Kind: captions
Language: en
in my last video I showed you five fun
physics phenomena and asked you how they
work you responded with thousands of
comments and some video responses well
here are my explanations let's start
with the cereal because it seems the
simplest but it turns out to be one of
the most surprising the simple
explanation goes like this so I showed
you cereal is magnetic I have ground up
all of this cereal into a very fine
pattern let us see what is in it I'm
going to take these strong magnets and
run them over the cereal powder and see
if anything is attracted to them look
how the cereal actually sticks to the
magnet and it's because there's pieces
of iron in this cereal they've been
added because iron is something we need
it's an essential nutrient that we need
to survive in fact this cereal has 60%
of your recommended daily intake of iron
but there's more to it I received a
video response from Martin be showing
non-magnetic objects like plastic and
paper also apparently being attracted to
a magnet so how does this work well
water is diamagnetic which means in the
presence of a magnetic field it
generates its own magnetic field in the
opposite direction this means the water
is very slightly repelled by the magnet
and this causes a depression in the
surface of the water into which a
floating object will slide you can even
see this depression by looking at the
reflections off the water
So Cal is attracted to a magnet due to
its iron content but when floating on
the surface of water there is an
additional effect the depression of the
water surface due to its
diamagnetism I showed you that you can
find the center of mass of a Cane or
another stick type object just by moving
your fingers in towards the middle from
the outside but how does this work even
when you start in say an asymmetric
position well one finger is closer to
the center mass and therefore it carries
more of the weight of the cane and so
the friction force between your finger
and the cane is greater until the point
where the other finger catches up which
point this finger slides in and
eventually they must meet in the middle
so this is a way you can find the center
of mass of any cane or Canik
object I showed you that if you try to
flip your phone end overend there is no
way to do it without it also rotating
around the short axis as well well why
is that the phone has three axes about
which it can rotate there is the long
axis which has the maximum moment of
inertia meaning it requires the most
torque to accelerate it in that
direction spinning about the short axis
has the least moment of inertia then
there is the intermediate axis which has
a moment of inertia in between the other
two now the intermediate axis theorem
says that if you try to flip any object
along its intermediate axis it will not
maintain simply that rot rotation it
will also get rotations in any of the
other directions that is if there is any
slight deviation from a perfect rotation
so why does this happen well the
mathematics is kind of complicated but
it's similar to the mathematics of a
rigid pendulum so if you're flipping the
phone along its long Axis or its short
axis the phone acts a little bit like
this pendulum in that any perturbation
will cause it simply to go back to where
it was before but if you're flipping it
along its intermediate axis it's as
though you're trying trying to balance
the pendulum on its end in which case
it's very unstable and any slight
perturbation may cause it
to exponentially increase so that is why
you can't just flip your phone along its
intermediate axis without it also
spinning along one of the other axes I
showed you that an electrically charged
object can deflect a stream of water but
it is not due to the common explanation
the common reason which is given which
is that water is a polar molecule
so what really is causing this water to
be attracted towards the cup well it is
charges but it is ions it is dissolved
ions in the water there will be some o
ions some H+ ions and there'll also be
some other impurity ions in the water so
what happens when you hold this
negatively charged cup up against the
water stream is it will repel the
negative charges the negative ions in
the water some of which will go back up
into the tap and that means the water
coming down will be slightly positively
charged and once it breaks up into
droplets those droplets have a positive
charge that they can't get rid of so now
those positive droplets are attracted to
the negatively charged cup and you can
see those droplets swirling around the
cup because they are so attracted to it
so this is not actually a very good
demonstration of the Polar nature of
water even non-polar substances with
some ions dissolved in them will deflect
in exactly this way so this is actually
showing us that water droplets are
charged they are charged by induction
and it is not due to the polar nature of
water that they are attracted to
electrically charged objects now you
know in the teabag rocket we showed that
if you light a teabag on fire from the
top it will actually take off into the
air that happens because as the tea bag
is burning all the air inside it heats
up and so it expands so it gets less
dense and it's pushed up by all the
cooler air around it you know sometimes
people talk about about how hot air
rises I mean that is what hot air does
but only because the cooler air around
it is pushing it up in essence it's like
a buoyant force because the hot air is
now less dense than the cooler air
around it so when the teabag Burns right
down to the bottom the remaining Ash is
so light that it gets swept up in that
convection current and you get a teabag
rocket so do you agree with all of my
explanations and did you get them right
the first time let me know in the
comments and thanks for watching