Pilot gig oar

 I was able to get up close to a pilot gig oar on Bryher. The oar is about 4 metres long and the pivot point is at about 1 metre. On a good day I could probably pull up 25kg so the force would be 250N. The blade is 3x further than the pull so the force on the water would be 750N. There are 6 oars in a gig so 4500N. The power will depend on the time to pull through. I should have timed that when I watched the race...

No parallax: navigation mark on St Agnes

On the end of the island is St Agnes there is a white pointed column with a black strip down it. If you line it up with the St Agnes lighthouse when you are out at sea, you can sail safely along that line. If two objects don't line up, we say that there is a parallax error. Parallax is used a lot in Physics. The parallax for stars caused by the Earth's motion around the Sun can be used to measure distances to stars. A no parallax method is used to find the position of a virtual image in a mirror.

Bishop Rock: testing the little finger method

From the top of Samson Hill on Bryher, I could see the Bishop Rock lighthouse in the distance. I used the idea that any little finger at arm's length subtends 1 degree. I enlarged the photograph and found that the lighthouse subtends 1/4 of a degree. 2*pi radians = 360 degrees so 1/4 degree = 0.0044 radians. Angle subtended in radians = height of lighthouse/distance to lighthouse using the arc length approximation. The lighthouse is 49 metres tall. That gives a calculated distance of 11.1km away. I got out the OS 1:25000 map and measured the distance as 10.8km. That means there is a percentage difference of 3% by my method. I'm quite pleased with that.

Does it get windier when a cloud blocks out the Sun?

On Porthcressa beach St Marys I once again noticed that it seemed to get windier on a hot sunny day when a cloud moved across the Sun temporarily. In the photos, the land near the beach was in shade and the beach itself was still in sunshine. In theory, there would be a drop in the upwards convection current from the nearby land shown in the top picture compared to the beach. This would mean that the land in the shade would have higher pressure air at ground level and thus a flow of air would occur from shaded area to sunlit. The Internet is divided about whether or not this effect is real but this isn't the first time I've experienced it.

Temperature: feeling the heat

When the temperature hits nearly 30 degrees Celsius in the uplands of northern England, it's big news. My car thermometer has a resolution of 0.5 degrees Celsius. But is that meaningful to me? Could I feel a change of 0.5 degrees? I wouldn't have thought so. But a result posted on https://ask.metafilter.com/291722/What-temperature-gradations-can-a-person-actually-feel argues that a change of 1 degree from 21 Celsius to 22 Celsius would reduce heat loss by 8%. This percentage will change depending on where on the scale the 1 degree change takes place.

Broken bridge

This bridge on Royal Deeside had been hit by a tree in flood waters and put out of action. Suppose the tree had a mass of 1 tonne (1000kg) and was travelling at 30km/h, which is about 8 m/s. That would give it a kinetic energy of 35000J. Elastic strain energy is given by 0.5kx^2 where k is the spring constant and x is the extension. Suppose the bridge is 15m long and the middle is displaced by 30cm. By Pythagoras, that means an extension of each half by about 0.5cm. That means spring constant would be 1.4 x 10^9 N/m.

Saharan dust and the jet stream

I took this photograph on the 22nd April. The dust on the car was said to be from the Sahara. Since then there have been two further events where blobs of hot air have moved north from the Sahara into Europe. I found https://www.eurekalert.org/pub_releases/2018-10/rai-pjc100218.php which suggests that it is to do with the jet stream. Weather forecasters refer to the jet stream a lot at the moment. Winds on Earth are driven by convection. At the equator, the Sun heats the air so that it expands and rises. This causes low pressure at ground level. The opposite happens at the poles. In between, the air from the equator falls down to the ground creating high pressure around the latitudes that contain deserts like the Sahara. At our sort of latitude, it is warmer than the poles so air pushed down from the Arctic by high pressure there can be warmed and rises. This means that in each hemisphere there are 3 convection cells. The polar jet stream occurs along the boundary between the two most northerly of the large scale convection cells in this hemisphere. I'm interested that it flows along the boundary rather than from one side to the other. This seems a bit like one of the hand rules for electromagnetism. Hot air is rising but the fast flow is perpendicular to this upwards flow. https://en.wikipedia.org/wiki/Jet_stream#Polar_jet_stream