Diffracted Moon

I also took a photograph of the Super Moon through net curtain. The weave on the net curtain is fine enough to act as a diffraction grating. There are both vertical and horizontal threads so the light is diffracted up and down by the horizontal threads and then side to side by the vertical threads. The result is the cross pattern. The weave is about 3 stitches per mm so say 0.3mm. Wavelength of light is of the order of 500nm. These figures give a tiny diffraction angle but since the Moon subtends 0.5 degrees, diffraction angle is of that order. The discrepancy is likely to be due to using a wide light source instead of a point source. I need to try it with a laser later.

Super Moon

The Moon doesn't orbit the Earth in a perfectly circular orbit. The orbit is slightly elliptical - in other words, slightly egg shaped. It takes 28 days for the Moon to go round once. In the meantime, the Earth moves round the Sun. A full moon occurs when the Sun, Earth and Moon are lined up. Because the Earth is going round the Earth, 28 days later the Moon won't be at the same point on its ellipse. So this month it happens that the Moon is at the part of its orbit where the ellipse comes closest to Earth in time for the full moon. This makes the Moon look a little bit bigger, they say. It's called a Super Moon and attracts a lot of media coverage. This one is called a Pink Moon apparently to call it a Super Moon in Spring when pink blossom is out, not because the Moon was coloured pink. There won't be a Super Moon next month for reasons outlined above, but it is not uncommon and there will be 2 more this year.

Rainbows again

This blog is still loving the proliferation of rainbow posters in support of NHS staff. It's not been raining recently, fortunately, so we haven't had a lot of rainbows to look at. As I remember, for a normal rainbow, red is at the top and following the mnemonic Richard Of York Gave Battle In Vain, the colours go down to violet at the bottom, as shown in the poster above. That's unless there is a double rainbow, when the colours in the upper bow are reversed, or indeed when there is a supernumerary rainbow when you get pastel colours like pinks. https://en.wikipedia.org/wiki/Rainbow#Supernumerary_rainbows and http://wigtonphysics.blogspot.com/2009/11/wigton-church-stained-glass.html I hadn't realised that these latter gave the first proof of the wave nature of light, being an interference pattern not a refraction effect. So not all rainbows need to follow the traditional order! Perhaps this might excuse the old stained glass in Wigton church http://wigtonphysics.blogspot.com/2009/11/wigton-church-stained-glass.html

Trying to improve my Foucault's Pendulum

 The first thing I did was to try to reduce the effective area of the pendulum. I tried a marble as a sphere has a smaller drag coefficient but it was much lighter so didn't get as much gravitational potential energy. I then went for the same stack of 1p pieces but with glue so I could do with the cork into which I'd screwed the fixing point.
I also worked on a better mathematical analysis of the work done by drag, as shown below. (A stands for cross-sectional area and a will be amplitude) The size of the drag force varies with displacement from equilibrium because the speed varies with displacement. In my last post, I used an average drag force to get a ball park figure.

 Here I've tried integrating between amplitude and -amplitude, half a time period.

                                      
So to reduce the work done by drag, amplitude is the key factor. A small amplitude is better. But I need an amplitude big enough to be able to tell that the direction has changed. Longer will also be better. That might just be possible if I can attach to the top rafter.

Trying to make Foucault's Pendulum

A proper Foucualt's Pendulum looks like this https://www.youtube.com/watch?v=aMxLVDuf4VY and has a frictionless magnetic drive at the top to keep it swinging 24/7. But then I found this https://www.youtube.com/watch?v=sWDi-Xk3rgw and decided to have a go with his design, because I realised that if I could get a pendulum to swing for maybe as little as half an hour, I might notice the change in angle as the Earth rotates underneath the pendulum. I built one about 2 metres long from a beam, suspended from a crocodile clip as was his.

 I don't have any proper pendulum bobs here so I stuck together a pile of 1p pieces and a piece of cork on top into which I screwed a hook. I tied it back and burned through the cotton as shown in the diagram so that it was able to swing true.

 The amplitude was large at first.

It was still swinging after 15 minutes but the amplitude was so small that it was hard to detect much change. To be fair, I realised that I didn't know in which direction it would rotate and thought I detected a very slight clockwise rotation. It turns out that this is correct for the northern hemisphere. However, I need to get it to swing for a lot longer. My first calculation will be the drag force, which = 1/2 x drag coefficient x cross sectional area x density x speed^2. The bob was a vertical cylinder of height 3cm and diameter 2cm. I got the formula for drag coefficient from http://documentation.dsaocean.com/tutorials/Tutorials/PDS-ACP.html = 0.66 approx. Density of air is about 1 kg per cubic metre. Time period is about 3 seconds and it swung about 50cm there and 50cm back so speed = 0.33 m/s approx. Putting the numbers in gives an average drag force of order = 0.00002N. Then average work done every 3 seconds = 0.00002J. So in 15 minutes that is 0.006J. Bob has mass of about 50 grams. If the work done by air resistance has dissipated all of the gpe put in at the start, then mgh = 0.006J and thus h=0.01 metres, or 10cm. It probably wasn't raised quite that high but the calculation does suggest that air resistance is the biggest cause of slowing. 

Carbolic soap

 The hand wash in my bathroom has run out. Then I remembered that we bought some carbolic soap from the a museum last year. It stinks! https://en.wikipedia.org/wiki/Carbolic_soap It is a noted anti-septic but the warning is that it could be a skin irritant.

 I had an idea that it contains carbolic acid so I decided to test it with my red cabbage indicator. I shaved some bits of soap of and made a weak watery solution of it, which was pink like the soap.

Then I added the red cabbage indicator. The result is actually an alkali, as is the case with soap. Searching around, it is likely that the carbolic acid (phenol) is reacting with the fats used and making a slightly alkaline product. This is Chemistry, though, so not my main subject. https://en.wikipedia.org/wiki/Phenol has the interesting anecdote that it was its early use in sewage treatment in Carlisle that alerted Lister to its possible use as an antiseptic. It seems that in Carlisle it all about the smell.

Stretching an elastic band

I got an elastic band and hung it from a wooden spoon from the kitchen between two chairs. I tied a water bottle to the bottom of the elastic band. Then I put a washing up bowl underneath the bottle in case I spilled when I was pouring water into the bottle.
I wanted to do an experiment where I added masses to the end of the elastic band to make it stretch but we don't have masses at home. Then I remembered that 100 cubic cm (which is 100ml) has a mass of 100 grams. So I found the measuring jug in the kitchen.

To make sure that the measurement is ACCURATE you have to get down to eye level to view it. Remember that the middle of a liquid sinks - this is called the MENISCUS - so you see a double line for the liquid in the jug. The lower line is the meniscus and it is this line that needs to sit on the 100ml mark.
First I measured the length of the elastic band with no water in it.

Then I added 100ml of water and measured the length again. Then I added 100ml more and measured the length. I could do this all the way to 500ml.
We don't normally use length, we use extension. You calculate extension by subtracting the first length (the length with no water) from every other length.
Next I turned the mass in grams into force in Newtons and produced a graph.

Notice that the line doesn't go through the origin - the line is much steeper at the start but then goes straight. (If you add even more mass to it, the line will go very steep at the end.) The gradient of the graph is calculated by drawing a big right-angled triangle onto the line and calculating (delta y)/(delta x). The gradient is called the SPRING CONSTANT and in this case is measured in Newtons per cm because gradient is always y-axis units divided by x-axis units.