Kind: captions
Language: en
[Applause]
In my last video, we performed an
experiment in which two identical wood
blocks were shot with the same rifle.
One through the center of mass and the
other one slightly off to one side. Now,
if you haven't seen that video yet, then
click here now and go and watch it. Or
if you're on a mobile device, click the
link in the description.
[Music]
So, the question was, compared to the
block shot straight through its center,
how high would the block shot to one
side go? Now, 60% of you predicted that
it wouldn't go as high.
Didn't seem as high.
Now, check out the high speed footage.
Oh, no.
You can clearly see that both blocks to
the exact same height.
Oh, come on.
19% said that it would go higher and
only 21% correctly predicted that both
blocks would go to the exact same
height. So, most of us got it wrong the
first time around. And that includes
Henry and me and Destin who has a lot of
experience with this stuff.
I mean, I can't tell you how many times
I've hit wood with a rifle.
So, even experienced and intelligent
people get it wrong sometimes. In this
case, I think the result was
particularly counterintuitive because
the spinning block clearly ended up with
more energy than the other block. So, I
asked you guys to make me a video
response and explain what is happening.
And this is what I received. If you
imagine the bullet traveling through the
center of the block, then after the
collision,
that is a lot of video responses. And
that's not even all of them. I've got to
say, I was blown away by all of the
great video responses I received. You
guys are awesome and I really really
appreciate all the effort you went to.
You know, some of you actually
replicated the experiment perhaps with
different setups.
The experiment was featured on blog
posts including Scientific American and
Wired. There's an interactive simulation
you can try. Link is in the description.
And someone even made a web comic. It is
so incredible. You know, watching all of
those video responses, and I did watch
all of them, that was my proudest moment
making this YouTube channel, seeing all
of you guys doing science and getting so
involved and putting in so much effort.
It just blows my mind. It's amazing.
Thank you guys so much. Thank you. Thank
you. I can't even put it into words how
awesome it is. Woo!
So, what is the answer? How could two
identical blocks shot with the same
rifle end up with different amounts of
energy? Well, some of you felt that the
amount of rotational energy would be
basically insignificant.
What if the the amount of energy
necessary to get that very small block
rotating really is negligible in
comparison to the amount of energy
necessary to lift it uh into the air and
resist gravity?
Well, we can calculate it because the
spinning block was rotating at about 11
times per second. So you can calculate
that its rotational energy would be
equal to 50% of the gravitational
potential energy when those blocks were
at their peak. So the spinning block
actually had 1.5 times the energy of the
non-rotating block. And in my books,
that is not negligible. So what else
could be causing this? Well, some of you
actually measured the height difference
using pixel counters and found that the
blocks didn't go to exactly the same
height. So, this raises an important
question, which is how different would
those heights have to be before you can
think that there really is a difference
there? Well, given that the rotational
energy is about 50% of the gravitational
potential energy, you might expect the
spinning block to go half as high. But
looking at the video footage, you can
clearly see it doesn't go half as high.
It goes basically to the same height. To
know for sure, you'd probably want to do
the experiment a number of times. In
science, you got to repeat things and
make sure what you saw is not just a
fluke. that will happen every time. So,
in fact, we did repeat the experiment a
number of times. And here are all of the
blocks lined up side by side so you can
compare their heights.
The blocks clearly do not all go to the
same height. So, you might say, "Derek,
you were tricking us. You just picked
the two that went the closest and used
those." It's true. I did do that. But
the point is, do the spinning blocks go
systematically to a lower height than
the non- spinning blocks? And I think
from the video footage, the answer is
clearly no. So why not? Well, another
common response I saw was that air
resistance must play a role. That the
spinning block exposes a smaller surface
area as it's traveling upwards. And this
means there's less air resistance
impeding it. So even with a lower
velocity, it could make it to the same
height as the non- spinning block. But
you can do the calculation and find that
the difference in air resistance would
be at most about 6% of the weight of the
block. In other words, negligible. So
what is the real answer? Well, there are
a number of ways of approaching the
problem and I saw you guys try almost
all of them. Force, torque, momentum,
energy. But the easiest one of these is
the law of conservation of momentum. You
see, when the bullet is fired out of the
rifle, it has a certain amount of
momentum entirely in the vertical
direction. And when the bullet becomes
lodged in the block, the bullet and
block together must have that same
momentum that the bullet had. Now, it
doesn't matter where on the block the
bullet hits because the bullet had a
certain momentum before it hit the
block. The block and bullet together
afterwards must have that amount of
momentum upwards. So this means
regardless of whether the block is
spinning or not, it must have the same
upwards velocity so that it has the same
momentum. So it will go to the same
height regardless of whether it's
spinning or not. It is simple
conservation of momentum. Momentum is
always always conserved. It is something
you can always rely on and it's why you
should think about it first. But some of
you may say okay well there's also
angular momentum in the spinning block.
But the point is angular momentum and
linear momentum are independent and
they're conserved independently. So you
can simply think about the upward
momentum of the bullet and that's going
to have to be the upward momentum of
those two blocks and it doesn't matter
that one's spinning because they have
the same total linear momentum upward.
So they have to go to the same height.
But if that is the case, why does the
spinning block end up with this extra
rotational energy? Well, the thing about
energy is it's a little bit tricky.
Although total energy is conserved, the
energy of motion which is called kinetic
energy is not conserved. So what happens
is as the bullet strikes the wood, a lot
of its kinetic energy is lost, heating
up the wood, creating sound and
deforming that wood block. So it would
be possible for the rotating block to
end up with more energy of motion, more
kinetic energy if the bullet loses less
energy deforming the wood and heating it
up and creating that sound. And this
kind of makes a little bit of intuitive
sense because if you imagine the bullet
entering the block on the side, the
block will be moving away from it faster
because it's rotating. So that may mean
that the bullet doesn't penetrate in as
far and so it wouldn't lose as much
energy as it's entering the wood, which
explains why the block has extra energy,
allowing it to rotate and go the same
height as the other block. Let's have a
look at how far in the bullet went into
each block. This is our first test. Here
we go. So this is the one where it went
straight in.
There.
Now we come over here to the off center.
This is where the bullet went in off
center.
It's not going in there. See where my
Yeah. Let's see the other one.
Okay.
I have a nice
It is all the way in.
There it is partway in.
See you tomorrow.
So the bullet didn't go as far into the
spinning block.
Right.
So that was pretty convincing evidence.
But just to be extra sure, we x-rayed
both blocks and this is what we saw. I'm
going to overlay the block so we can see
the difference in penetration depth.
Hang on. It looks like both bullets
penetrated each block to the same depth.
So, does that mean our theory is wrong?
Well, let's think about how different
the bullet depths should be. I mean,
when the bullet strikes the block, it
actually loses 97% of its original
kinetic energy. And the amount of
rotational energy the spinning block has
only works out to about 1% of the
original kinetic energy of the bullet.
So we would only expect a difference in
depths of about 1%.
And for our measurements that would be
about a tenth of a millimeter. That
would be immeasurable and that is what
we see. So although this answer is a
little bit unsatisfying at least it is
the truth. As an extra challenge, I want
you guys to think about how we could
perhaps modify this experiment so the
difference in bullet depth would be
measurable. You can leave your answer in
the comments or make me another video
response cuz I love those so much.
[Applause]
This episode of Veritasium was brought
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