Kind: captions
Language: en
hey everyone it's me Derek from the
channel veritasium and I've been
following this series by Destin on
smarter every day about helicopters and
gyroscopic procession is just one of
those things that still blows my mind as
it did Destin so I'm here at the
University of Sydney my old stomping
grounds I'm going to borrow some of
their lecture Dem equipment to hopefully
teach you a bit about how gyroscopic
procession
works the first thing I need to do is
talk about vectors because a lot of
things in physics are vectors that means
they have Direction so an example would
be momentum and force those are two
examples of some pretty common vectors
so if I have a cart moving along here it
has some momentum in that direction and
um if I apply a force to it I can change
its momentum so for example if I push it
to the right then its momentum will
increase to the right it all makes a lot
of sense well there's a similar
statement we could make about rotating
bodies which is that a torque increases
the angular momentum of an object in the
direction of that torque so let's try to
figure out those terms if I apply a
force down on this side of the wheel I
create a torque now torque is force
times the distance from the rotating
axis you could call it R so torque is
the force applied times the radius away
from the Turning axis now what's the
direction of that torque well we
actually use a right- hand rule to
Define this and so what you do is you
put your fingers in the direction of the
radius from the Turning axis
and curl them in the direction of the
force and your thumb points in the
direction of the torque so the torque is
actually out this way at 90° to the
force the force is down that way but the
torque Vector is actually pointing out
this way so the angular momentum of this
wheel is being increased in that
direction so the more I apply this
torque the more I increase the angular
momentum of the wheel in the direction
of the torque so I'm making this wheel
have a very large angular momentum out
towards the camera out towards you now
before I go any further let's have a
look at this setup that I've got I have
a wheel here hanging from only one side
by this rope and so right now there's
actually a torque on the whole system if
I let go because the weight of this
wheel is pulling down and that force is
applied at this distance away from its
pivoting point there so if I let go well
it does exactly what you'd expect the
wheel uh swings down this way but I want
to see that as an example of rotational
motion there's a torque which is
pointing out this way which is trying to
increase the angular momentum of the
system in that direction now angular
momentum in that direction requires that
the whole system starts swinging
anticlockwise and so that is what
happens but what happens if I only let
go after I've already spun up the
bicycle wheel well in that case the
bicycle wheel would already have angular
momentum this way and so a torque
pushing that way actually swings this
angular moment momentum round that way
okay so let me try to get the wheel
spinning and make it
work come
on look at that it's rotating as we
predicted see the angular momentum
Vector is pointing out this way but
torque is pushing it this way so angular
momentum torque and torque is pushing
that angular momentum Vector around
except not for long because this appears
to be quite frictional friction full
wheel so uh check out D's other videos
about helicopter physics and then come
check out my channel veritasium where
you can find more like this except I
don't often nearly get my head cut off
by a bicycle wheel
so