km/h

A rare km/h speed limit sign for the UK. But on a water way. 8km/h seems a random choice but the significance is that it is 5mph.

Stationary waves on the Eden at Rockcliffe

This morning the tide was coming in up the Eden backed by a strong wind. As we walked downstream, the water stopped flowing out to sea. Instead we found a patch of choppy water where the waves didn't make any progress. Stationary waves form when two sets of waves travelling in opposite directions run into each other. In the lab, these have to be of same speed and frequency. I doubt that's strictly true here. In such a stationary wave pattern, I wonder how deep it goes down and whether there is a net flow of water below. It would need to be out to sea because otherwise the water down the Eden would spill over the sides. Also whether the particles at the surface move through the pattern in equal numbers in opposite directions or whether they are stalled and vibrating up and down.

Literal geological map

Outside the Bill Bryson library in Durham there is a geological map of the British Isles set into the pavement. What is best about it is that each part is made of the rock that is found there. This is Cumbria. So there is the ring of sandstone with limestone flanks and different igneous in the middle. A great way to visualise what is going on.

Generator and diodes

I loved this dynamo setup in the museum in Kirkwall. The lights are diodes so they only allow current to flow in one direction. One has been set to allow current forward and the other to allow current back. It is a brilliant way of showing that the direction of current depends on the way the dynamo is rotated.

Barytes - Potts Gill mine

Potts Gill mine above Caldbeck produced barytes. I've known the name for decades but never studied it. Turns out it is barium sulfate. It has a whole series of applications including fillers in paint but I have come across barium meals for CT scans because it does not allow X-rays through. I've also been reading about its use in drilling fluid where its density helps to stop fluids leaking out of the rocks into the borehole and causing a blow out.

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.