Centripetal acceleration

As part of the Christmas Elf Trail at Watchtree last December, this poor elf was lashed to a vent. It was rotating rapidly. I filmed it and counted 12 rotations in 6 seconds so time period = 0.5s. Angular velocity  = 2pi/T= 13 rad/s to 2sf (time period is certainly not to 3sf). Angular acceleration = (omega)^2 x radius = 13 x 13 x 0.1m = 1700ms-2.

Dobsonian Reflector

The telescope in the window was down as a Dobsonian reflector and I wondered what that meant. Turns out that it is not to do with the optics inside but to do with the mounting. This article explains the history. The significance is that it made it easier to mount bigger mirror reflector telescopes so reducing the cost for amateur astronomers. The advantage of reflectors is that the heavy glasswork is at the bottom of the light gathering tube so they are easier to support. This means reflectors are easier to build with larger diameters so have more light gathering power. 

Rufus and Newton's Third Law

Rufus was trying to pull a toy towards himself. To do this he needed to plant his claws in the carpet and then push back with his leg muscles. Trouble was that the rug was on an almost frictionless surface. In the basic "to every action, an equal and opposite reaction" version of the Third Law, the backwards pull has a forwards push. You'll notice below how this results in the rug sliding forwards.

Voltmeter

I loved seeing this demonstration voltmeter in a window in Kirkwall. I used these to show readings to classes when showing them circuits. My one had insert cards so that you could change the scale and swap from volmeter to ammeter. This looks to be fixed. Looks like it might just be possible to read to the nearest 0.1V.

 

Orbital

This is the brilliant book that won the Booker Prize this year. It is about 24 hours in the life of the International Space Station and details the thoughts and relationships of the six people on board. Really interesting insights into training and the psychology of it all. There are some lovely bits of physics. On page 109, she explains that the feeling of weightlessness is not due to lack of gravity but about falling towards the Earth constantly; not hitting the Earth because the Earth is curved by an amount that means the Earth's surface falls away at the same rate that the spacecraft falls. I wish I'd understood that at school.

Red Sky in the Afternoon

For the second day running there was a clear band of red sky towards the south in the afternoon. The sun was behind cloud at the top of the photograph. Now any colours we can see must be due to light coming directly towards our eye. Outside, the source of that light must be the sun. So light must be heading from the sun and then getting either reflected or absorbed and reemitted in our direction. Something must be happening to the sunlight to filter out the other colours. Normally, red sky and night and in the morning are due to dust in the direct line of sunlight filtering out the other colours. This is not in direct line. Maybe the clouds in the way are filtering colours. Mrs B suggested that it was maybe snow-laden clouds. 

More interesting rime ice

If rime ice builds into the wind, then wind flow round this fence was interesting. It is away from me lower down but the top wire has it towards me. Swirling round the fence??