Official Answer Key

Q	Ans	Q	Ans	Q	Ans	Q	Ans
1	C	11	A	21	A	31	D
2	A	12	C	22	C	32	B
3	A	13	C	23	D	33	A
4	B	14	C	24	A	34	D
5	A	15	D	25	C	35	D
6	D	16	C	26	B	36	A
7	D	17	C	27	D	37	D
8	A	18	B	28	A	38	C
9	C	19	C	29	D	39	C
10	A	20	B	30	C	40	C

Detailed Explanations

Q1 — Correct answer: C

The height of a clothes peg is small, so a micrometer is suitable for an accurate measurement.

Why others are wrong:

• A, tape: not accurate enough for a small object.
• B, metre rule: can measure it, but less accurately than a micrometer.
• D, measuring cylinder: measures liquid volume, not length.

Q2 — Correct answer: A

Vectors have magnitude and direction.

Option A contains only vectors:

• acceleration
• displacement
• velocity

Why others are wrong:

• B: chemical energy, mass and power are scalars.
• C: force is vector, but extension and gravitational potential energy are not all vectors.
• D: weight is vector, but kinetic energy and work are scalars.

Q3 — Correct answer: A

Average speed:

average speed = total distance / total time

From the graph:

total distance = 28 m
total time = 14 s

Average speed:

28 / 14 = 2.0 m/s

Answer A.

Why others are wrong:

• B, 2.3 m/s: too high.
• C, 2.4 m/s: too high.
• D, 2.5 m/s: too high.

Q4 — Correct answer: B

The question asks where the magnitude of velocity is decreasing. Magnitude means size only, ignoring positive or negative direction.

In section P, velocity is positive but moving towards zero, so speed is decreasing.

In section Q, velocity becomes more negative, so the magnitude is increasing.

In section R, velocity is negative but moving towards zero, so the magnitude is decreasing.

Therefore, the answer is P and R.

Why others are wrong:

• A: Q is wrong because magnitude is increasing there.
• C: misses R.
• D: misses P.

Q5 — Correct answer: A

When the metal disc is squeezed, its shape changes.

The radius, distance between atoms and forces between atoms may change.

But the mass remains unchanged because no material is added or removed.

Answer A.

Why others are wrong:

• B: radius can change when squeezed.
• C: distance between atoms changes under compression.
• D: forces between atoms change when the disc is deformed.

Q6 — Correct answer: D

Mass is the amount of matter in an object. It does not depend on location.

Weight depends on gravitational field strength:

weight = mass × gravitational field strength

As the satellite moves away from Earth, gravitational field strength decreases, so weight decreases.

Mass is unaffected.

Answer D.

Why others are wrong:

• A: weight is affected.
• B: mass does not decrease.
• C: mass does not increase and weight is not unaffected.

Q7 — Correct answer: D

Convert:

1 g/cm³ = 1000 kg/m³

So:

2.4 g/cm³ = 2.4 × 1000
= 2400 kg/m³

Answer D.

Why others are wrong:

• A, B and C: all use incorrect conversion factors.

Q8 — Correct answer: A

The object falls downwards.

Forces acting:

• force from Earth/weight acts downwards
• air resistance acts upwards, opposite to motion

Diagram A shows air resistance upwards and weight downwards.

Why others are wrong:

• B: air resistance is shown downwards, wrong.
• C: weight is shown upwards, wrong.
• D: both forces are wrongly arranged for a falling object.

Q9 — Correct answer: C

Moment of a force:

moment = force × perpendicular distance from pivot

The perpendicular distance is given as d, not l.

So:

moment = F × d

Answer C.

Why others are wrong:

• A and B: divide instead of multiply.
• D: uses l, but l is not perpendicular to the force.

Q10 — Correct answer: A

Work done to increase speed is the work done by the resultant force.

Engine force = 3.5 MN upward
Weight = 1.9 MN downward
Air resistance = 0.10 MN downward

Resultant force:

3.5 – 1.9 – 0.10 = 1.5 MN

Distance = 100 m

Work done:

1.5 MN × 100 m
= 1.5 × 10⁶ × 100
= 1.5 × 10⁸ J
= 150 MJ

Answer A.

Why others are wrong:

• B: uses an incorrect resultant force.
• C: uses engine force only and ignores weight and air resistance.
• D: adds forces instead of subtracting opposing forces.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Q11 — Correct answer: A

Efficiency is:

efficiency = useful energy output / total energy input

Answer A.

Why others are wrong:

• B: upside down.
• C: mixes power input with energy output.
• D: multiplies energies instead of forming a ratio.

Q12 — Correct answer: C

Power:

power = work done / time

For lifting:

work done = mgh

Since g is constant, compare:

m × distance / time

A:

2.0 × 2.0 / 3.0 = 1.33

B:

1.5 × 2.5 / 2.0 = 1.875

C:

2.0 × 2.0 / 1.5 = 2.67

D:

1.5 × 2.5 / 1.5 = 2.5

Greatest value is C.

Why others are wrong:

• A: too small due to longer time.
• B: smaller mass and longer time.
• D: close, but still less than C.

Q13 — Correct answer: C

The temperature remains constant, so Boyle’s law applies:

P1V1 = P2V2

The container has the same cross-sectional area, so volume is proportional to height.

P1 = 1.0 × 10⁵ N/m²
V1 ∝ 0.18 m
V2 ∝ 0.15 m

P2 = P1 × V1 / V2

P2 = 1.0 × 10⁵ × 0.18 / 0.15
P2 = 1.2 × 10⁵ N/m²

Answer C.

Why others are wrong:

• A and B: pressure should increase because volume decreases.
• D: too large for this volume change.

Q14 — Correct answer: C

The gas is kept at constant volume.

When temperature increases:

• gas particles move faster
• collisions with the walls become more frequent and more forceful
• pressure increases

Answer C.

Why others are wrong:

• A: decreasing temperature would decrease pressure.
• B: decreasing temperature would not leave pressure unchanged.
• D: increasing temperature does not leave pressure unchanged at constant volume.

Q15 — Correct answer: D

To convert °C to K:

K = °C + 273

Gold melts at:

1064 + 273 = 1337 K

Answer D.

Why others are wrong:

• A: subtracts 273 instead of adding.
• B and C: incorrect conversions.

Q16 — Correct answer: C

We check the states at each temperature.

At -190 °C:

Argon:

• melting point = -189 °C
• -190 °C is below melting point
• so argon is solid

Nitrogen:

• melting point = -210 °C
• boiling point = -196 °C
• -190 °C is above boiling point
• so nitrogen is gas

Oxygen:

• melting point = -218 °C
• boiling point = -183 °C
• -190 °C is between melting and boiling
• so oxygen is liquid

The three elements are in different states.

Answer C.

Why others are wrong:

• A, -220 °C: all are below melting point, so all solid.
• B, -205 °C: not all three are in different states.
• D, -175 °C: all are above boiling point, so all gas.

Q17 — Correct answer: C

Evaporation is reduced by limiting escape of molecules from the surface.

Putting a lid on the beaker reduces evaporation.

Answer C.

Why others are wrong:

• A: blowing air removes vapour and increases evaporation.
• B: heating increases evaporation.
• D: larger diameter means larger surface area, increasing evaporation.

Q18 — Correct answer: B

The best absorber of infrared radiation is:

• black
• dull

Answer B.

Why others are wrong:

• A: black is good, but shiny is a poor absorber.
• C: white and shiny is a poor absorber.
• D: dull helps, but white is not the best absorber.

Q19 — Correct answer: C

Ultrasound has frequency greater than 20 000 Hz.

Use:

frequency = velocity / wavelength

Convert wavelength from cm to m.

A:

330 / 0.066 = 5000 Hz, not ultrasound.

B:

890 / 0.89 = 1000 Hz, not ultrasound.

C:

1500 / 0.060 = 25 000 Hz, ultrasound.

D:

4000 / 0.25 = 16 000 Hz, not ultrasound.

Answer C.

Why others are wrong:

• A, B and D: frequencies are below 20 kHz.

Q20 — Correct answer: B

The 30° shown is between the incident ray and the mirror surface.

Angle of incidence is measured from the normal.

Normal is 90° to the mirror.

Angle of incidence:

90° – 30° = 60°

Angle of reflection = angle of incidence

So angle of reflection = 60°.

Answer B.

Why others are wrong:

• A: uses the angle with the mirror instead of the normal.
• C and D: incorrect reflection angle.

Q21 — Correct answer: A

A plane mirror forms an image:

• same size as the object
• upright
• same distance behind the mirror as the object is in front
• laterally inverted

Diagram A shows the image upright and the same distance behind the mirror.

Why others are wrong:

• B: image is inverted, wrong.
• C: image is not the same distance behind the mirror.
• D: image is inverted and incorrectly placed.

Q22 — Correct answer: C

The diagram gives angles measured from the surface.

Angle of incidence in air from the normal:

i = 90° – 42.0°
i = 48.0°

Angle of refraction in medium Q from the normal:

r = 90° – 49.5°
r = 40.5°

Refractive index of Q:

n = sin i / sin r
n = sin 48.0° / sin 40.5°
n ≈ 1.14

Critical angle c satisfies:

sin c = 1 / n

sin c ≈ 1 / 1.14
c ≈ 60.9°

Answer C.

Why others are wrong:

• A, B and D: do not match the refractive index found from the given angles.

Q23 — Correct answer: D

Electromagnetic waves include:

• radio waves
• microwaves
• infrared
• visible light
• ultraviolet
• X-rays
• gamma rays

X-rays and microwaves are both electromagnetic waves.

Answer D.

Why others are wrong:

• A: sound is not electromagnetic.
• B: ultraviolet is electromagnetic, but ultrasound is sound, not electromagnetic.
• C: infrared is electromagnetic, but water waves are mechanical.

Q24 — Correct answer: A

An induced magnet is formed when a magnetic material becomes magnetised due to a nearby magnet.

In diagram A, the iron nail is near a permanent magnet, so it becomes an induced magnet.

Why others are wrong:

• B: iron bar alone does not induce magnetism in the nail.
• C and D: switch is open, so no current flows in the coil; the electromagnet is not working.

Q25 — Correct answer: C

Iron is a magnetic metal.

Copper is non-magnetic.

Answer C.

Why others are wrong:

• A: aluminium is not magnetic.
• B: copper is not magnetic, steel is magnetic.
• D: steel is magnetic, but iron is also magnetic, not non-magnetic.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Q26 — Correct answer: B

The magnetic field lines point downwards.

A freely suspended magnet turns so that its north pole points in the direction of the magnetic field.

The north pole must turn downwards, so the magnet rotates anticlockwise through 90°.

Because the field is uniform, there is no resultant force causing translation, only a turning effect.

Answer B.

Why others are wrong:

• A: wrong direction of rotation.
• C: wrong rotation and wrongly says it moves towards U.
• D: correct rotation but wrongly says it moves towards V.

Q27 — Correct answer: D

If the plastic rod becomes positively charged, it has lost electrons.

Protons do not move between objects during charging by rubbing. Electrons are transferred.

Answer D.

Why others are wrong:

• A: losing protons does not happen in electrostatic charging.
• B: losing electrons makes the rod positive, not negative.
• C: gaining protons does not happen.

Q28 — Correct answer: A

The four resistors are in series.

Total resistance:

Rtotal = R + R + R + R = 4R

Current in the circuit:

I = E / 4R

In a series circuit, current is the same through every resistor.

So from left to right, each current is:

E/4R, E/4R, E/4R, E/4R

Answer A.

Why others are wrong:

• B and C: current does not change from one resistor to the next in series.
• D: ignores the total resistance of 4R.

Q29 — Correct answer: D

Potential difference/voltage is:

voltage = energy transferred / charge

So:

1 volt = 1 joule per coulomb

Answer D.

Why others are wrong:

• A: coulomb per ampere is a unit of time.
• B: coulomb per joule is the reciprocal of volt.
• C: joule per ampere is not volt.

Q30 — Correct answer: C

Resistance of a wire:

R = ρl / A

Wire X has resistance R.

For wire Y:

length = 4l
area = 4a

So:

RY = ρ(4l)/(4a)
RY = ρl/a
RY = R

To make total resistance R/2, two equal resistances R must be connected in parallel:

Rtotal = R/2

Answer C.

Why others are wrong:

• A and B: length 2l and area 0.25a give resistance 8R, not R.
• D: R and R in series give 2R, not R/2.

Q31 — Correct answer: D

Circuit D contains:

• a diode symbol
• a fuse symbol

Why others are wrong:

• A: contains a diode, but not a fuse.
• B: contains resistive components, but no diode.
• C: contains an LED/diode-type symbol and variable resistor, but not the required fuse and diode combination.

Q32 — Correct answer: B

A kettle powered by mains electricity transfers energy by electrical work done.

The useful output is heating water, so the energy is transferred to the thermal store.

Answer B.

Why others are wrong:

• A: kettle does not store energy chemically.
• C and D: not mechanical work; it is electrical work.

Q33 — Correct answer: A

By Lenz’s law, the induced pole opposes the motion/change causing it.

When the magnet enters the coil, the north pole approaches end P. End P becomes north to repel the approaching north pole.

When the magnet leaves the coil at Q, the south pole is leaving last, so end Q becomes north to attract it back and oppose the leaving motion.

So:

P = N
Q = N

Answer A.

Why others are wrong:

• B, C and D: give one or both induced poles incorrectly.

Q34 — Correct answer: D

Use the right-hand grip rule.

The current in the vertical wire is downwards. From the observer’s view, the current is going away from the observer, into the page.

For current into the page, the magnetic field is clockwise.

Diagram D shows the correct circular field pattern and direction.

Why others are wrong:

• A and B: wrong direction/pattern.
• C: not the complete correct circular pattern/direction.

Q35 — Correct answer: D

In a simple d.c. motor, the split-ring commutator reverses the direction of current in the coil every half-turn.

This keeps the coil rotating in the same direction.

Answer D.

Why others are wrong:

• A: armature is the rotating coil/core, not the current-reversing part.
• B: brushes maintain electrical contact.
• C: slip rings are used in a.c. generators, not for reversing current in a d.c. motor.

Q36 — Correct answer: A

In nuclide notation:

R = nucleon number = protons + neutrons
Z = proton number

Isotopes of the same element have the same proton number but different nucleon numbers.

So R is the nucleon number, and it is different for different isotopes of X.

Answer A.

Why others are wrong:

• B: R is not the proton number.
• C: Z is not the nucleon number.
• D: Z is the proton number, but it is the same for isotopes, not different.

Q37 — Correct answer: D

Background = 28 counts/min.

No sheet = 750 counts/min.

Thin card = 750 counts/min, so the card stops nothing. Therefore, there are no alpha particles, because alpha would be stopped by thin card.

5 mm aluminium = 478 counts/min, still much greater than background. Aluminium stops beta but gamma passes through.

So the source emits:

• beta particles
• gamma rays

Answer D.

Why others are wrong:

• A: includes alpha, but card caused no decrease.
• B: includes alpha and misses gamma.
• C: includes alpha and misses beta.

Q38 — Correct answer: C

For a medical tracer:

• radiation must escape the body to be detected
• gamma rays are suitable because they are penetrating
• half-life should be short, a few hours, to reduce radiation dose to the patient

Answer C.

Why others are wrong:

• A: alpha is too weakly penetrating to be detected outside the body.
• B: beta is less suitable and a few years is too long.
• D: gamma is suitable, but a few years is too long and increases risk.

Q39 — Correct answer: C

Check the statements:

1. False. Light from the Sun takes about 500 s, not 3.0 × 10⁸ s.
2. True. Earth rotates once in about 24 hours.
3. True. The Moon takes about 1 month to orbit Earth.
4. False. Earth orbits the Sun, not the Sun orbiting Earth.

Correct statements are 2 and 3 only.

Answer C.

Why others are wrong:

• A: includes statement 1, which is false.
• B: includes statement 4, which is false.
• D: includes statement 4 and misses statement 2.

Q40 — Correct answer: C

A red supergiant can explode as a supernova.

A supernova can form:

• neutron star
• black hole
• new heavier elements

A white dwarf is formed from a lower/middle-mass star, not from the explosion of a red supergiant.

Answer C.

Why others are wrong:

• A: black holes can form after massive star supernovae.
• B: neutron stars can form after supernovae.
• D: heavier elements are formed in supernova explosions.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change