(Describer) A man stands on a turntable and holds a spinning wheel.
When I hold
the spinning wheel,
(Describer) ...sideways.
nothing crazy happens.
But when I flip it,
I spin,
(Describer) He does.
(Describer) ...on the turntable.
But back...
and I stop.
Flip it, I spin.
(Describer) He does it again.
So the question is,
"Why am I spinning?"
(Describer) He spins clockwise, the wheel counter-clockwise.
I have to force the wheel
to turn over.
(Describer) ...to clockwise.
Newton's third law says
for every force,
there's an equal
and opposite force.
So when I force
the wheel,
(Describer) ...flipping it.
the wheel is applying force
equal and opposite onto me,
causing me to turn.
(Describer) He gets off the turntable.
The same applies
when I'm on the ground,
but friction between
my shoes and the ground
stops me from spinning.
Another way to explain
what's going on is through...
Now, angular momentum
tells you how hard it is
to either start or stop
something from spinning
and depends on two things.
One: How fast it's spinning.
We call that the...
Two: Its shape,
or what we call the...
So this wheel is rotating
counterclockwise.
Here's a trick.
Get your right hand
and curl your fingers
in a counterclockwise direction.
You see that your thumb
is pointing upwards.
We'll call that
the positive direction.
So, see, this wheel has
positive angular momentum
with a magnitude of L.
Angular momentum is conserved.
As long as there are
no outside forces,
the total angular momentum
of the system stays the same.
When you flip this over
and do the right-hand trick,
your thumb points down
or in the negative direction.
The angular momentum of
this wheel is now negative L.
So the whole system--me,
the turntable, and the wheel--
starts out at positive L.
But when you flip the wheel,
the wheel now has negative L.
(Describer) ...and he turns...
The whole system
needs to stay at positive L
because of conservation
of angular momentum.
So I need to spin with
angular momentum of positive 2L.
That's why I spin
the other way.
(Describer) Title: Made with love at MIT.
Accessibility provided by the U.S. Department of Education.
Accessibility provided by the
U.S. Department of Education.
