Tracking the sunrise in Mablethorpe

The structure above is on the seafront at Mablethorpe in Lincolnshire. I thought it was a sundial. It does indeed work by projecting a shadow of the Sun, but it has been set up to record the position of the Sun at sunrise on specific dates throughout the year. So it's like a latter day Stonehenge. The lines on the pavement are labelled with dates.

The line furthest right in the top photograph is labelled "True South". In other words, that is the position of the Sun at midday (allowing for the analemma issue). Summer Solstice is the line nearest to it, and Winter Solstice is the line furthest away. This because on the Summer Solstice, the sun rises the furthest north of any day as it has further to go across the sky to reach south by midday. The Sun is out for more hours on that day. Hence its shadow at sunrise must be closest to the south. It is almost counter-intuitive and it took me a while to understand it.

Back to Roger Bacon

I found this in the Technical Museum in Berlin. Roger Bacon is quoted as saying that it will be possible to make flying machines in which humans can sit.They will have artificial wings, like a bird in flight. Either perceptive or lucky for 1285. I visited his memorial in Oxford last October and wrote a post about it. Since then, the Westgate car park has been demolished to extend the shopping centre. I hope they preserve the memorial. The site is a mess at the moment.

Physics and the new Weston library

This was the outside of the New Bodleian library in Oxford 3 years ago. I loved the hoardings and spotted a couple of Physics related letters in their alphabet.

 

It's been finished! Sadly we didn't have time to go in on this visit but next time...

Beckley mast - plane polarised TV waves

Here's the view from Debenham's café in Oxford. Between  the church tower and the tree you might just be able to pick out Beckley mast, the TV transmitter for Oxford. It is a primary transmitter and the TV electromagnetic waves from it are horizontally plane polarised. The TV aerials on houses will need to have the metal rods in the horizontal plane to detect them. There will be places hidden by hills where you cannot see the mast. TV waves probably have too short a wavelength to diffract round the hills and into the valleys - I need to check this. So they use repeater antennae called relay stations. These take in the signal from Beckley and re-broadcast it into the valleys. The re-broadcast signal is sent out with vertical polarisation. I have not seen this said anywhere, but I imagine that is so that there is no interference between the two versions of the same signal. I found a marvellous website about all of these issues http://www.aerialsandtv.com/aerials.html so expect more!

Centre of gravity on Cornmarket

We saw this entertainer on Cornmarket in Oxford. This is obviously a very simple trick: you need to get your centre of gravity over the rope and then you balance... Very difficult in practice. First there is the distinction between centre of mass and centre of gravity. It depends what you are doing. If you are trying to accelerate something, then inertial mass becomes important and it is the centre of mass that you want. Here, we are interested in the weight, so centre of gravity will be a more sensible usage although I think they are in the same place. If the centre of gravity is not above the rope, there will be a perpendicular distance from the rope, which will act as a pivot, and the line of action of the man's weight. There will thus be a resultant torque which will make him spin one way and fall. The balancing act involves producing a movement to make an equal and opposite torque every time you wobble slightly.

Hareshaw Linn: minimising potential energy

Hareshaw Linn is a waterfall in an impressively gothic ravine near Bellingham in Northumberland. This picture doesn't really do it justice. Solar energy lifted the water up from the sea by evaporation, doing work against gravity and giving it a lot of gravitational potential energy. Then the water begins to try to minimise its potential energy. It always seeks out the lowest point. This waterfall is an impressive example of it doing that. Yet even on the waterfall itself, there were certain channels that must have lower potential energy than other parts as the water sticks to certain routes across the fall.

Crumple zone

The engine compartment at the front of the car is designed to crumple up in the event of a crash. This means that it takes a longer time to come to a standstill. The same momentum is lost over a longer time so we say that the rate of change of momentum is reduced. By Newton's Second Law, a lower rate of change of momentum means that the force of the crash is smaller so you are less likely to get hurt.