Amazing ice crystals in Boredale

 

We walked down Boredale and found these amazing ice crystals that had formed around grass blades by a stream. The water must splash over the sides and freeze against the grass blade, then providing a bigger target for further droplets to freeze onto.

Water valves on Boredale Hause

 

The mains water signage on Boredale Hause is always strange to see. There seems to be a lot of speculation about the precise nature of the path - some say aqueduct from Hayeswater but I can't find a map showing its course and the photographs showing aqueduct near Hayeswater are much lower down the hill. However, it seems that the pipes continue down into Boredale. It'ds also quite hard to find information about the signage. It seems AV means air valve, so that gas that has built up in the system can be bled. This would normally be at the high point in a system because air is less dense and floats up. SV is sluice valve and allows the water flow to be stopped. The top number is apparently pipe diameter in mm and the lower number would be distance to the valve - but 100 metres would be a long way. These might not even be metric.

Double glazing; triple reflection

 

I noticed that the halogen bulbs produced a triple reflection in the double glazing. The nearest reflection must come from the front glass and the second bright reflection will be from the outer layer of glass. But I noticed a third, dimmer reflection. This must be when some of the light coming back from the outer pane reflects back out off the outer side of the inner pane. It is then reflected back our way again. I wonder if by drawing it is possible to figure out how the path differences result in the different angles.

Investigating the varifocals

 I noticed that the left hand lens will focus the ceiling lights onto the carpet. This suggests a convex lens producing a real image.

The right hand lens produces a bright boundary outside the dark image of the lens which suggests a concave diverging lens.

I will have to look more carefully at how the two different lenses affect the optical coronae.

Making a noise with the hot chocolate tub

 The empty tub of hot chocolate flakes has a membrane at the end labelled A.

When I push in the bottom of the container (red arrow below) the membrane makes a noise when I push it in fast but not when I push it in slowly.

There is a volume of air trapped inside the container. It is able to get out through the tiny gap between the two parts of the container, as shown by the blue arrows. There is clearly a maximum flow rate through this gap. Pushing in fast means that the squeeze of the air into the top of the case happens faster than the air can leak out of the gap. This increases the pressure enough to pop the membrane out, making a noise. It then flexes back, so you get a double sound. It only happens by being pushed outwards. Pulling the bottom half out again fast makes no sound. 

Varifocal coronae

 

The optical coronae through steamed-up glasses have been keeping me entertained. This morning I succeeded in photographing the effect of varifocal lenses. I've circled one light and shown what it looks like through different parts of my glasses. The top picture shows the lower part of my glasses being used. This spreads the light out more in the corona. It's the part of the glasses that are designed for reading so should be more convex. 

The Packwoman's Grave in Rossett Gill: satellite resolution

 

In Wainwright's Guide to the Southern Fells, he mentions on page 4 of the Rossett Pike page about the grave of packwoman from the late 1700s. I read about it over 40 years ago but because he declines to reveal the exact location, I assumed I'd never visit. It turns out that it is 300 metres higher up than I had imagined as a child and is considerably bigger than I was expecting. In fact, it is so big that it is visible from space on the Google Maps satellite image. The large stone at the end is covered by 9 pixels at the highest magnification so that suggests a pixel covers a 10cm square on the ground. I have now checked here and they state the resolution to be 15cm so I'm pleased with that estimate. 

How hard was that kick?

 I wanted to know how many Newtons of force I use to kick a football. I wrapped the ball in foil and weighed it. It has a mass of 0.385kg.

I needed to time the contact between my foot and the ball. I used the Scaler/Timer on "timing" mode with the leads conected to Start and Earth. In this mode, it times how long the two leads are in contact when they touch.

Next I put foil on my shoe to kick the ball.

The contact time recorded was 0.039 seconds.

I then filmed how far along the ball went. 

Stopping the film showed that the ball had gone 3.25 metres. The table was 0.85 metres high. Using the suvat equation s=ut+1/2at^2 for the vertical drop of the ball, it was in the air for 0.41 seconds. Assuming constant horizontal speed, the ball left my foot with a speed of 3.25/0.41=7.8m/s. So after the kick, the ball had a momentum of 0.385 x 7.8 = 3.0kgm/s. It had 0 momentum before the kick so the change in momentum was 3.0kgm/s. The kick lasted 0.039 seconds so using force = change in momentum/time taken, F=3.0/0.39 = 77N.

Conservation of linear momentum

 I did an experiment to attempt to verify the Principle of Conservation of Momentum using the linear air track.

The vacuum cleaner's outlet vent is connected to the air track so that air comes up through a series of tiny holes in the track. V-shaped plastic gliders can then travel almost friction-free along the layer of air. The only problem is that getting it level so that the gliders stay in place when not moving is difficult. I think the track is slightly warped because two next to each other can drift off in opposite directions.

                                        

            In the experiment, I pushed B from the right so that it travelled through the first light gate. A was stationary between the two light gates.

                                         

It stuck to A using Velcro attachments.

The two travelled together through the second light gate. The data is below.

For Conservation of Linear Momentum, the combined speed shown on the left on the screen should be half the velocity of B on its own beforehand. You can see that this worked for the second attempt.

We did some calculations based on the mass of the gliders being 50 grams. I was pleased with my second example where the gliders were both pushed so they they collided into each other between the two light gates.

A milk drop rebounding from the surface of the water

 

Taking inspiration from Day 2 of this year's Physics in Advent, I filmed a milk droplet in slow motion as it dropped into a glass of water.

Above shows the drop falling.

Next it has just hit the surface of the water.

Now it goes a little way down into the water, opening up an envelope in the surface of the water.

In this final picture the white drop has started to go up a little way. The solution film for the Day 2 question shows it much more clearly. They must have a better camera, and their drop bounces much higher. The reason they give is that the surface tension is strong enough to push the drop back up. There must be elastic stored energy in the distorted surface that can then be transferred back to kinetic and then gravitational potential in the drop.

More colour filter experiments

 

After considering the stained glass, I was back with the classic colour filter experiment. As shown above, only blue will get through a blue filter. The other colours are absorbed. This suggests that the scattered light from the clouds contains blue but the tree is not reflecting any blue in my direction.

                                     

Same with the red filter.

And when you put the two together, one absorbs all colours but red and the other absorbs all colours but blue so so see black.

When I've done it in class, they have always pointed out that they can see violet lights. I've never been able to explain that. How can violet light avoid BOTH filters? Is it to do with the intensity of the beam? It can't be ultraviolet because I can't see ultraviolet. I can't find an explanation.

Stained glass inside out

 This week I went round to the outside of the stained glass that I wrote about last week. The reason it looks black from the inside is that the light that hits it passes through and here it is, visible from the outside. The stained glass is translucent.

A wad in Keswick

 

The window of the Graphite Gallery in Keswick has a wonderful display. Wad was the local name for the graphite that was mined at the far end of Borrowdale. It was incredibly valuable at the time for military reasons. It has had many uses in Physics. One is as a moderator for nuclear fission reactions. For uranium-235, fission is more likely with slower neutrons but those released tend to be fast. A moderator slows them down. This article explains why graphite is a good moderator. It explains that if you throw a tennis ball at a wall, it bounces back at roughly the same speed. The large mass of the wall means that it stays still. It would be modelled as an elastic collision where both momentum and kinetic energy are conserved. But for two snooker balls, the one moving stops on collision and the stationary ball moves on. So carbon works as a moderator because it is not too much bigger than a neutron. The cited article explains brilliantly about carbon not being good at absorbing neutrons.

Newton's Third Law experiment

 

Put two people on skateboards. Get them to pull on each other with a spring balance. The forces measured can be shown to be equal and opposite. Eventually they pull themselves together.

No colour in the stained glass

 

It occurred to me during the Advent Carol service that stained glass must let light though it, or absorb it. It certainly doesn't reflect much. The walls around were white with reflected light but the window was resolutely black. Certain colours will be absorbed but most light must just be making its way outside.

Why are snowflakes white?

 

This Christmas decoration got me thinking about snowflakes. It turns out that the actual crystals are colourless but they have many surfaces that reflect. Tere are two types of reflection shown below. The top one is called specular reflection. It is the sort of reflection you get from mirrors that enable you to see your reflection. As you can see, the flat surface reflects all rays incident at the same angle away at the same angle of reflection. The bottom one is called diffuse reflection and shows what happens when the surface is uneven. As ever, angle of incidence = angle of reflection, but the different surfaces mean that the angles of incidence are actually all different. The result is no clear image. If all wavelengths are reflected in this way, the result is white.

Glencoyne hydro-electric

                                  

We found this sign as we walked up to Seldom Seen.

Here's the view over the houses at Seldom Seen. Glencoyne is the perfect location for hydro-electric because of the very high level of the hanging valley.

In fact there was a dam up here in lead mining days. The new water intake is just visible in the mid-right of the picture.

There was a building at the end of the new track at the lower level which could be the generator shed. We didn't go down.

400000kWh per year for 120 houses is 3300kWh per house. This is 380 Watts averaged across the year for a house. Not too big. It means generating at nearly 50kW on average. Checking here it turns out th.at the max power generation is 100kW.

Scattering experiment in the kitchen

 I filled a tall glass with tap water and shone a white LED head torch up from below. From the side, the water looks clear.

From above you can still see the torch underneath. The LED is clearly white.

Then I stirred in a small amount of milk powder, maybe quarter of a teaspoon.

From above the bulb now looks red. This the sunset effect. White light is made up of a mixture of many colours of which red had the longest wavelength. The milk powder particles are able to knock the shorter wavelengths sideways. This is called scattering. Only the longer wavelength red wavelengths can reach the top.

Viewed from the side, the bottom of the glass had a bluish hue, that hasn't photographed well. The shorter blue wavelengths are scattered sideways most easily. Towards the top of the glass the edge looks yellowish.

A success for the Newton's Second Law experiment

 

In this experiment I take a 900 gram dynamics trolley and have it pulled by weights on a string through two light gates so that the acceleration can be measured. The first time I ever did this, I made the mistake of just adding weights to the hanger on the end of the string using weights from a stack on the table. That did make the pulling force go up but it also increased the total mass being accelerated, because these hanging masses are accelerated just the same as the trolley. So now I use a total of 2000 grams. I start with 1000 grams on top of the 900 gram trolley, with 100 grams hanging from the string. I then transfer across 100 grams at a time so that the pulling force increases by 1N every time but the mass stays constant.

The force is actually supposed to be the resultant force. This would need to include friction, meaning that the actual force wouldn't be the 1N, 2N etc on the string but would be less than that. You get rid of friction not by eliminating it but by compensating for it. You do this my propping the book up ever so slightly so that a trolley without string would roll down at constant speed.

The reward was a decent straight line of best fit through the origin. It doesn't happen very often!