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Physics Challenge

AS Challenge

A2 Challenge

Experimental Project

BPhO ◦ Round 1

◦ Round 2

◦ Training Camp

◦ IPhO

www.bpho.org.uk Oxford 24th June 2014

Robin Hughes

King’s College School Wimbledon

British Physics Olympiad www.BPhO.org.uk

Rutherford Schools Project www.Rutherford-Physics.org.uk

“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

rwh@kcs.org.uk

What makes a student competitive

in physics and engineering?

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

Transferable skills

Clarity of thought

Perseverance

The buzz of success

Confidence

Interest

Empowering

Our people are our greatest asset

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.

• 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

5012 – 4992

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

0. 4

Any good ideas?

Any numbers we know?

Is it too hard?

Is the hard way the only way?

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?

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.

𝑚 = 𝑓(𝑣,ρ, g)

6

3

20

30

1

,,

][][][][

][][][][

231

T

L

M

TLMofpowersequating

LTMLLTM

gvM

3

6

g

vkM

Mass of rock swept down by a flooding river:

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?

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.

2

5

5

2

5

1

5

1

t

REortEconstR

),,( tEfR

E = 1.2 x 4.2 x 1013 J

= 5 x 1013 J

= 5 x 1013 / 4 x 109 T TNT

= 12 kilo tonne TNT

ρair=1.2 kg m-3

1 tonne TNT = 4 x 109 J

0.006 ms 16 ms 25 ms

53 ms 62 ms 90 ms

Trinity Atomic Explosion

R5 = 4.2 x 1013 t2 (+ 6 x 109) R2 = 0.996

0.0E+00

5.0E+10

1.0E+11

1.5E+11

2.0E+11

2.5E+11

3.0E+11

3.5E+11

4.0E+11

0 0.002 0.004 0.006 0.008 0.01

R5 /

m5

t2 / s2

Trinity Explosion

y = 0.367x + 2.7 R² = 0.997

1.8

1.9

2

2.1

2.2

2.3

2.4

-2.4 -2.2 -2 -1.8 -1.6 -1.4 -1.2 -1

Lo

g(R

/m)

Log(t/s)

Trinity Explosion

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?

GPE lost in forming a star of mass M, of radius R, and of uniform density ρ is given by

R

GM 2

5

3

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.

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

Tea

Social event

Portfolio of questions

Pupils are the key asset

Teacher role

Overall winner of the 1988 IPhO Competition

Conrad McDonnell (UK)

O levels 1986

A levels 1988

Special Paper 1988

Ox Entrance Paper Nov ‘87

Overall winner of the 1988 IPhO Competition; Conrad McDonnell (UK)

O levels 1986, A levels 1988, Special Paper 1988, Ox Entrance Paper Nov ‘87

O & C Special Paper 1988

Bathed in the Glow of Success