Roundabout and angular momentum

Our playgrounds have been open again for a few months and my sons have been getting progressively more adventurous with climbing, sliding and spinning around on roundabouts.  I thought I'd show my 4 year old a new way to play on a roundabout, and demonstrate conservation of angular momentum in the process!

The idea here was that if you set the roundabout spinning whilst you're standing up and holding on, you can change the speed it spins at by either leaning inwards (faster) or outwards (slower).  This is the same principle that figure skaters use to make themselves spin faster or more slowly as they extend limbs or tuck them in.  

Experimenting on the roundabout

Conservation of angular momentum is pretty tricky to explain to a 4 year old, so I didn't really try, I just sought to demonstrate how we can see it in action.  I got him to sit in the middle of the roundabout whilst we spun together and I leaned inward and outwards - as an adult who is pretty heavy and can lean a long way outward, the effects were easily noticeable and he seemed impressed.  He then had a go himself, standing at the edge of the roundabout whilst it spun and leaning in and out.  The effect was still there, and he liked making it speed up again by leaning inwards.

As an attempt at a simplified explanation for why this happens, angular momentum is related to the mass, speed (more properly, velocity) and distance from a fixed point.  Angular momentum is conserved i.e. stays the same, so if you change the distance (but the mass stays the same) the velocity changes.

This was a nice demonstration of some physics in action, but most importantly added some more fun to our afternoon excursion to a playground!  I have no doubt it will be tried again soon.