Everything is a teaching opportunity…

Posted: 18 November 2020

Viral toys especially are a teaching opportunity in physics.

One of the dangers of always “doing physics” with expensive equipment in lab conditions is that the subject looses it’s relevance, it becomes something that only happens in lab conditions. As a counter to this I encourage anyone to have a go at “found physics” and use teaching toys and pound-shop items to teach physics. There are some great examples out there and I’ll try to document a few as I come across them. The first one I want to look at is the fidget spinner;

The fidget spinner was the toy of 2017-2018, there can’t be anyone not familiar with these toys but the fidget spinner is essentially a low-friction bearing in a case that allows the outer part of the bearing to have increased mass and therefore increased inertia. It’s an old toy and the main craze is past, however I want to use three experiments to illustrate how far you can go with something that looks so simple at first glance.

Experiment 1 – Inertia

Inertia is an interesting property of matter, it is the resistance of any physical object to any change in its velocity. Remembering that velocity is a vector quantity – ie direction and magnitude, this includes changes to the object’s speed, or direction of motion. There is a link between inertia and the quantity of energy stored in the movement of the object – the more inertia an object has , the more energy you need to transfer to get the object to speed up, or to stop.

By changing the mass of the spinning part of the toy then you should be able to change how long the toy will spin for – we’re assuming here that the only thing slowing it down is a little air resistance and the resistance of the bearing and that these forces are roughly constant regardless of mass of the spinner. If the resistive forces are roughly constant then the rate at which they convert the kinetic store to thermal store of the room is constant. In this case, the more mass that’s spinning, the more resistant to change, the more energy is stored for a given speed of rotation and therefore the longer the spinner will spin. Add blu-tack or remove the masses from the outside edge to experiment!

Experiment 2 – How fast?!

There are a few ways to measure the speed of rotation. you could simply set up a light gate and time the length of time that the beam is broken for by one “arm” of the spinner. Measuring the dimension of the arm will allow you to calculate speed!

If you don’t have a light gate you could use a small magnet on each arm and a field sensor:
The eagle-eyed amongst you will have spotted that I’m not actually doing a simple calculation here – I’m using the datalogger to measure the frequency of the pulses of light – or the pulses of induced current, and then using this frequency to measure the rate at which the thing was spinning. If you look closely at the image above you will see that the peak in the graph is at 32HZ – a frequency of 32 “events” per second. The event in question is any of the three arms of the (slowly spinning) spinner passing the field probe. Each arm has a magnet attached and therefore causes a reading. 32 Hz and 3 arms means the spinner is rotating about 10 times per second in this photograph!
Not immediately curriculum but a great way to talk about frequency in a new context.

Experiment 3 – The Wagon-wheel effect:

The wagon-wheel effect is a really neat optical illusion in which a spoked or striped wheel appears to rotate differently from its true rotation. The name comes from the early film or television depictions of stagecoaches or wagons in Western movies, where the rotating wheel was captured only intermittently by the frames of the film. The rotation of the wheel and the intermittent capture means that when played back we appear to see the motion distorted.

There are several great examples of this on youtube, where the rotation of a helicopter blade and the shutter-rate of the camera match and therefore the blades appear to be frozen in time. This one is really good: https://www.youtube.com/watch?v=yr3ngmRuGU

You can make your own example with two fidget spinners above each other too:

Triangle mountains