
wheel you could actually get the train
to move forwards if you pulled backwards
with enough Force for all normal
bicycles pulling back like this on the
bottom pedal will cause the bicycle to
move backwards but depending on the gear
ratio you can get the bike to move
forwards so it was those people who said
it depends on the ratio of these gears
and the size of this crank to the radius
of the back wheel that were actually the
most
correct