Trying to find the mass of the roundabout

 I was back on the roundabout. I timed myself as 7 seconds per rotation near the centre (say 0.5m from axis) and 8 seconds when I was about 1.5m away.

The movement should involve conservation of angular momentum. I have added my point moment of inertia to that of the roundabout, which I have taken to ne a disc of radius 1.5m. The result is a mass of 730kg for the roundabout. It might be a bit high but is not totally unrealistic.

Vivid blue from polarised sunlight

 Through a polarising filter, the reflected sunlight from the shallow water at Fleswick Bay was a vivid blue - a sort of metallic indigo colour. It was very striking seen live and some evidence of it has come out in the photographs in the middle of each picture. I have seen it before but always associated with reflections from water through polarising filters. The colour is always this deep blue or indigo.

Even more evidence from the mirror

 It was a bit later in the morning today so the sun had moved round to a different angle. Light was hitting the other mirror door. The direct shadow of the blinds can be seen with the reflected image on the lower left. Again the spectrum is at a different angle to the reflections. There were also curved spectra in mid left.

More about the refraction from the bevelled mirror edge

 I posted last month about the prism effect from the bevelled edge of a mirror. This time I noticed that the spectrum cuts across the pattern of the reflected blind slats. In other words, the process of producing the spectrum is pulling the light through a bigger angle than the reflection is. I a suspecting that there is some refraction involved with reflection from the back of the mirror but will need to think further about a possible ray diagram.

Stress patterns in the ice

 I know that it is possible to see places of high stress in plastic using polarising filters, but it looks as though I have managed to see them in ice today. It only worked through my polarising sun glasses. 

A moment of inertia in Patterdale

 I timed 2 circuits of the roundabout as shown below.

Then moving towards the centre, I timed again. This time each rotation was 1 second faster.

The reason is that for circular motion, it is not just mass that affects motion but distance from the middle. The combination of the two is called moment of inertia. Angular momentum is conserved in the movement to the middle. Angular momentum = moment of inertia x angular velocity. Moving towards the centre reduces the moment of inertia because it is easier to accelerate a mass closer to you in a circular motion. If momentum is conserved but moment of inertia is reduced, angular velocity must increase thus each rotation takes less time.

Miners' Balcony Path at Glencoyne: another equipotential

 The miners who worked at the Greenside Lead Mine certainly ensured that they didn't waste energy traversing the hillside on their way to work. The path around Glencoyne follows the contour faithfully - a spectacular equipotential to walk.

It leads to the hole dug in the 1950s as a safety evacuation route from the mine.