Tricky Spool Riddle SOLUTION – ft Anne Wojcicki



How did you answer the spool riddle?
Try SquareSpacepace at http://squarespace.com/physicsgirl
Special guest Anne Wojcicki, entrepreneur, biologist and CEO of 23andMe, joins Physics Girl for the answer(s?!) to the spool riddle and for a genetics riddle!
First Video: https://youtu.be/BnxG0RqP080
Second Video:

Creator: Dianna Cowern
Editor: Jabril Ashe
Animator: Kyle Norby
Thanks to: Anne Wojcicki, the 23andMe staff and Dan Walsh

I first heard this riddle from Derek Muller of Veritasium during a talk he gave. It stuck with me as a fun hands-on riddle to ask in person. Plus he gave me his blessing to use it in a video. He hates riddles anyways.

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Keep up with my less sciencey shenanigans:
INSTAGRAM: http://instagram.com/thephysicsgirl
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FACEBOOK: http://facebook.com/thephysicsgirl

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Fahad Hameed

Fahad Hashmi is one of the known Software Engineer and blogger likes to blog about design resources. He is passionate about collecting the awe-inspiring design tools, to help designers.He blogs only for Designers & Photographers.

47 thoughts on “Tricky Spool Riddle SOLUTION – ft Anne Wojcicki

  • September 24, 2017 at 6:59 am
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    Electrician here…

    Unrolling wire. You can only unroll a spool so far without it spinning around, hence putting it on an axle to get the wire off without twisting. Just lying it on the ground, at first the wire pulls off, but as the wire spins off… poof, the wire starts trying to roll up the spool, spins around and twists up.

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  • September 24, 2017 at 6:59 am
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    Funny enough i didn't expect the result in the spool experiment, but i nailed the mule chromosome riddle 😉

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  • September 24, 2017 at 6:59 am
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    not exactly fair to ask a physics girl a biology question.

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  • September 24, 2017 at 6:59 am
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    I think it will move opposite of inertia of all three forces. The person pulling the spool the earths spin on its axis and gravity

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  • September 24, 2017 at 6:59 am
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    The genetic part was straight out of high school in the 90s. You may need to revisit the basics of genetics to understand the differences in the different living things on this planet. You could, in fact, consider the whole planet as one living being made up of billions or trillions of separate entities.
    Or, you could think backwards: Those same entities as one living being as a whole.
    Don't take my word for it, though. It's my intuition.
    My common-sense tells me these things without proof.
    Prove me wrong, I beg you.

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  • September 24, 2017 at 6:59 am
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    hey diana can u make a video recommending some good books on physics in general..?

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  • September 24, 2017 at 6:59 am
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    That's not the entire answer… let's call pulling force "Fp"; friction force "Ff"; radius of the big wheel "R"; radius of the cilinder "r". We can get 4 cases:

    1) Fp*r > Ff*R > This means primarily that Fp > Ff and the object will move forward. In this case the torque producted by Fp wins and the spool will rotate clockwise. In a realistic world it would probably be very bouncy.

    2) Fp*r = Ff*R > Fp > Ff like before. While the spool will still go forward the torque by Fp will be equal to the Ff one and we will get no rotation.

    3) Fp*r < Ff*R > two cases: 3a) Fp =< Ff > no movements.

    3b) Fp > Ff > the spool go forward but the torque producted by Ff will overwhelm the Fp one
    and it will rotate counterclockwise

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  • September 24, 2017 at 6:59 am
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    You didn't explain why the torque from friction wins (τ = F * r); you just alluded to the explanation in the visuals. If people don't understand the answer, they haven't really learned anything, so this was a pointless exercise.

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  • September 24, 2017 at 6:59 am
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    @physicsgirl could you do a video on the physics behind a boomerang please 😎

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  • September 24, 2017 at 6:59 am
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    It's just a horizontal yo-yo. How could anybody possibly get this wrong?

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  • September 24, 2017 at 6:59 am
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    Ok. Let's get some math down. We assume that the spool rolls without slipping, so there is static friction, not kinetic friction. By applying newton's second law of motion, we get ΣF=ma=> F-2Τστ=m*a. This is equation 1.By applying the law for rotational movement, we get Στ=Ιολ*a/R. Στ is the sum of all torques, Ιολ is the total rotational inertia of the spool. So we have 2Τστ*R – F*r=Ioλ*a/R. We'll call this equation 2. Keep in mind that Forces and torques are vectors, as is acceleration. If we add the two, and solve for a(acceleration), we get : a=F*(1-r/R)/(mολ+Ιολ/R²). This is always bigger than 0. It's always a positive value. Thus the spool goes backwards (We set the direction of F as the positive one). That's the solution in a nutshell. No reason to further complicate things.

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  • September 24, 2017 at 6:59 am
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    Is my gaydar too sensitive, or is Anne like "How YOU doin?"

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  • September 24, 2017 at 6:59 am
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    to be a planet ther has to be nothing in your atmisfer . but almost every planet in ower galexy has moon in its atmisfer.

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  • September 24, 2017 at 6:59 am
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    Uneven audio. Please correct it in post. Adding the music makes it even more unintelligible. Thanks for using the body mikes but their response was poor (deficient treble, an impedance mismatch?).

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  • September 24, 2017 at 6:59 am
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    thats kindoff the reason why you shall study all subject well in school life …… ha ha ……… well just joking . how much time has it been since you studied bio dianna ?

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  • September 24, 2017 at 6:59 am
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    Another way of looking at it. Suppose the spindle has radius r and the rim has radius R. For one rotation of the spool, the centre moves 2 * pi * R and the extra rope is 2 * pi * r. Thus the end of the rope moves 2 * pi * (R-r). Turn it around and pull on the rope instead. Since the rim is larger than the spindle, the amount of free rope decreases and the spool moves towards you.

    Now consider when the rim is supported on rails with a gap between them so that the rope can be wrapped to a larger radius than the rim. When you start pulling, the spool will move away. When it reaches the point where the spool and the rim have the same radius then, assuming infinite friction, the spool will stop experiencing a net force. However, if it has momentum then it will continue to roll. Then the rim will be larger than the rope on the spindle so the spool will start moving back. Rinse & repeat. Congratulations! You have just built a horizontal yo-yo.

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  • September 24, 2017 at 6:59 am
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    You didn't tell Diana that mules are all sterile because of this.

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  • September 24, 2017 at 6:59 am
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    3:14
    "I've ridden a mule"
    "Did you have donkeys?"…
    The look on physics girls face says it all. TBH i am surprised you didn't get demonetized XD

    Reply

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