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