British Physics Olympiad 2018-19

BPhO Round 1
Section 2
16th November 2018

This question paper must not be taken out of the exam room
Instructions
Time: 5 minutes reading time (NO writing) and 1 hour 20 minutes for writing on this section.

Questions: Only two questions out of the five questions in Section 2 should be attempted.
Each question contains independent parts so that later parts should be attempted even if
earlier parts are incomplete.

Working: Working, calculations, explanations and diagrams, properly laid out, must be shown for full
credit. The final answer alone is not sufficient. Writing must be clear. If derivations are required, they
must be mathematically supported, with any approximations stated and justified.

Marks: Students are recommended to spend about 40 minutes on each question. Each question in
Section 2 is out of 25, with a maximum of 50 marks from two questions only.

Instructions: You are allowed any standard exam board data/formula sheet.

Calculators: Any standard calculator may be used, but calculators cannot be programmable and must
not have symbolic algebra capability.

Solutions: Answers and calculations are to be written on loose paper or in examination booklets.
Graph paper and formula sheets should also be made available. Students should ensure that their name
and their school/college are clearly written on each and every answer sheet.

Setting the paper: There are two options for sitting BPhO Round 1:
a. Section 1 and Section 2 may be sat in one session of 2 hours 40 minutes plus 5 minutes reading time
(for Section 2). Section 1 should be collected in after 1 hour 20 minutes and then Section 2 given out.
b. Section 1 and Section 2 may be sat in two sessions on separate occasions, with 1 hour 20 minutes
plus 5 minutes reading time allocated for Section 2. If the paper is taken in two sessions on separate
occasions, Section 1 must be collected in after the first session and Section 2 handed out at the
beginning of the second session.
 Important Constants

Constant Symbol Value

Speed of light in free space c 3.00 × 108 m s−1

Elementary charge e 1.60 × 10−19 C

Planck’s constant h 6.63 × 10−34 J s

Mass of electron me 9.11 × 10−31 kg

Mass of proton mp 1.67 × 10−27 kg

atomic mass unit u 1.661 × 10−27 kg

(1 u is equivalent to 931.5 MeV/c2 )

Gravitational constant G 6.67 × 10−11 m3 kg−1 s−2

Acceleration of free fall at Earth’s surface g 9.81 m s−2

Permittivity of free space ε0 8.85 × 10−12 F m−1

Permeability of free space µ0 4π × 10−7 H m−1

Avogadro constant NA 6.02 × 1023 mol−1

Mass of Sun MS 1.99 × 1030 kg

Radius of Earth RE 6.37 × 106 m

T(K) = T(◦ C) + 273

Volume of a sphere = 34 πr3

For small angles, sin θ ≈ tan θ ≈ θ

For x  1, (1 + x)n ≈ 1 + nx
Blank page

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 Section 2
Question 2

a) A planet has a radius R and the gravitational field strength at its surface is g. A satellite is placed
in a circular orbit of radius r around the planet.
Obtain an expression for T , the period of orbit of the satellite, in terms of R, g and r, explaining
the relevant physics in each of your steps.

b) Both the Earth and Jupiter can be assumed to have circular orbits about the Sun. Jupiter has an
orbital period, TJupiter = 11.9 years. What is the orbital radius of Jupiter in au?
Note: an au (astronomical unit) is the average distance between the Earth and the Sun.
(4)

c) The density of the Earth changes sharply between the core and the mantle. By taking just two
suitable values of the density of the core and mantle of the Earth from the graph in Figure 1,
estimate a value of the mass of the Earth using this data. Your values and calculation must be
shown.
(3)

Figure 1: credit: AllenMcC. [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons

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d) A planet of mass Mp has an orbiting moon of mass Mm . The mass of the moon is about 1/8 the
mass of the planet.

(i) Sketch a set of equipotential lines for the planet-moon system. Ignore the orbital motion of
the moon. Without detailed calculation, indicate on your sketch the relative positions of the
neutral point (a point of zero field strength) and the centre of mass of the planet-moon
system.

A rocket of mass m is launched from the planet. The rocket engine is used very briefly at
take-off when it is close to the planet’s surface, and it almost immediately reaches speed v,
after which the fuel is exhausted and there is no further propulsion. It is then in free-fall and
will have just enough energy to reach the moon. Assume that you can ignore relative motion
of the planet and moon, and ignore air resistance.
Obtain expressions and calculate values for:
(ii) The distance, d, from the planet’s centre, where the speed of the rocket is lowest.
(iii) The launch speed v of the rocket from the planet, in terms of the variables Mp , Mm , the
radius of the planet, Rp , and the separation of the centres of the moon and planet, D.
For this question part, you may take Mp = 6.00 × 1024 kg, Mm = 7.50 × 1023 kg,
D = 3.84 × 108 m. (9)

e) The Earth can be considered to consist of concentric spheres of material, the outer sphere being
the crust with a density of 2500 kg m−3 . If the mass of the Earth is ME = 6.0 × 1024 kg, what
will be the fractional change in the gravitational field strength when descending to the bottom of
a gold mine, a distance d = 3000 m below the surface of the Earth?
Hint: the field inside a hollow spherical shell is zero.
(4)

f) A planetary sized object similar to the Earth was formed from a very large spherical cloud of
interstellar dust collapsing under gravity to form a spherical body of uniform density ρ and
radius R

(i) Find an expression for the change in gravitational potential energy as the radius of the proto-
planet increases from r to r + δr by the formation of a thin spherical shell of material of
mass δm.
(ii) Now derive an expression for the total transfer of gravitational potential energy in the
formation of the planet. Use the following terms: the gravitational constant G, and the final
radius and mass, R and M respectively.

(iii) If all of this energy was used to heat the planet formed at a very low initial temperature,
estimate the temperature that the planet would have reached.

For this planet, take R = 6400 km, M = 4.0 × 1026 kg, and the average specific heat
capacity of the material of the planet is 700 J kg−1 K−1 .
(5)

[25 marks]

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Question 3

a) A particle is moving in a horizontal circle and is kept in its path by a string tied to a point at a
height h above the centre of the circle.

(i) Find the period of rotation in terms of h and the gravitational field strength g, explaining the
physics of your derivation in words.
(ii) If the tension in the string is three times the weight of the particle, find the length of the
string `, in terms of h.
(5)

b) (i) A particle of mass m is suspended by a thin elastic string of natural length a in which a
tension mg would produce an extension a. Write down an expression for the period of
oscillation of the mass in terms of a and g.

(ii) Two particles of equal mass m on a smooth horizontal table are connected by the same thin
elastic string of natural length a. The particles are held at rest at a distance 3a apart.
i. Describe their motions between the time they are released and when they collide.
ii. If the particles are released simultaneously, calculate the time elapsed before they
collide, given a = 0.20 m.
(7)

c) A piece of elastic cord of length ` and cross-sectional area A is stretched to twice its natural length.
Assuming that the total volume remains constant, and that cross-sectional area is uniform along
its length, and the Young’s modulus E for the material remains constant,

(i) obtain an expression for the final tension in terms of E and A.

(ii) Show that the work done in stretching the elastic is given by E`A(1 − ln 2).
(5)

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d) The behaviour of the boxing training ball shown in
Figure 2 can be modelled as two identical vertical springs
attached to a mass, m. The springs each have a natural
length `, and a spring constant k, such that an applied
force of mg doubles the length of spring to 2`. The top
and bottom springs are attached to the ceiling and floor
respectively in a room of height 4`.
Assume the radius of the ball, r  `.

(i) Determine the equilibrium position of the ball

below the ceiling in terms of `.

(ii) The ball is given a small vertical displacement and

released. Find an expression for the period Tv for
the vertical oscillations.

(iii) The ball is given a small horizontal displacement. Figure 2

Using a suitable approximation provide an
expression for the horizontal force Fh exerted on
the ball by the springs.

(iv) Find an expression for the period of the horizontal

oscillations Th and evaluate the ratio Th /Tv

(Image credit: Kangrui Sports http://kangruisport.com/sport/punchingbags )

(8)

[25 marks]

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Question 4

a) (i) A tall cylinder of circular cross sectional area A and height h is filled with a liquid of uniform
density ρ.
i. Derive an expression for the pressure P at the bottom of the liquid in terms of the
variables ρ, g and h. Ignore atmospheric pressure.
ii. Show that the units for each side of your derived equation are the same.
iii. What would your result be if the liquid changed in density linearly from ρt at the top to
ρb at the bottom of the cylinder?
iv. If you used your formula for the pressure at ground level, at the bottom of the Earth’s
atmosphere, which has a height of about 200 km, it would give an answer which is very
far from correct. Explain why.

(ii) A water tank which sits on the ground is kept full of water to a depth of h. Water is issuing
horizontally from a hole which is in the sidewall of the tank, a distance h0 below the surface.
Determine the maximum horizontal distance H from the hole in the can where the jet of
water strikes the floor, by using arguments which assume only
i. conservation of mechanical energy, assuming the water falls down through
height h0 ,
ii. conservation of momentum (using the pressure at depth h0 ).
(8)

b) An empty, hollow cuboid container of mass M and height h, width a, depth b, is suspended upright
from a thin cable in a liquid of density ρ` , as shown in Figures 3a and 3b.
(i) Find an expression for the tension, T , in the cable by considering the force due to the pressure
of the liquid on the top of the container, and the force due to the pressure of the liquid on the
bottom of the container.
(ii) The cuboid container now has a small hole pierced in the bottom surface and the liquid leaks
in until the air now confined to the top part of the container, occupying a fraction f of the
volume, with a fraction (1 − f ) of the volume inside now being filled with liquid. The
pressure of the atmosphere at the surface of the liquid is P0 and the density of the liquid is
ρ` . Obtain an expression for the depth, h0 of the bottom surface of the container in terms of
P0 , ρ` , g and h.

hollow
h container
a liquid
(a) (b)

Figure 3
(7)

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c) A rod of length λ and material density ρr is pivoted by a hinge at a point on the wall of a container
which has liquid of density ρ` , below the level of the pivot point. The rod floats without touching
the bottom of the container. If a fraction λ/n of the rod is above the surface, determine n in terms
of ρr and ρ` .

rod

liquid

(a) (b)

Figure 4
(5)

d) A solid circular cone of height h, mass m, radius of base, r, is floating in a liquid of density ρ`
with its axis vertical and vertex downwards, as shown in Figure 5. In the liquid, a height h/n is
submerged below the surface of the liquid.
h
(i) Obtain an expression relating the mass of the cone to the fraction submerged, n , in terms of
r, h, n and ρ`
The volume of a circular cone is given by V = 13 πr2 h.
(ii) A weight equal to the weight of the cone is placed on the base whilst it is floating, and it
sinks until the base is just level with the liquid, and then it rises again. For this condition,
show that
n3 + n2 + n = 7
You may neglect any effects of the inertia and viscosity of the liquid. Energy considerations
may be helpful.

2r

air

liquid
h
h/n

Figure 5

(5)

[25 marks]

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Question 5

a) In the two circuits shown in Figures 6a and 6b below, a cell of e.m.f. E and negligible internal
resistance is connected across a pair of resistors, R1 and R2 . Resistor R1 is a length of uniform
resistance wire, and R2 is a fixed resistor.

(i) For the circuit shown in Figure 6a, sketch two graphs on the same axes showing how the
potential V across R2 varies with position of the sliding contact on R1 for the cases
i. when R2  R1
ii. when R2  R1
(ii) Repeat part (i) above for the circuit shown in Figure 6b.
(iii) What are the relative advantages of each of these circuits when used to control the potential
difference across R2 ?
R1

E
R1 E

R2
R2

(a) (b)

Figure 6
(5)

b) A resistance network is shown in Figure 7.

a 30

b c d
X Y
15
15
15

e 30

Figure 7

(i) A total current of 10 mA flows through the circuit. Determine the current in each of the
resistors, a to e.
(ii) Hence find the total resistance between X and Y.

(4)

c) A strain gauge consists of a straight metal wire of length 0.30 m, diameter 0.028 mm and
resistivity 5.0 × 10−7 Ω m. Calculate the resistance of the gauge when

(i) unstrained and

(ii) stretched by 1.0 mm. You may assume that the diameter of the wire does not change.
(iii) Calculate the resistance if the stretched wire maintained the same volume i.e. the diameter
decreased in the process of stretching?

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 (iv) Four identical strain gauges as in c) (ii) above, labelled R1 to R4 , are arranged in a bridge
circuit as shown in Figure 8, with a cell of e.m.f 9.0 V and of negligible internal resistance.
R1 and R4 are stretched by 1.0 mm. Assuming the internal resistance of the power supply
is negligible, calculate the reading on the voltmeter (which can be considered to have an
infinite resistance).

Figure 8
(6)

d) The following network shown in Figure 9 is infinite. Find an expression for the resistance between
A and B in as simple a form as possible, and determine the resistance when r1 = 4 Ω and r2 =
15 Ω. Hint: consider adding new resistors r1 and r2 at the left end of the circuit shown.

Figure 9
(4)

e) The network shown in Figure 10 consists of an infinite number of cells of e.m.f. ε1 and ε2 with
corresponding internal resistances R1 and R2 respectively. A resistor R of value R = 1.0 Ω, is
connected to the end of the series. If the values of the e.m.f.s and resistances are ε1 = 1.0 V,
ε2 = 2.0 V, R1 = 4 Ω, R2 = 6 Ω, determine the value of the current flowing through resistor R.

1 1 1 1

Figure 10
(6)
[25 marks]

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Question 6

a) Two uncharged capacitors with values C1 and C2 , are connected in series with a battery of
e.m.f. V , and a switch. When the switch is closed there is a charge of Q1 and Q2 on the
capacitors respectively.

(i) State the relationship between Q1 and Q2 .

(ii) State expressions for the potential differences across each capacitor, V1 and V2
(iii) Hence derive an expression for the equivalent capacitance, Ceq produced by C1 and C2 in
series.
(iv) Write down expressions for the total energy stored in the two capacitors, and the energy
supplied by the battery, in terms of Ceq and V , and comment on the difference
(6)

b) A 15 nF parallel plate capacitor is connected across a 70 V battery, as in Figure 11. How much
work must be done in order to double the plate separation

(i) with the battery disconnected,

(ii) with the battery connected?

15 nF 70 V

Figure 11
(2)

c) (i) Two capacitors, of capacitance 2.0 µ F and 4.0 µ F, are each given a charge of 120 µ C. The
positive sides are then connected, as are the negative sides. Calculate:
i. The new potential difference across the plates.
ii. The change in the energy stored.

(ii) A conducting metal sphere is insulated from the ground. It is initially charged by contact
with a small metal disc which carries a charge q, so that the sphere gains charge Q. The
metal disc is then again given a charge q, and is then touched on the sphere a second time. If
this process is repeated indefinitely, what would be the final charge on the sphere, Qf ?

(iii) Three capacitors with leads AB, CD and EF of capacitances 1 µF, 2 µF and 3 µF respectively
are each connected separately to a battery and charged to a potential of 12 V. The positively
charged plates are A, C and E. The capacitors are joined in series so that B is connected to
C, D is connected to E, and F is connected to A.
i. Calculate the charges on each of the capacitors, indicating the values and signs on a
diagram.
ii. Sketch a graph to show the variation of potential around the circuit from A to F.
(9)

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d) A pair of concentric conducting spheres have radii of 10 cm and 15 cm respectively. They are
insulated from each other, and the potential difference between the spheres is 120 V. The spheres
carry equal charges ±Q, with the inner sphere being positive..

(i) Sketch the shape of a graph of the electric field strength between r = 0 cm and r = 30 cm.
(ii) Sketch a graph of the potential between r = 0 cm and r = 30 cm.
(iii) Write down expressions for the potential on the inner charged sphere if the outer sphere
were not present, and the potential for the outer charged sphere if the inner sphere were not
present.
(iv) From these results, calculate the magnitude of the charge Q

(4)

e) An LED is to be lit with a constant current of 800 mA supplied by a charged capacitor, C, in series
with a variable resistor, R, as shown in Figure 12. The diode has a forward bias of 2.7 V, which
means that when the diode is conducting a current, it has this fixed potential difference across it.

(i) What is the variation R(t) of the variable resistor against time, such that the constant
current is maintained?

The initial potential difference across the capacitor is 5.0 V.

(ii) For how long would the LED remain lit if the value of the capacitor is 0.4 µ F?

C
R

Figure 12
(4)

[25 marks]

END OF SECTION 2

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