Rowing and Newton's Laws

The pictures are of rowers on the Thames at Oxford last Sunday along with a view of the college boat houses. When you pull the oar, you start to push the water. The water is very heavy and has a lot of inertia so it doesn't move. By Newton's 3rd Law there is an equal and opposite force pushing back from the water on the oar. This is what pushes the boat. The boat accelerates as described by Newton's 2nd Law. Newton's 3rd Law was written as "To every action, there is an equal and opposite reaction" when I was at school. These days I teach it as "If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type on object A". I think that the change is to emphasise that two objects are involved. The type of force involved here will be an electromagnetic contact force caused by pushing the atoms closer together so that the nuclei repel.

 

Lower Sixth estimation question #1

The picture below shows the tower and spire of the library of Lincoln College, Oxford. If you can magnify the picture, you'll see that there is a lightning conductor going from the point at the top and then down the tower, to the left of the window, until it reaches the ground. You need to look up the material used to make lightning conductors, and its resistivity. Then you'll need to estimate the length of the lightning conductor and the dimensions needed to calculate the cross-sectional area. The two close-up photographs below will help with this. CALCULATE THE RESISTANCE OF THE LIGHTNING CONDUCTOR AND JUSTIFY THE SIZES THAT YOU HAVE ESTIMATED.

Estimation is an important skill in Physics and Engineering.

 

Breakdown field and pylons

 

This pylon is on the flood plain of the River Thames at Kennington, an easy walk down the river from the centre of Oxford. Note the massive insulators that keep the wires from the pylons. I'd say that at all times, the wires are about 2 m from the pylon even when the wire loops down around the insulators. If the wire are at 425,000V, then the field strength is 212500 Volts per metre. The breakdown field for air is about 3 million Volts per metre, so it is well below the level that would give sparks. Calculations suggest that the insulators would need to be about 15cm long before you would get electrical breakdown.

Another benchmark

We were given a tip off about a bench mark at Carfax, the main meeting point in the middle of Oxford. It took a while to find it because it is below knee height. It's on the street side end of the buttress to the left of the door. Apparently they mapped the UK in the 1800s by going from bench mark to bench mark. I know about triangular points but I suppose I'd never stopped to think about how they filled in the contours on the map (and remember that contours were invented by physicists in about 1790 - see October 2009 piece about Schiehallion on this blog). Here is an example of one of their bench mark journeys that started at Carfax - though I think it was at a different marker. http://www.bench-marks.org.uk/line398

 

How video didn't kill the Radiohead star

We went to a charity concert in the suburbs of Oxford at the weekend. It started with an interview with Colin Greenwood of Radiohead. I just wanted you to know that I had been in the same room as somebody famous! He came across as generous, witty and intelligent. He was there because he is an ambassador for the charity being supported by KOA this year: http://www.childrensradiofoundation.org/ I wanted to post about this, but how to get some Physics into my name dropping? I thought about doing colour filters with the stage lights. But I then thought that it might be good to start a series of posts about radio. Imagine the world without the harnessing of electromagnetic waves for communication by humans! We'd be back in 1850. Radio waves are very low frequency electromagnetic waves. You set up such a wave at a particular frequency and then you either vary its amplitude in time to the voice or music (this is AM) or you slightly vary the frequency in time to the voice or music (this is FM). The electromagnetic carrier wave has a varying electric field at 90 degrees to a varying magnetic field. To detect the waves, you either get the electric field to get electrons to wobble in a pole aerial, or you get the varying magnetic field to induce electricity in a magnetic coil aerial. Colin Greenwood is almost my age. He said how much he'd enjoyed radio for music as a child. It was the same with me. I still love radio more than video for music. More to follow.

 

 

The final furlong

Physicists love measurements. There are all sorts of distance measurements out there. The furlong is one that I have never understood. It turns out that it is one eighth of a mile, which is about 200 metres. I took these photographs at Cheltenham racecourse. The first is from the finish line looking down the course. Having tried to understand a plan of the course on a racing website, I think a furlong is down to the second white stripe across the course. The second picture is from that place looking back up. Although I like metric units professionally, there is some cultural capital in the continued use of a variety of measures.

 

 

 

New Brian Cox TV programme

Brain Cox presented the first programme of a series called Science Britannica last night. It wasn't what I'd expected but it was all the better for that. I was expecting a reverential trawl through Isaac Newton and Robert Hooke. But we were straight into the difficulties of scientific ethics through GM crops, animal experimentation and the atomic bomb. So brave and clever, I thought. Well worth a look if you didn't see it http://www.bbc.co.uk/programmes/p01d56dn

Benchmarks

 

A winning analytical technique in Physics is breaking angled vectors down into horizontal and vertical components. Finding the truly horizontal can be important. I took this picture at Shap Church last week with some vague memory of what it was from childhood. Mrs B did the research after we had discussed the trig point network and found out that it is a benchmark. In surveying, the height of each horizontal line is worked out by triangulation from other marks. Church towers are useful because they are very prominent landmarks. We found info on http://www.bench-marks.org.uk/

The Tour of Britain in Wigton

Just got back from seeing The Tour of Britain cycle race come through Wigton, replete with all the big names. If you can figure out who's who then you're doing better than me. It brings up another hoary old chestnut in Physics - what keeps a bicycle stable when you're going along? I'd have gone for angular momentum. Apparently most theories until recently have centred on the motion of a gyroscope. This is something I used to understand but since I've never taught it I'm struggling a bit at the moment. However, look at this: http://www.popularmechanics.com/outdoors/sports/physics/physics-of-a-riderless-bike It looks reliable as there is a link to a scientific paper on the Science website.

 

The curvature of the Earth

This is a picture of Blackpool Tower taken through my binoculars on the beach at Conishead Priory near Ulverston on our trip to the Buddhists yesterday. Sorry it's a bit fuzzy - difficult conditions for photographing. By measuring a road atlas, I reckon that it is about 25 miles distant. Blackpool Tower is a bit like the Eiffel Tower. You can't see the wider base, which shows that some of it is below the horizon. Apparently the Greeks knew that the Earth was round because the first thing they saw of a ship was the top of the sail and then the rest appeared. So how widespread was the flat Earth belief in the Middle Ages?? Did sensible people really understand from evidence like this that the Earth was round? Wikipedia suggests that the horizon should only have been about 3 miles away, but we had good views to Heysham, which is 10 miles. I will have to look into those calculations too. http://en.wikipedia.org/wiki/Horizon

The Physics of Buddhism?

We went for a heritage open day at Conishead Priory, Ulverston. It is a Buddhist centre. They have lovingly repaired the old stately home and built a modern temple in the grounds. It was the light reflecting that I noticed. So today's question was "why is metal shiny?" I knew that it was because metal has free electrons. But Internet searches reveal that the full answer requires a full understanding of quantum mechanics. My degree is in Engineering Science, so the explanations are a little beyond me. Try this: http://www.thenakedscientists.com/forum/index.php?topic=6530.0 It's a wonderful website put up by Cambridge science enthusiasts. My normal teaching involves simple atoms like hydrogen. An incoming photon of light would excite an electron up several energy levels. It would de-excite and release a photon. But in metals, there are many free electrons. They occupy energy bands rather that precise levels, so the situation is more complicated. I do like the explanation that says that the emission is because the electrons move to cancel the electric field induced by the incident electromagnetic wave.

 

 

This photograph was deliberately staged. I wanted to capture the reflection from the bronze statue, but also the reflection in the glass. It shows that glass is partially reflective. Some photons are reflected but some are transmitted.

Winds

 

We went to ring the bells at the church in Shap. Here's the weather vane on the roof. It comes from the church at Mardale that was demolished along with the village when the Haweswater Reservoir was constructed.

In our part of the world, the prevailing wind is from the south west. These prevailing winds stem from convection in the Earth's atmosphere driven by the Sun. The Equator receives the greatest solar energy, so the air expands and rises there. What goes up must come down, once it has cooled away from the surface of the Earth and become more dense. In the northern hemisphere it comes down in a belt that includes the Sahara. The air pushing down causes high pressure and hence the desert. When the falling air hits the Earth it has to go somewhere. Some goes south back to the Equator to complete the convection cell, known as a Hadley Cell. Some comes north towards us. From our point of view it is coming from the south - it is a south wind. But the Earth is spinning which by means of the Coriolis Effect drags it into a south west wind.

Why is the sky a paler blue nearer to the horizon?

One of the great things about trying to post daily is that it is forcing me to notice things. We went to walk along the mid section of Hadrian's Wall yesterday. Not much Physics to comment on but I began to notice through the gaps in the clouds that the sky was always a paler blue further away towards the horizon. I know that the sky is blue because oxygen and nitrogen molecules scatter the shortest wavelengths of light. This is called Rayleigh Scattering. The darker blue is closer to indigo and will have a shorter wavelength. The paler blue has a longer wavelength. Now the reason given for the sky being red at sunset is that more colours have been scattered as the light has to come further through the atmosphere, leaving only red visible. But surely this would need bigger particles. It is true that there are aerosols of larger particles closer to the ground. This is the reason given on one website I found for the paler sky colour http://www.webexhibits.org/causesofcolor/14D.html I was a bit dubious about this source at first but it claims to be part financed by the US Dept of Education which sounds OK. They suggest Mie Scattering, but that should lead to white like clouds, not paler blue. I'm wondering if it is just because the light has travelled obliquely across the sky a longer distance and had more chance for the shorter wavelengths to be scattered away. It is true that to reach my eye the photons will have come through aerosols closer to the ground when coming from nearer the horizon. I will check out their claims that the sky goes a deeper blue after rainfall and that it is a deeper blue at altitude.

 

Here is the photographic evidence.

 

I like this picture because you can pick out the Cheviots in the distance.

 

 
 
 
 When I got home I photographed the sunset. Note that the sky is bluer higher up where the light has followed an oblique trace. I suppose you'd argue that there aren't the aerosols up there. And you can also pick out the lights on the Anthorn aerials, as mentioned on Monday.

A clock controlled from Anthorn

 

When I thought about it, I remembered that the clock in the ringing room at church receives radio signals from Anthorn aerials to make sure it stays correct to UK time, as set by the physicists at NPL. My Tower Captain likes to make sure the clock on the outside of the church tower is set correctly. In the old days, it would have been set to the time at which the Sun was highest in the sky locally. Few people would have owned their own watch or clock, so it was vital to the town.

The time signal

We walked across the salt marsh from Silloth to Abbeytown and had great views of Anthorn aerials. They show up well from our back garden too, but are much more distant. One of the things that they do is to broadcast the national time signal using measurements from the National Physical Laboratory, the UK's temple of measurement. 200 years ago, each town in the UK had its own time, set by local midday, when the Sun is highest in the sky. Carlisle was 8 minutes behind London if I remember rightly. This was no good for the railways, so time needed standardising across the country. This could be done by telegraph. Anthorn is now the chosen vehicle of communication. It is vital that places in the UK have standardised time for transport timetables but also for timed financial transactions.

 

Here's my picture of the aerials. Not the storm brewing. We had to shelter under a gorse bush.

 

Cantilever roof

The Grandstand at Cheltenham Racecourse is a great example of a cantilever roof. A cantilever is an overhang that is only anchored on one side. Early stadium roofs had pillars to support the open end of the roof. The problem with that is that sometimes your view of the action will be obscured if it happens to line up with a pillar. This is still a problem at Barrow AFC, the team I support. I decided to estimate the mass of the concrete section of the roof. I enlarged the photograph and counted 20 seats along in each of the blocks with coloured flags in. There are 6 of these blocks under the concrete roof. Say each person gets about 60cm to sit in, then you might get up to 15m per block including the stairway on the side. So let's say the roof is 90m long. It might by 1/4 of that length sticking out - so let's say the overhang is 15m. The concrete needs to be deep enough to cover the steel reinforcement and prevent rust. Let's say 50cm on average. That gives 675 cubic metres. Now concrete has a density of around 2.4 tonnes per cubic metre. That means the roof is at least 1500 tonnes. If you study Civil Engineering, you'll need to be able to make good estimations and will be taught how to keep the roof up!

 

Why I didn't see the Loch Ness Monster

The week after we returned from Loch Ness, a photograph was published purporting to show more evidence of the Loch Ness Monster. http://metro.co.uk/2013/08/26/do-new-pictures-from-amateur-photographer-prove-loch-ness-monster-exists-3938074/

 

I think the picture shown in the link is a stationary wave. Whilst we were at Loch Ness, a motor boat went past, trailing a wake behind it. Five minutes later, the wake was still there. The conditions had formed a stationary wave which did not decrease in amplitude for some time. At that point I decided I needed to photograph it, but by the time I'd got the camera from the van some minutes later, it had decreased almost to nothing, as shown in the picture below. So that's why I don't think the new picture shows the monster.

 

Video of the edge of space

At a local school, 3 Sixth Formers were sponsored by the Ogden Trust to send up a weather balloon carrying a box full of computer sensors. It included a video camera. The Youtube footage can be seen from the link below. You can see the balloon bursting as the air pressure outside becomes too low to resist. I love the fact that you can see the thickness of the atmosphere and the curve of the Earth. I met the students involved and was immensely impressed. They tell an amazing story of how they tracked the box to the top of a tree in Yorkshire, spotted it from a micro-light and climbed the tree to recover the camera. Congrats to all involved! http://www.youtube.com/watch?v=m9UB_ENmWwY

Spring tides

We went to Silloth for chips a couple of weeks ago. The tide was out further than I think I've ever seen it. The were sand banks visible that I have never seen before.

 

It had been cloudy in Cumbria but the following night in Cheltenham I noticed that there was an almost full Moon. It's the white globe in the middle over Cleeve Hill.

 

The two things are not unconnected. It is logically obvious that when the Moon and the Sun are on the same side as the Earth, their respective pulls on the water will add up to make a bigger bulge - so higher tides but low water will be less. What is less obvious is that they do the same thing when they are on opposite sides of the Earth but still in line. This is the case with the full Moon. It's called a Spring Tide as in springs of water open up, not the season. I've still not fully grasped how the fields add up when the pulls seem to be in opposite directions but there's something for me to work on this autumn!

Expansion on the railway

On the way back from Cheltenham, we stayed at Beddgelert in Snowdonia. The Welsh Highland Railway is a recently re-installed steam line. We noted the expansion joint between the rails on the platform. On a hot day, rails expand which would make them to deform and cause derailments. To prevent that, it was traditional to leave frequent gaps between the rails to give the rails space to expand into. As the train rolled over the gaps, you got the traditional "clickety-clack" noise. I had noticed that it had disappeared but hadn't thought much of it. Apparently we now have Continuous Weld Rails which have to be securely bolted to the sleepers. This bolting down prevents distortion due to expansion.

 

 

This is looking along the track from where the first picture was taken. I think the next joint was before the hut.

 along th