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. 

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??

Snow Moon

This week there was a lot of stuff about the full moon being the Snow Moon. I has become very common to name the full moons now - I think the Internet has spread this from North America. I found this lovely page about the Moon. Most of the full moon names come from Native American culture. In Europe we have been based on a solar calendar.

Aldebaran

Orion is prominent in the winter sky. The star Aldebaran on one shoulder is clearly red to the naked eye. It is a red giant, a star that is running out of hydrogen fuel. It begins helium burning which causes the star to expand. As it expands, it cools. This cooling is relative - it still has a surface temperature of several thousand degrees. But the colour of the light emitted is a clue. Red photons have a lower frequency than the yellow ones from the Sun and thus carry less energy. Hence the red giant has a cooler surface. It is so bright because the expansion has vastly increased the size of the surface and light can only escape through the surface. The bigger the surface, the more light emitted at once.

Rime ice again

This amazing formation on the trig point of Crag Hill is likely to be hard rime. It is not clear and it is not spiky like soft rime. It seems to show wind flow around the column. In fact, the individual bits are a bit like a vector field. What causes the separate parallel sections to develop is not clear. Is it only certain bits of the stonework that can take the first water droplets or is it just chance? Also, does a bigger chunk indicate a faster wind speed? This would be explained by more frequent arrival of supercooled water droplets.

Setting the clocks in Kendal

Not sure that this is exactly calibration because I'd expect that to fix a range of points, but this system was to ensure accuracy in clocks around the Kendal area. Resetting every day reduces the risk of systematic error. An odd system - a cannon was fired at 1pm every day. And how did they know when 1pm was? Telegraph from Greenwich!

Planetary conjunction

The sky was magnificent on Tuesday, maybe the best ever. Venus was west of the Moon and then Jupiter and Mars were on a clear arc through.

The conjunction has also included Saturn, Uranus and Neptune but just the three planets together was spectacular and too vast for my camera. All are on the same side of the Sun as us at the same time. I need to think some more about how to work out the next time it will happen from first principles.

Angel of the North wind protection

The Angel of the North must have a large wind load and the largest area for catching the wind is higher up, giving a big torque that could pull it over. According to this, the ribs direct wind flow downwards to push on the foundations. This would produce an opposite torque to keep it from falling.

Percussion installation

I tested out the hollow metal of this installation at Durham Botanical Gardens after accidentally knocking it and getting an impressive sound.

Knocking it in different places gave different notes. The highest note from the narrowest section.

This was a bit deeper.

This was a very low note. Since they are all part of the same space, it can't be the air that is vibrating. Maybe stationary waves are forming across the metal. Wider metal would be bigger wavelength and thus deeper note.

Low clouds in climate models

These low clouds over Longsleddale are the opposite in climate models to the high clouds. They are thicker and the cloud tops reflect more solar radiation back out into space. However, global warming models suggest that a warmer earth will push clouds higher on average which will then increase the warming. See January 2025 Physics World for the details.

High clouds in climate models

There's a lovely article in the January 2025 edition of Physics World explaining the differing effects of clouds in climate models. High clouds are cooler being higher up in the atmosphere and absorb more of the outgoing radiation from the Earth's surface, reradiating a lot of it back down in the classic greenhouse warming effect that makes our planet habitable. These high clouds - like the alto-cumulus in the picture of Skeggles Water - are thinner so they also let more sunlight through which is another warming effect.

Venus

Venus has been very prominent in the early evening these last couple of months. It was the subject of a feature in the last series by Brian Cox on the Solar System. I was interested in the apparent "snow line" effect on the mountains of Venus. It's a kind of metallic frost. It has to be something that melts and evaporates at 460 Celsius at the lowest levels on Venus but solidifies at cooler temperatures with altitude. Brian Cox names lead sulfide as a possible candidate. Looking that up, I see a melting point of over 1000 Celsius. This must be at Earth's standard atmospheric pressure and will be different on Venus. This article names bismuth sulfide and tellurium as other possible candidates.

Retrograde motion

Mars has been in retrograde motion recently as Earth has been overtaking it. Earth has a higher angular velocity. Mars is still going forward but looks like it is going backwards relative to a fixed reference point. OK, there are no fixed reference points in space but the background of stars are so far away that it doesn't make much difference. Here Mrs B was overtaking a lorry coming up to a bridge. Above, the cab lines up with a bridge in the distance. 

In the second view there is daylight between the cab and the bridge and below the cab is nowhere near lining up with the bridge. You could achieve the same effect if we were still and the lorry went backwards. But all the while it was moving forward at 50mph. This is what we mean by retrograde motion.