BALLISTIC PENDULUM

The ballistic pendulum consists of a suspended block which is struck by a fast-moving projectile. It is an example of "combined mechanics" meaning that it involves physics concepts from a range of topics.
The topics are:
conservation of momentum (when the slug hits the block)
conservation of energy (when the block rises up after being struck by the slug).
Data collected by 12PHYMC classes:
Mass of slug = 0.50g
Mass of block = 252.0g
Height change of block after being struck = 6.0mm.


Calculations
As the block moves upwards, kinetic energy is converted to potential energy
½mv²=mg∆h
=> v=√(2g∆h)
=> v=√(2*10*0.006)=0.3464m/s
So we now know the block had momentum immediately after being struck of 0.2525*0.3464=0.08747kgm/s.
As the block had no momentum before being struck, this must also be the momentum of the slug immediately before it hit the block.
So now we can work out the size of the slug's velocity:
ρ=mv
=>v=ρ/m=0.08747/0.00050=174.9m/s (170m/s to 2 sf)

Ballistic pendulum
Video sent by johnmc2

"Physics for Future Presidents"

An interesting lecture which touches on many of the topics we are looking at this term.

NASA SPACE SCHOOL

Akshay and Leela were interviewed by the Daily News yesterday and their story appeared on page two of this morning's edition. Congratulations to these NPBHS and NPGHS students on being selected to represent us at the NASA Space School later this year.

Photo: Daily News

MOMENTUM PROBLEM

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

10SCAMC modelling the latest in laboratory eyewear


















Come on guys let's be serious!

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.