Year 12 Applied Science: Glycolysis reaction

This is elderberry wine on its way to completion. I added sugar to the elderberries and then added yeast. Yeast is a living things, a single-celled type of fungus. Like us it needs to do some form of respiration to turn its food into useable energy. Unlike us, it can only do a type of anaerobic respiration - respiration without oxygen. For yeast, the full reaction is called FERMENTATION.
However, it does involve the GLYCOLYSIS reaction in the CYTOPLASM of the cell that we have studied. Here it is...

The energy supply to start the reaction comes from ATP. This contains 3 phosphate groups but bonds are broken as it loses a phosphate group to become ADP. Breaking these bonds releases some energy and this energy is used to turn glucose into glucose phosphate, Glucose has SIX carbon atoms as does glucose phosphate. But then glucose phosphate splits into two TP molecules. Each TP molecule contains  only THREE carbon atoms.
Each TP molecule now needs to turn into pyruvate. To do so, it needs to lose a hydrogen and a phosphate. Bonds are broken so the TP loses energy. That energy has to go somewhere. It is used up turning ADP back into ATP. And the NAD+ mops up the spare hydrogen atom.
We can get away with saying that ATP is high energy and ADP is low energy. Going from ATP to ADP releases energy. Going from ADP to ATP needs energy. Overall glycolysis makes more ATP than it uses up. So the cell ends up with more high energy ATP than it had at the start. So we say that the cell has gained energy from the breaking up of glucose into two pyruvate molecules.

Custard/corn flour with water is not thixotropic

 We added water to a custard/corn flour mix. Above, it was flowing freely. Then we tried to stir it (below). It became solid and the stirrer wouldn't move.

I confidently told my class that the mixture is thixotropic. But apparently it isn't http://www.thenakedscientists.com/forum/index.php?topic=3697.5;wap2 It is similar but undergoes shear thickening instead. I'd never heard of this. Here's what I found https://en.wikipedia.org/wiki/Dilatant The custard mixture is essentially a suspension - ie tiny solid particles floating in water. Applying a shear stress (that's basically a tearing force) the particles get pushed close enough together that instead of electrostatic repulsion, they start to attract each other using forces called Hamaker forces. I've come across Van der Waals forces before where a negative charge on one particle repels electrons to the far side of the neighbouring particle, leaving the near side positive so that - hey presto! - they attract. Hamaker forces are this done with bigger particles. The particles in the suspension are pushed close enough together for this to happen and that grabs the particles into to order to be like a solid.

Total Internal Reflection: DVD player

I was interested to find out that the indicator LEDs for the DVD player were actually some way down under the display, soldered to the main circuit board. Light is taken up and along to the front of the display by some solid chunks of clear plastic. The light goes vertically up and then hits the outer slanted edge of the plastic. The angle of incidence will be 45 degrees. If the refractive index is the right size then the critical angle will be lower than 45 degrees so the light will reflect. Angle of reflection will be 45 degrees meaning the light is bent through 90 degrees and exits horizontally. The bigger semi-transparent window is to allow infra-red from the remote control to be detected by the sensor on the circuit board.

Centre of mass of the UK

 To find the centre of mass of an irregularly shaped object, you hang it so that it can swing freely and let it settle under gravity. There is then a plumb line hanging down from the suspension point. Draw a line vertically down the plumb line. Repeat for a different suspension point. Where the lines cross, that is the centre of mass. So the centre of mass lies directly below the suspension point.

 The reason is shown below. If I pull the object to the left side and out of equilibrium, there is more weight on the left of the plumb line that on the right. Each of the two divided sections of weight are act as a turning force (a moment). The moment on the left is an anti-clockwise moment, The moment on the right is a clockwise moment. But the anti-clockwise moment is bigger so there is a resultant moment pulling the shape back down. There is no resultant moment when equal weights are on either side of the plumb line. That can only happen when the centre of mass is below the suspension point.

If you suspend the object through the centre of mass, there will always be equal weights on either side of the middle. You can swing it to any position and it will stay there. There will never be a resultant moment.

Finishing the birthday balloon calculations

I modeled the balloon as a flattened cylinder. I calculated the volume as 0.0113 cubic metres. Air has a density of about 1.2 kg per cubic metre so mass of air displaced = 0.0135kg of air displaced so upthrust = 0.135N. Density of helium is 0.164 kg per cubic metre so there is 0.002kg of helium in the balloon. But the foil weighs something too. I calculated the surface area of the balloon as 0.301 square metres. I looked for possible thicknesses of foil on the Internet and came up with 0.03mm. Density of aluminium is 2700 kg per cubic metre giving the mass of the foil as 0.024kg. Weight of the balloon = 0.26N ie more than the upthrust so the balloon shouldn't float. The thickness must be wrong. On yesterday's post, I think the derivation of the equation was correct, so I stuck with the measured time period of 7 seconds and worked backwards to a thickness. It came out as foil of thickness 0.014mm. That doesn't sound impossible.

Birthday balloon simple harmonic motion analysis

The first part of the document below shows how I do an SHM analysis for a pendulum. The restorative force pulling it back to equilibrium acts along the radius of the motion so perpendicular to the string. This means that no component of tension is involved, The resultant force turns out to be the sine component of the weight mg. But sin(theta) = x/L. For small angles x is approximately equal to the true displacement.

The balloon analysis takes up the bottom two thirds of the page. It works in much the same way except that two forces act on the balloon vertically - the upthrust and the weight. I have turned them into a resultant F. Hence the intertial mass m does not cancel this time although the solution as the same form. I got a theoretical answer of 1,1 seconds having measured it as 7 seconds. I will post later about how I got estimates for the upthrust and mass.

Birthday balloon pendulum

I received a helium balloon on my birthday! I found that it had a very damped oscillatory motion. I counted 5 cycles in 35 seconds. This was an occasion on which I should have used a fiducial marker. This is an indicator of the centre of the oscillation. Oscillations are timed from the fiducial marker. This is because the object is moving fastest at that point. I timed from the amplitude. That seems better but the problem then is judging the exact time at which it stops moving and turns to go back. With fast oscillations there is little difference but with slow oscillations using a fiducial marker is more accurate. So for my balloon, one time period = 7 seconds. For a normal pendulum, time period T = 2pi x square root (L/g). So if this equation applied to the balloon, L=T squared x g / 4 x pi squared, which comes to 12.4 metres. The string was less than 2 metres long so the normal pendulum equation does not apply.