How The Turntable Paradox Works

Leave most objects on top of a turntable, and set it spinning, and they’ll fly off in short order. Do the same with a ball, though, and it somehow manages to roll around on top for quite some time without falling off. [Steve Mould] set about unpacking this “Turntable Paradox” in a recent YouTube video.

In the basic case, the fact that the ball rolls is what keeps it on the turntable. As the turntable spins, the ball spins in the opposite direction, as per Newton’s first law of motion. As long as the ball is allowed to roll up to the same speed as the turntable, it will pretty much stay in place in the absence of any other perturbing forces. In the event the ball is nudged along the turntable, though, it quickly ends up in a more complicated circular motion, orbiting in a ratio to the speed of the turntable itself. [Steve] explains the mechanisms at play, and dives into the mathematics behind what’s going on.

Sometimes, demonstrations like these can seem like mere curiosities. However, understanding physical effects like these has been key to the development of all kinds of complicated and fantastical machinery. Video after the break.

35 thoughts on “How The Turntable Paradox Works

    1. “Extrapolate”? How??? That’s like “extrapolating” from a toy car to the corona vaccine: The two things have absolutely nothing to do with each other, so that’s not a valid extrapolation.

    2. Newtonian physics don’t really corelate to planetary motion, as time-space distortion doesn’t apply on such a miniscule scale. Gravity is only working in one measurable direction when you have a tiny ball on a turntable that’s placed on a planet. There is no giant mass below our solar system so we aren’t on a turntable. We’re freewheeling with a central force holding us. Gravity holds us in, we continue around the sun. But we move forward. The ball stays relatively still, while the orbit spins. It’s a difference of point of reference and physics behind it. Gravity is locked in a singular direction on the turntable, with nothing in the room massive enough to exert enough gravitational force to overcome Newtonian physics.

      1. Thanks for the thoughtful reply. Wasn’t dark matter to postulated to explain the lack of correlation between Newtonian physics and motion in galaxies? Specifically, that stars further from a galactic center are traveling at the same speed as stars closer to a galactic center, when they should not according to Newtonian physics. I believe the hypothesized dark matter is supposed to be essentially in the plane of the stars in the galaxy, and with a mass much greater than the stars. Which would exert a local gravitational effect downward (point of view one side or the other of the middle of the disc). This experiment reminded me of that concept. Maybe it’s not at all relational, but again I appreciate your thoughtful response.

  1. Many moons ago I was watching an ant on my turntable, as I was playing an LP. The poor thing was in the centre of the record near the spindle, going round at the same rate. Every so often it would set off for the edge. It’s path would be a perfect small circle, entirely contained within the surface of the LP, arriving back at the spindle in the middle again.

  2. So this video really screwed with me when I first watched it… I get why the ball rolls “forward” when closer to the axis of rotation, and “backward” when closer to the edge of the disc… I still can’t wrap my head around why the ball rolls towards/away from the axis. Away makes a certain amount of sense, as that’s what everything does, but I suspect that that intuitive answer is incorrect for the ball (as it stays stationary when “rolling” in place).

    1. Because it has some rotational inertia in a tangential direction and it either lags behind or overtakes the disk’s rotation when it is orbiting at a varying distance from the axis.

      When it overtakes or lags behind, it ends up rolling over to a portion of the disk which is not going in the same direction but curves away from the ball’s path because the tangential direction has changed. This pushes the ball in a new direction.

  3. Centrifugal force on the nonsphere increases as the disk spins faster and that pulls the object out unlike a sphere which creates a centripetal force pulling it inwards countering the centrifugal force.

    1. No centrifugal force “pulls” a nonsphere Tn this example. The object is being pushed in a straight direction (although the force direction changes as platform spins). But that straight motion keeps moving it towards the edge of the platform.

  4. I feel like there might be some relationship to the non-intuitive wibble-wobble that a spinning tennis racket makes while in a zero gravity environment (dzhanibekov effect?)… I’m looking specifically at the disc, and at the orbit pattern of the solid ball over the turntable’s center, and seeing some similarities.

  5. Why is this a paradox? It is explainable with rudimentary physics. It is simply because the rolling speed and the direction of the ball changes due to the turning of the table, varying from the center to it’s edge, while the ball is trying to roll in a straight line by it’s inertia.

  6. I just want to say how refreshing it is for me and a bit of relief when I read forms like this. Everyday I find myself dumb founded my the ignorance that seems to be over taking our world. I dont know if ignorance is even a proper word to descried their stupidity. Lol. But OMG, right when I start to give up on the future of humanity, stumple across something on my news feed that interest me and there are the beautiful minds I started to think where becoming instinct. So thank you all for sharing, comparing, debating….everyone has valid points by the way. Im very interested to see what all played out from this. Beautiful Minds!

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