DIANNA COWERN: This episode is supported by Squarespace.
Hi, I'm Dianna.
You're watching "Physics Girl," and this
is the third and final and last video with Anne Wojcicki
answering physics riddles.
So here's a recap of the last riddle.
And it's going to go straight into the answer.
So if you want to think about it first, you can't.
The question is, the riddle is--
I'm going to lay it down on the ground
so that the rope is coming from underneath the spool.
And I'm going to pull it this way.
And this doesn't rotate.
It's like--
It's fixed.
--stuck on here.
Which way is this going to roll?
It's going to roll backwards.
OK.
Potentially.
Why do you think that?
It's not going to just pull it forward
because it's going to force a rotation.
It's been a really--
a while since I've had to think about these things.
This is a riddle.
It's a tricky one.
It's a tricky one?
OK, put it down.
Let me see this again.
I'm not going to do anything.
You hold it.
I'm holding it, though.
So I don't cheat.
So there's no cheating, yeah.
Come on.
Let me do it.
I'm dying.
Just let me do it.
That's my experimentalist coming.
So if I pull it, it's going to create--
create the torque.
OK.
And it's going to--
Do you want to try it?
I do.
So badly.
OK, go ahead.
Oh, wow.
So it does roll towards me.
Oh, interesting.
Yeah.
Wrong.
Not wrong.
Surprising.
ANNE WOJCICKI: Yeah.
I would think it would go-- come on.
[INAUDIBLE] backwards, man.
Make me look bad.
So tell me, what's going on here?
Yeah.
So you're applying a force in that direction.
Yeah.
DIANNA COWERN: That force acts here.
So the torque from it about the center of the spool
tends to make the spool rotate clockwise.
But there's also a friction force
between the spool and the ground keeping it from slipping.
The torque from that force tends to make
the spool rotate counter-clockwise.
And it wins.
Another way to think about it is to consider the spool
like an upright pole.
Tugging on the rope will tend to tip
the pole over, which is like the spool rolling to the left.
This wouldn't happen if the rope was attached near the ground,
as in spools we're more familiar with.
You're pulling it this way and it starts rolling that way.
You need an opposite force down here to create--
To push it.
--torque to spin it.
That's why the friction?
Yeah.
Maybe, can I try it on the rug?
You can try it on the rug.
Let me try it on the rug.
Do you think it's going to be different?
You know, it's a lumpy rug.
Come on.
Come on, little guy.
No.
It's clearly not--
What do you think is going to happen
if you pull it like this?
Or like this?
So some angle.
So yeah, what do you think will happen
if we get it straight up, pull it straight up,
versus [INAUDIBLE].
ANNE WOJCICKI: I think then potentially,
it has more of a chance of going a different direction.
It all has to do with where your net force is
being applied.
I love it.
Physics.
Nice job.
We love physics.
Good job with the riddles.
Good job.
Awesome.
Is there a riddle that you want to ask me?
Just to make it fair.
All right, physics girl.
We're expanding our world of chromosome knowledge.
OK.
How many chromosomes does a mule have?
All right.
So I'm going to have to give you a--
Oh, wait.
OK, wait.
OK.
So a mule is a cross between a horse and a donkey.
Yes?
You are brilliant.
Yeah.
Yes.
DIANNA COWERN: I had horses when I was a kid.
ANNE WOJCICKI: Oh, you did?
DIANNA COWERN: I didn't have mules, though.
I've ridden a mule.
Oh.
Very--
Did you have donkeys?
That experience is coming in handy right now.
We didn't have donkeys, no.
No.
Hence, you had no mules.
True.
Yeah.
OK.
So you're completely right there.
So do you have any ideas about horses number of chromosomes,
or donkeys?
No idea.
I feel like it's got to be something similar to humans,
just because there are genetic similarities between--
So how many chromosomes do you have?
--all animals.
23?
Awesome.
Sets?
You have 23 pairs.
23 pairs.
Yeah, 23 pairs.
Most people have 23 pairs.
Sets of two.
Some people might have a few extras,
but you have 23 pairs, most likely.
Some people have extra?
That's cool.
Some do.
But that usually resolves in some kind of other conditions.
Yeah.
OK.
I would imagine that--
I don't know, there's some similarities
between horses and humans.
There's like the mammal thing they've got going on together.
I'm going to guess 23.
No.
But you are-- I love the thought process.
I love the guessing.
OK.
So I'll give you a few clues here.
You look kind of surprised that I was wrong.
So a horse has 64.
DIANNA COWERN: Wow, that is a lot.
ANNE WOJCICKI: That's a lot of chromosomes.
DIANNA COWERN: OK.
ANNE WOJCICKI: A donkey has 62.
DIANNA COWERN: Those are good things to know.
And so now if you mix them, then--
I would imagine you would get the average.
You'd get 63.
But why would you get 63 if that's the answer?
Well that is the answer.
The mule has 63.
I don't know enough about how you
get your sets of chromosomes from your parents.
Well, you're getting one set from one parent
and one set from the other.
Oh, OK.
Wow, that's a big learning experience here.
Thank you so much for going through this with me.
Of course.
Super-fun.
I love Physics Girl.
She is cooler than Santa.
Thank you, guys, so much for watching.
This is a lot of fun.
Thank you for having me.
(BOTH): Happy physicsing.
Thank you.
Woo.
Thanks to Squarespace for sponsoring this episode.
Whether you need a domain, website, or online store,
Squarespace can help you make your next move.
Squarespace provides an all-in-one platform
with templates that allow you to easily set up a website.
There is nothing to install, patch, or upgrade ever.
It's about time you made that website
to host the eclectic collection of riddles you have, right?
Start your free trial with Squarespace
at squarespace.com/physicsgirl, and enter offer code
physicsgirl to get 10% off your first purchase.
[MUSIC PLAYING]