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学習統計
CEFRレベル
難易度
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[Music]
hey Vsauce Michael here do you want to
see the most illegal thing I own it's a
penny from 2027 that's right it is a
piece of counterfeit US currency or is
it there are no 2027 pennies today which
means that this is a counterfeit of an
original that doesn't exist yet
I mean sure if you didn't look at the
date this could pass is real but will it
not truly be counterfeit until 2027 well
what we do know is that it's novel team
has a lifespan it's really cool to show
people today it's a penny from the
future but in 2027 it will become
indistinguishable from a common penny
and it will cease to be as interesting I
cannot tell you where I got this but the
point is I can break the law I am free
perhaps not free from punishment but
nonetheless here we are I cannot break
the laws of physics
nothing can why today I want to look at
the lawful behavior of spinning let's
start with a physics classic as a
spinning ice skater pulls their body in
closer they spin faster and faster it's
really cool but what is accelerating
their rotation what is pushing them
around faster and faster you can try
this at home with a chair that spins and
some really heavy books in both hands
now to get started spinning I'm gonna
have Jake give me a push
I don't need Jake no no watch this with
him gone I can nonetheless speed myself
up by just pulling the books in if I
move the books out I slow down somehow
pulling the books in speeds me up around
now if you look up why this happens
you'll probably be told that it is
because of the conservation of angular
momentum
oh the conservation of angular momentum
I love demonstrations of it and this is
one of my favorites this is a Hoberman's
sphere a toy that collapses and expands
doesn't it sort of remind you of me with
the books far from my body and in the
books close to my body and it behaves in
much the same way I need to give it some
angular momentum to start but once I do
if I bring all of that mass towards the
axis of rotation it speeds up and then
it slows down and then it speeds up and
then it slows down speeds up slows down
I love this thank you
conservation of angular momentum but
what are you well if you look it up this
is what you will find for a particle in
circular motion around some center of
rotation see the angular momentum of the
particle is defined as the product of
the particles mass the particles
instantaneous velocity B and the
particles distance from the center of
rotation are M V R this is angular
momentum if you divided me and the chair
and the books I'm holding into a bunch
of tiny little particles and found the
MV ours for every one of those particles
and summed them all up you would have
the angular momentum of the entire
system of me the chair and the books but
notice that this is just a mathematical
expression combining three different
measures it's not a physical substance
you could pull out of a particle and
hold in your hand or put in a jar and
study it's kind of like oh I don't know
taking like my weight and multiplying it
by the number of countries on earth and
then multiplying it by the time however
unlike something like that
this concept is really useful because we
have found that in our universe over
time it is conserved if the particle is
pulled towards the center of rotation
its our value will go down but angular
momentum is conserved so one of these
other variables must go up well at low
speeds mass is essentially constant so
the only option is for the velocity of
the particle to increase for the
particle to spin around faster than it
was before if the R value gets smaller
the velocity must get larger or else a
law has been broken but how do atoms and
molecules know to follow this law I mean
is there some kind of physics police
force in the universe
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