Explained: 5 Fun Physics Phenomena


– 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 powder. 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 Maarten Baert 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 cereal 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’s 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 of 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 at which point
this finger slides in and eventually they must meet in the middle. So this a way you
can find the center of mass of any cane or cane-like object. I showed you that if you
trt to flip your phone end over end, there is no way to do it without it also rotating
around the short axis as 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 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
kinda 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 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 OH 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’s 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 teabag 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 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.

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