Tree rings in Ennerdale

 

This is an odd way of timing things. I love the term "dendrochronology". Apparently there is more growth in the spring so the wood is less dense and lighter. Later in the year, the opposite is true and a darker section is formed. I believe that you can tell what kind of a year it has been by the relative amounts of growth. I forgot to see if these photographs were of the same species. I suspect not because I think that the rings are wider in the bottom picture which might represent a  faster growing tree. These sorts of ideas are useful in analysing things like climate change.

Our crumple zones

 

We made crumple zones in class. The aim was to keep a marble on the trolley even after a crash. The brick wall caused the front to deform. You can argue by energy that the deformation uses up energy that would otherwise have been transferred as kinetic. But we used a momentum argument. Same mass and speed meant same change in momentum as the trolley came to a stop. But the crumpling means that it takes a longer time to stop. There is a smaller rate of change of momentum and thus a smaller force.

 

I did like the following design!

Napes Needle: Components of gravity

 

We picked our way round the Climbers' Traverse to the famous landmark of Napes Needle on Great Gable. No one was climbing it because of the horrible weather. We were out in the rain for 8 hours - not pleasant. I had been worried about the steepness of the slopes. They look as though they might be 75 degrees or something similar. However, looking at sideways photographs taken by other people suggests much less. I used my walking pole to try to gauge the slope and I'd say actually about 45 degrees. Why does it look so much bigger? It's probably to do with "distance foreshortening" but I need to think about it more. If you fall vertically, gravity makes you accelerate at a rate of 9.91 metres per second per second. But a 45 degree angle means that the acceleration would only be 9.81 x cos45 = 6.93 metres per second per second. I've heard this called "dilute gravity". And friction would reduce this further. However, that won't help if you hit a rock.

Condensation

 

Using the kettle in the hide at Threave caused problems. The hot water vapour hit the cold windows. Somehow the water vapour loses thermal energy and cools so much that it condenses back into liquid water. Is it conduction into the glass? Probably, because it works by contact. The window will then dissipate the thermal energy by conducting through and trying to heat the air outside.

Answer to the U6 estimation question #3

 

The trick is that the area of my hand doesn't matter. The flux has to pass through a metal loop so we are only interested in the flux passing through the ring itself. I used http://www.ngdc.noaa.gov/geomag-web/#igrfwmm to calculate the strength of the Earth's magnetic field for 55 degrees north and 3 degrees west. It gave me 49,623 nanoTeslas. The area through the middle of the ring through which flux can pass: well, radius is 1cm so area = 3.1 x 10^-4 square metres. Flux = field strength x area = 1.6 x 10^-8Wb. Flux linkage = flux x number of turns. There is only one turn here, so flux linkage = 1.6 x 10^-8 Wb(turns). By Faraday's Law, emf = rate of change of flux linkage. If I move my hand through 90 degrees, I go from full flux to no flux through the ring. It takes 0.1s. Hence emf = 1.6 x 10^-7V.

 

This emf makes a current flow round my ring. The resistance of my ring = (resistivity x circumference) / cross sectional area of the metal itself. Gold has a resistivity of 2.44 x 10^-8 Ohmmetres. The ring is 5mm deep by 1mm wide so cross-sectional area = 5 x 10^-6 square metres. I calculate a resistance of 3 x 10^-4 Ohms.

Current = emf/resistance so a current of 5 x 10^-4A flows. Not much to worry about there.

 

The only thing to add is that I have assumed you knew I was standing facing north, in line with the field.

Escapement in Penrith

 

I was in Penrith to ring bells and found this clock mechanism inside the front door of the church. I blogged about escapement mechanisms earlier in the year and got a good chance to inspect one here. In the top picture, there is a curved piece of metal above the top brass cog. It is attached the pendulum. What happens is that you wind up the clock. The place where you attach the winding handle is apparent in the bottom photograph (it wasn't clear whether you are winding a spring or winding up a weight but you give the mechanism some form of potential energy). This then drives the top brass cog round. The cog locks with the escapement mechanism, the curved piece of metal. This momentarily stops the cog, but then the cog pushes the curved piece out of the way and in so doing pushes on the pendulum. The pendulum then swings back and catches the cog on the other side. The process repeats. A pendulum would naturally stop swinging after a while but the mechanism is a way of taking the potential energy and giving it to the pendulum as extra kinetic energy. So the clock drives the pendulum. I'd always thought that it was the other way round. So what's the point in the pendulum? Well, by catching on the cog wheel all the time and momentarily stopping it, it forces the cog wheel to keep to the speed of the pendulum. So the pendulum regulates the speed at which the cog wheel turns and hence keeps it in time if the hands are attached to the cog. The tick and the tock are when the escapement mechanism hits the cog wheel to stop it.

Life in the flatlands - Goole

 

We went to the flatlands near Goole in East Yorkshire. It is very different to north Cumbria. Not just flat, but intensive arable farming. The visibility wasn't very good. It was a murky day. I wondered how far you could see. The furthest pylons in the distance were about 1 little finger width high - so about 1 degree. The usual calculation, based on the fact that a pylon is about 45 metres high, suggests they might just be 3km away. That' not very far considering that I could see a mountain 50 miles away in Scotland last week. Next time I visit I shall hope for better visibility and repeat the exercise.