### Physics Workshop presentation

```www.bpho.org.uk
Oxford 24th June 2014
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Physics Challenge
AS Challenge
A2 Challenge
Experimental Project
BPhO
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Round 1
Round 2
Training Camp
IPhO
“Moreover a physics problem should be difficult in
order to entice us, yet not completely inaccessible,
lest it mock at our efforts. It should be to us a guide
post on the mazy paths to hidden truths, and
ultimately a reminder of our pleasure in the
successful solution”.
David Hilbert
Robin Hughes
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King’s College School Wimbledon
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Rutherford Schools Project www.Rutherford-Physics.org.uk
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www.BPhO.org.uk
[email protected]
What makes a
student competitive
in physics and
engineering?
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Problems that demand understanding?
Linguistically stylised – interpretation & recognition
Massless pulleys
Infinite planes
Inextensible massless string
Point particles
Zero friction
Etc.
Superfluous information
Occurs in the real world
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Transferable skills
Clarity of thought
Perseverance
The buzz of success
Confidence
Interest
Empowering
Our people are
our greatest
asset
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Explanations
Computations & calculations
Estimates & Fermi problems
Technique spotting
Proofs
Bookwork
Data analysis
Recent research by SEPnet (from ASE EiS April 2011)
Employer views of the skills of physics graduates
indicated that the three aspects most highly prized
were those of
 mathematical competence
 the ability to use equipment to produce evidence
 being good at problem solving.
What was disturbing was the view that the only one
that employers felt they were getting was the first.
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Requires a knowledge of physics ideas
Requires a “feel” for some of the ideas
Requires putting in numbers
Requires a feel for the physics and what
seems reasonable
2
501
–
2
499
0. 4
Is it likely that you breathe in a molecule
from Caesar's last breath?
Estimate the mass of the earth's atmosphere
Estimate the temperature of a newly formed
star
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Any good ideas?
Any numbers we know?
Is it too hard?
Is the hard way the only way?
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When a river floods, the debris that is left
behind is often seen in the form of large
boulders. Most rivers do not flow very much
faster when the river floods as the slope of the
river bed remains the same.
What is the physics?
What are the variables?
Are they related?
What is the result?
Is this what we
observe?
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Mass of the boulder rolled m
Speed of the river flow v
Density of boulder (and river combined into
some density parameter) ρ
field strength g
Derive a dimensionally homogeneous
equation for m in terms of v, ρ and g.
= (, ρ, g)
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[ M ]  [v ] [  ] [ g ]
1 
3 
2 
[ M ]  [ LT ] [ ML ] [ LT ]
equating powers of M , L, T
M 1 
L
0    3  
T
0    2
  3
 6
Mass of rock swept down by a flooding river:
v 
M k 3
g
6
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What is the (simple) physics?
Is it a fundamental physics idea?
What are the variables?
Are they related?
What is the result?
Is this what we observe?
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An explosion produces a pressure wave and
the speed of the wave is determined by the
nature of the surrounding medium and the
energy of the explosion.
Explosions producing pressure waves in the
air can be can be caused by atomic bombs,
exploding petrol cans, nitroglycerine, etc.
R  f ( E, t ,  )
R 
R  const E  t or E  2
t
1
5

1 2
5 5
5
E = 1.2 x 4.2 x 1013 J
ρair=1.2 kg m-3
1 tonne TNT = 4 x
= 5 x 1013 J
109
J
= 5 x 1013 / 4 x 109 T TNT
= 12 kilo tonne TNT
Trinity Atomic Explosion
0.006 ms
53 ms
16 ms
62 ms
25 ms
90 ms
R5 / m5
Trinity Explosion
R5 = 4.2 x 1013 t2 (+ 6 x 109)
R2 = 0.996
4.0E+11
3.5E+11
3.0E+11
2.5E+11
2.0E+11
1.5E+11
1.0E+11
5.0E+10
0.0E+00
0
0.002
0.004
0.006
t 2 / s2
0.008
0.01
Trinity Explosion
y = 0.367x + 2.7
R² = 0.997
2.4
Log(R/m)
2.3
2.2
2.1
2
1.9
1.8
-2.4
-2.2
-2
-1.8
-1.6
Log(t/s)
-1.4
-1.2
-1
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A star of uniform density is formed from a
very large cloud of gas
The loss of gravitational potential energy
appears as thermal energy of the star
Average stars radiate due to fusion
processes going on internally. But how does
this start?
Do the “hot” protons get close enough to
fuse, and then start the exothermic
(nuclear) reaction?
Mass dm falls from a great distance to radius
r and forms a thin shell of thickness dr
Integrate up from 0 to R to determine the
total gpe lost.
GPE lost in forming a star of mass M, of radius
R, and of uniform density ρ is given by
3 GM 2
5 R
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For the sun, M = 2 x 1030 kg
no. of protons, N (1.2 x 1057 )
Average ke of a proton (3.3 x 10-16 J ≈ 2.2
keV)
Temp of star (1.6 x 107 k)
Closest approach of protons (3.5 x 1013 m)
Range of strong nuclear force ≈ 10-15 m
de Broglie wavelength ≈ 6 x 10-13 m
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Tea
Social event
Portfolio of questions
Pupils are the key asset
Teacher role
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Overall winner of the 1988 IPhO Competition