LEN LYE

Not really about physics (any more than anything isn't!) but this picture is historically interesting, especially to residents of New Plymouth. It shows Len Lye (in black) attempting to erect an early (12m) version of his famous "Windwand" in New York in 1960. The technology back then simply wasn't up to Lye's vision. Now you can see a 45m Windwand on the New Plymouth foreshore. Lye's sculptures show an instinctive understanding of physics: if you get the chance, go and see his works. They are particularly interesting to students studying harmonics and wave-forms. The gentleman assisting him in the photo, wearing a white suit, is the English writer Robert Graves (author of "Goodbye to All That" and "I, Claudius"). In 1942 Robert Graves' daughter Catherine married the New Zealand nuclear physicist Dr. Clifford Dalton, who was later to invent the fast-breeder reactor. After Dalton's death in 1961, Catherine alleged that he had been murdered for his part in breaking the American monopoloy on nuclear technology, and even that the death of another New Zealand-born physicist, Dr. Gilbert Bogle, in 1963 was part of a cover-up to prevent Bogle from investigating Dalton's death. However it is generally accepted that Dalton died of cancer, and that Bogle's mysterious death beside a Sydney river was probably caused by hydrogen sulphide poisoning.

GOOGLE PHYSICS VIDEOS

Hi everyone
I have added a new "element" to my blog - videos from Google with the keyword "physics". I don't yet know how useful/suitable they will be, but the first lot are pretty good.
I hope you enjoy these videos, and that they make you want to know more about the physics behind them.
There is one here which is silent and therefore requires some explanation - it involves pouring liquid air (or nitrogen) onto a material which then becomes supercooled, and shows the phenomenon called "superconductivity" - essentially it has no resistance.
The silvery disc is a magnet - when it is placed above the superconductor it will fall towards it, inducing a current in the superconductor which will oppose the change inducing it and produce a magnetic field that opposes the field of the magnet, making it "levitate" . Year 13 students who have studied electromagnetism (Physics 3.6) will recognise this as Lenz's Law in action.
You can achieve the same effect by replacing the superconductor with a permanent magnet, but in this case you have to spin the "levitating" magnet so that gyroscopic precession takes care of any derangement of the floating magnet's spin.
Towards the end of the clip, the magnet is pulled upwards away from the superconductor. In response to this, the currents induced in the superconductor set up a magnetic field that tries to hold the magnet closer - again opposing the change!
Then as the superconductor heats up again on the side of the vessel, the currents in it will reduce because its resistance is increasing, and the magnet gently falls.

IMPULSE (CHANGE IN MOMENTUM)

When a force acts on an object for a period of time, it will change the object's momentum.
This quantity (change in momentum, or the product of force and time) is called IMPULSE.
We can easily derive a formula from Newton's second law. F=ma
a=Δv/Δt so F=m Δv/Δt
gives FΔt=mΔv
mΔv is the change in momentum, Δρ
In your textbooks the formula is given as FΔt=Δρ.

SAFETY FIRST

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USING COMPUTER DATA-LOGGING

Data-logging

Using the "datastudio" software we can find the acceleration of a free-falling object. The sensor we used was a picket-fence and photogate.
Angus shows the screen display of graphs and tables produced.

CENTRIPETAL FORCE (YR12)

The Yak 'planes looping over New Plymouth demonstrate circular motion.
Circular motion requires a centripetal force, which is a force directed towards the centre of the circular motion. This force is provided by the control surfaces of the aircraft pushing against the air, and by the aerofoil effect (lift) of the wings
At the top of the loop, gravity will provide some of the centripetal force, so the pilot will feel some "weightlessness"; but at the bottom of the loop, the 'plane has to have enough lift to overcome gravity and provide the necessary centripetal force and the pilot will experience a strong upward force from the seat, making him feel "squashed down".

Looping is also interesting from an energy perspective.
As the 'plane climbs into the loop it loses kinetic energy and gains gravitational potential energy. It needs enough kinetic energy at the bottom, to supply the potential energy at the top, and still have enough kinetic energy to keep flying.

DOPPLER EFFECT (YR13)

This video shows the Doppler effect, and explains the ideas of red-shift and blue-shift.
BUT...
Don't forget, that as the entire electromagnetic spectrum produced by a star will be shifted, the star as seen from Earth will NOT change colour!
For red-shift, for example, the UV will be shifted into the visible, so you will still see the violet end of the visible spectrum.
It is the shifting of spectral lines (gaps in the spectrum) that enable us to measure red-shift or blue-shift.


Here's a neat animation of the shifting of spectral lines

PHYSICS (YR10)

The Bugatti Veyron has a top speed in excess of 400km/hr. You need a good broadband connection to watch this movie. You may get better results by double-clicking the screen and watching it direct at dailymotion.

Bugatti Veyron at top speed

It accelerates from 200km/hr to 300km/hr in 9.4s

To convert from km/hr to m/s you need to divide by 3.6.

1) What is the average acceleration in m/s² over the 9.4s?
2) If the mass of the Veyron is 1950kg, what is the average force applied by the car on the road over the 9.4s?

Post your answers as 'comments'.

DIFFRACTION GRATING (YR13)

The diffraction grating (a thin film with hundreds or thousands of tiny parallel lines) produces interference patterns (Young's Experiment) with very sharp, bright fringes compared to a double-slit apparatus.
This is because each fringe produced by a grating is a result of the superposition of light from hundreds of lines.
The first-order fringe is produced when the path difference for light coming from adjacent lines is one wavelength.
The second-order when the path difference is two wavelengths, and so-on.

If the diagram on the left shows the first order fringe, then:

bc=λ
de=2λ
fg=3λ.

If it shows the 2nd-order, then:
bc=2λ
de=4λ
fg=6λ.

The angle of the n-order fringe can still be found using nλ=dsinθ, or the approximation nλ=dx/L

Young's type interference can also be seen with CD's, in which the pits act like the lines of the diffraction grating. The photograph on the left shows incident laser light (strong vertical beam) striking a CD and producing antinodal lines. The zero, first and second-order lines can be seen very clearly.

Homework:
If light of λ=632nm is incident on a CD, and the distance between the two first-order fringes is 98.4cm on a screen 1.20m from the CD, what is the distance between pits?



Link to Hyperphysics

Link to Diffraction Grating Applet

RUSTING (YR10)

The results of our rusting experiment:

The tube on the left has a stopper, and a small amount of Calcium Chloride to exclude and absorb any air (Oxygen)
This nail has no rust.

The middle tube has no stopper and is half-full of plain water. The nail is therefore open to reaction with Oxygen and Water. This nail had a lot of rusting.
The tube on the right is filled with boiled water (to remove dissolved Oxygen) and covered with a layer of oil. This nail had only a very small amount of rust

Conclusion:

Both Oxygen and Water (as well as Iron, of course! - Thanks to Ken "what's the frequency?" Lockhart) are needed for rusting to occur.

Wikipedia link

BEATS (YR13)

• At the top of the diagram, you see the waveforms created by two sources with similar, but slightly different frequencies. Notice how they are sometimes in phase, and sometimes out of phase.

• The blue waveform at the bottom of the diagram, shows the superposition of the two.

• The antinodal forms that result are called 'beats'. The frequency of the beats is equal to the absolute difference between the frequencies of the two sources.

• fb=f1-f2

Check out this link to Hyperphysics website