Kind: captions
Language: en
So in the spinning disc trick, we saw
that an asymmetrically weighted disc
when spun actually flips so that the
lighter side goes towards the bottom.
Now this is a variation on something
called the tippy top. A little spinning
toy that spins upside down so that its
heavy side actually lifts up. Now this
toy was played with by far more
accomplished physicists than me, people
like Neils Boore and Wolf Gang Pie. So,
I have a little confession to make that
when I made this spinning disc trick
video, I didn't actually know how it
worked. So, when I said I don't want to
give you the answer right away, what I
actually meant was, "Can you guys help
me out and figure out how this works?"
Now, in the comments, there were 25
comments pertaining to centrial force
and 56 comments related to centrifugal
force. So, clearly, I need to do a video
about those things. I think most of the
time you guys were thinking an
explanation like this.
Rotational systems tend so that their
moment of inertia increases. So I'm
thinking that as this starts spinning
like this and is uh wobbling around. You
think about there's a difference moment
of inertia of this thing spinning around
compared to at the bottom where there's
more of the meat
at the top up the top. And intuition is
that takes effort for the ice skater to
bring her arms in. Whereas just relaxing
it tends to increase moment of inertia.
Tribal force pushes it out.
Now an additional 52 comments mentioned
inertia betting that that's my answer
for everything. Unfortunately, I don't
think any of these things explains
really what's going on with the spinning
disc. And so I had to set out and do my
own investigations. So the first thing I
did was I talked to the smartest
physicists I know. What do you think
it's going to do if I if I rotate?
It's going to spin around and it's going
to turn upside down. So, the mass is up
the top. I know. I bug it if I know why.
[laughter]
I filmed the disc in 100 frames per
second and 300 frames per second slow
motion. And we labeled some points along
the disc so we could really see what was
happening. But even then, it was tricky
to figure out. I took it out to the ice
rink to see what role the surface played
in terms of the spinning disc. So, what
is my final explanation? it is that when
the disc is spinning, the first thing we
should notice is that the angular
momentum remains constant the whole
time. So when I spin it clockwise from
above, the angular momentum is pointing
vertically downwards. And that remains
the same even as the disc flips over. So
what is causing the disc to flip? Well,
if the disc wobbles a little bit, it's
off center. So its center of mass about
which it's spinning is not located
directly above the point of contact with
the surface. And so as the disc spins
there's some sliding friction which acts
to create a torque on the disc. Now that
torque reduces the angular momentum in
the three direction of the disc and
increases it in the one two direction of
the disc which causes the disc to turn
over like this. And once the disc goes
past that point, the frictional torque
at the bottom continues to rotate the
disc until it is vertical like that.
So it's really friction that is
responsible for flipping this disc over.
So the heavy part goes up. Now, there
was an excellent video response made by
Zog from Beetlejuice, so you should
definitely check that out. Why don't I
have any hair in that? Anyway, it is an
amazing and hilarious response which is
actually very physically accurate. So,
check it out when you get a chance. Now,
I have my doubts that this explanation
is really the whole answer because I
noticed that if I spin it above the
ground, the hole still seems to go
towards the bottom. That's kind of odd.
And when I took it to the ice rink,
well, sometimes the disc would behave as
expected. And sometimes it just did
strange things where the hole would go
down and then up and then down and then
up. I think maybe it's a sign of
procession or perhaps mutation, but
those are things for another video. So
until next time, keep spinning those
discs.