Simple accelerometer and circular motion

I’ve just started teaching circular motion to my Year 12s. There are some obvious demonstrations you can do when teaching this topic, such as spinning a bucket of water around your head, but I’m somewhat ashamed to admit that I’ve only just discovered the floating cork accelerometer which can be used to illustrate a key idea for this topic. Watch the video to see what I mean.

Massive thank you to my colleague Ronan McDonald for making the big accelerometer and volunteering to get dizzy.

[Edit 18/6/2013 – this post inspired a lively discussion at the Institute of Physics PTNC mailing list for physics teachers, which is a hidden gem of a community and a list every teacher of physics should at least be aware of. Sign up via the web interface. Thanks to everyone who cross-posted their comments here.

Joe Rowling had a nice blog post a few days before this, too – well worth a look if circular motion is your thing.


9 thoughts on “Simple accelerometer and circular motion”

  1. Cheers.

    Think the video (or me!) is wrong. If you take a car perform donuts your inertia ‘pushes’ you outwards.

    Isn’t the case here that theres more water moving to the edges creating high pressure on the ‘outside’ of the bottle pushing the less dense cork towards the centre?

  2. I like this demo. It will help with the “but I feel pushed outwards when I’m swung in a circle” discussion. It is also easy to set up.
    After the centripetal acceleration is established it would be nice to come back to the question of how the force is applied to the cork. This will require a discussion of pressure in a liquid and buoyancy.

  3. With a burning candle firmly planted in the bottom of a 250ml beaker, in order to get a good flame displacement, I found a record turntable needed to rotate at 78 rpm, so the beaker in which the candle was burning had to be securely blu-tacked down! The candle flame was at an estimated angle of 40 degrees or so from the vertical towards the centre of rotation and it was definitely one of those ‘ahhh’ moments for the students.

    As others have commented in respect of corks, balloons etc., the explanation is not obvious and if one compares it also with water in a beaker then it’s an excellent starting point for discussion or independent research before the next lesson.

  4. It might be better to begin with an even simpler accelerometer – a ball on a string, for example, in which the forces on the ball are more easily identified. The ‘ball in a denser fluid’ examples add value as extension or follow up and are certainly fun, but if they are your students’ first or only experience of applying Lab physics ideas to circular motion, they might prove counter productive.
    I can certainly imagine students getting in a tangle if they weren’t alerted to the need to consider differences in pressure in this example.

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