Official Answer Key

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

Detailed Explanations

Q1 — Correct answer: B

The micrometer reading is:

Main scale reading = 7.50 mm
Thimble scale reading = 25 × 0.01 mm = 0.25 mm

Total reading:

7.50 + 0.25 = 7.75 mm

So the answer is B.

Why others are wrong:

• A, 7.25 mm: misses the half-mm main scale reading.
• C, 10.25 mm: reads the main scale incorrectly.
• D, 25.77 mm: treats the thimble reading as a main length, which is classic micrometer chaos.

Q2 — Correct answer: B

The two forces are perpendicular:

• vertical force = 1.5 N
• horizontal force = 2.0 N

Use Pythagoras:

Resultant² = 1.5² + 2.0²
Resultant² = 2.25 + 4.00
Resultant² = 6.25
Resultant = 2.5 N

So the answer is B.

Why others are wrong:

• A, 0.50 N: this is the difference between the forces, but they are not in opposite directions.
• C, 3.0 N: incorrect calculation.
• D, 3.5 N: this adds the two forces directly, but they are perpendicular, not in the same line.

Q3 — Correct answer: D

Acceleration is defined as:

acceleration = change in velocity / time taken

Velocity is a vector, so it includes both speed and direction.

Why others are wrong:

• A: change in distance / time is not acceleration.
• B: change in displacement / time gives velocity, not acceleration.
• C: change in speed / time is only valid when direction is unchanged; the full definition uses velocity.

Q4 — Correct answer: B

Weight = mass × gravitational field strength

On Earth, 1 kg weighs 9.8 N.

On the Moon, gravitational field strength is 1/6 of Earth’s:

9.8 / 6 = 1.63 N

Approximately:

1.6 N

So the answer is B.

Why others are wrong:

• A, 0.98 N: divides by 10 instead of 6.
• C, 9.8 N: Earth’s weight, not Moon’s weight.
• D, 16 N: multiplies instead of dividing.

Q5 — Correct answer: A

The ultrasound travels down to the sea bed and back up again.

Total distance travelled by pulse:

distance = speed × time
distance = 1500 × 0.52
distance = 780 m

But this is the return journey, so depth is half:

depth = 780 / 2
depth = 390 m

So the answer is A.

Why others are wrong:

• B, 780 m: forgets to divide by 2.
• C, 1600 m: incorrect multiplication/rounding.
• D, 5800 m: completely wrong scale.

Q6 — Correct answer: C

Mass is the quantity of matter in an object.

Mass is measured using an electronic balance.

So the correct row is C.

Why others are wrong:

• A and B: “force experienced by an object” describes weight, not mass.
• D: a force meter measures force/weight, not mass.

Q7 — Correct answer: C

Initial water volume = 35.0 cm³
Final volume after adding rock = 59.0 cm³

Volume of rock:

59.0 – 35.0 = 24.0 cm³

So:

water = 35.0 cm³
rock = 24.0 cm³

Answer C.

Why others are wrong:

• A: reads the initial water level incorrectly.
• B: treats the final reading as the rock volume, which is wrong.
• D: gives the final water level as the rock volume instead of subtracting.

Q8 — Correct answer: D

A force can change:

• shape
• velocity
• volume

A force cannot change mass. Mass is the amount of matter in the object.

So the correct list is shape, velocity, volume.

Answer D.

Why others are wrong:

• A: includes mass.
• B: includes mass.
• C: includes mass.

Q9 — Correct answer: A

Resultant force decreases steadily to zero.

Since F = ma, acceleration also decreases steadily to zero.

So the speed should:

• increase at first
• increase more slowly as acceleration decreases
• become constant when force becomes zero

Graph A shows this curved increase followed by constant speed.

Why others are wrong:

• B: shows speed decreasing, but the force is still in the direction of motion.
• C: shows constant acceleration, but force is decreasing.
• D: shows speed decreasing linearly, not correct.

Q10 — Correct answer: D

The object is at rest on a table, so forces are balanced.

The forces are:

• weight downward
• contact force upward

Since it is at rest:

contact force = weight

Diagram D shows equal-sized opposite forces with correct labels.

Why others are wrong:

• A: labels are reversed.
• B: labels are correct, but the arrows are not equal in length.
• C: labels are reversed and forces are not correctly shown.

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: B

Point X is where the graph stops being a straight line.

A straight line means load is proportional to extension. After X, proportionality breaks down.

So X is the limit of proportionality.

Why others are wrong:

• A: elastic deformation means the spring returns to its original length after the load is removed; X does not directly show that.
• C: maximum extension would be the furthest extension shown, not X.
• D: spring constant is the gradient of the straight-line section, not a point.

Q12 — Correct answer: B

A car moving in a circle at constant speed has a resultant force towards the centre of the circle.

This force is called centripetal force.

The centre of the circle is to the right of the car, so the force acts in direction B.

Why others are wrong:

• A: points away from the centre.
• C: points partly along the motion, not towards the centre.
• D: points downward, not towards the centre.

Q13 — Correct answer: C

Take moments about the pivot.

Moment from X:

140 × 1.2 = 168 Nm anticlockwise

Moment from Y:

220 × 0.55 = 121 Nm clockwise

Anticlockwise moment is larger by:

168 – 121 = 47 Nm

So the third object must produce a clockwise moment, meaning it must be placed to the right of the pivot.

Required distance from pivot:

110 × distance = 47
distance = 47 / 110
distance = 0.43 m

This is to the right of the pivot but before Y, so region C.

Why others are wrong:

• A and B: left side would add more anticlockwise moment.
• D: too far to the right, producing too much clockwise moment.

Q14 — Correct answer: B

An object is more stable when:

• its base is wider
• its centre of gravity is lower

So objects with the same centre of gravity height are more stable when the base is larger.

Answer B.

Why others are wrong:

• A: larger base makes objects more stable, not less stable.
• C: higher centre of gravity and smaller base makes objects less stable.
• D: for identical bases, lower centre of gravity is more stable, not higher.

Q15 — Correct answer: A

Batteries, fuel and food store energy chemically.

So the energy store is chemical.

Why others are wrong:

• B, electrostatic: energy stored by separated charges.
• C, nuclear: energy stored in atomic nuclei.
• D, thermal: energy store due to temperature/internal energy.

Q16 — Correct answer: C

The question asks about the kinetic energy of the rocket, not the rocket plus fuel.

Initial rocket kinetic energy:

KE = 1/2 Mv²

Final rocket kinetic energy:

KE = 1/2 M(2v)²
KE = 1/2 M × 4v²
KE = 4 × 1/2 Mv²

So the kinetic energy of the rocket increases by a factor of 4.

Answer C.

Why others are wrong:

• A: doubling speed does not double kinetic energy; kinetic energy depends on speed².
• B: it does not halve.
• D: it does not stay the same because speed changes.

Q17 — Correct answer: C

Efficiency is:

efficiency = useful energy output / total energy input

So the correct option is C.

Why others are wrong:

• A: reverses useful input/output and total output.
• B: puts total input over total output.
• D: uses total output instead of useful output.

Q18 — Correct answer: B

To lift an object:

Work done = gain in gravitational potential energy
Work done = mgd

Power = work done / time

So:

P = mgd / t

Answer B.

Why others are wrong:

• A: has md / gt, which is dimensionally wrong.
• C: has dt / mg, inverted and incorrect.
• D: has m / gdt, also incorrect.

Q19 — Correct answer: D

Pressure = force / area

Given pressure = 1.0 Pa.

1 Pa = 1 N/m²

So a possible combination is:

force/weight = 1.0 N
area = 1.0 m²

Pressure = 1.0 / 1.0 = 1.0 Pa

Answer D.

Why others are wrong:

• A: 1.0 kg has weight about 9.8 N, and 1 cm² is tiny, so pressure would be huge.
• B: 1.0 kg has weight about 9.8 N over 1 m², so pressure is about 9.8 Pa.
• C: 1.0 N over 1 cm² gives 10 000 Pa, not 1 Pa.

Q20 — Correct answer: D

Force on bottom = pressure × area

Liquid pressure = density × g × depth

Since g is the same, compare:

density × depth × area

A:

1.3 × 50 × 10 = 650

B:

0.80 × 80 × 20 = 1280

C:

1.0 × 60 × 40 = 2400

D:

0.92 × 75 × 50 = 3450

Greatest is D.

Why others are wrong:

• A: high density but small area and lower depth.
• B: larger depth but lower density and area.
• C: large area, but D has greater overall product.

Q21 — Correct answer: B

The table describes:

• random arrangement
• particles moving freely
• weakest forces between particles
• greatest expansion when heated

This describes a gas.

So:

state = gas
forces = weakest
expansion = greatest

Answer B.

Why others are wrong:

• A: gas is correct, weakest is correct, but gases do not have the smallest expansion; they have the greatest.
• C: gas does not have strongest forces.
• D: liquid particles do not move as freely as gas particles, and liquids do not have the weakest forces.

Q22 — Correct answer: C

The trapped air has constant temperature, so Boyle’s law applies:

P1V1 = P2V2

The tube has constant cross-sectional area, so volume is proportional to length.

P1 = 1.0 × 10⁵ Pa
V1 ∝ 50 mm
V2 ∝ 45 mm

P2 = P1 × V1 / V2

P2 = 1.0 × 10⁵ × 50 / 45
P2 = 1.11 × 10⁵ Pa

Approximately:

1.1 × 10⁵ Pa

Answer C.

Why others are wrong:

• A: pressure would not halve from such a small volume change.
• B: pressure should increase, not decrease, because volume decreases.
• D: too large for a change from 50 mm to 45 mm.

Q23 — Correct answer: D

From the diagram:

X: gas → liquid = condensation
Y: solid → liquid = melting

So the answer is D.

Why others are wrong:

• A: boiling is liquid to gas, not gas to liquid; freezing is liquid to solid, not solid to liquid.
• B: boiling is wrong for X.
• C: condensation is correct for X, but freezing is wrong for Y.

Q24 — Correct answer: A

A black and dull surface is the best emitter of thermal radiation.

Since all surfaces have equal temperature and equal area, only colour and texture matter.

Answer A.

Why others are wrong:

• B: black helps, but shiny surfaces are poor emitters.
• C: dull helps, but white surfaces emit less effectively than black surfaces.
• D: white and shiny is the poorest emitter among these choices.

Q25 — Correct answer: D

Infrared emission increases when:

• temperature increases
• surface area increases

The greatest emission comes from the sphere with:

• highest temperature = 1000 °C
• largest surface area = 480 cm²

Answer D.

Why others are wrong:

• A: lower temperature and smaller area.
• B: large area but lower temperature.
• C: high temperature but smaller area than D.

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: C

Ultrasound has frequency greater than 20 000 Hz.

Use:

frequency = speed / wavelength

Wave 1:

340 / 0.19 = 1789 Hz, not ultrasound

Wave 2:

1500 / 0.050 = 30 000 Hz, ultrasound

Wave 3:

4000 / 0.23 = 17 391 Hz, not ultrasound

Wave 4:

6700 / 0.31 = 21 613 Hz, ultrasound

So waves 2 and 4 are ultrasound.

Answer C.

Why others are wrong:

• A: includes wave 1 and wave 3, both below 20 000 Hz.
• B: wave 1 only is not ultrasound.
• D: wave 4 is ultrasound, but wave 2 is also ultrasound.

Q27 — Correct answer: D

For light entering a transparent material from air:

n = sin i / sin r

Rearrange:

sin r = sin i / n

So the answer is D.

Why others are wrong:

• A: gives r = n sin i, which is not Snell’s law.
• B: says i = n sin r, missing the sine on i.
• C: inverts the relationship incorrectly.

Q28 — Correct answer: D

For reflection from a plane mirror:

angle of incidence = angle of reflection

Both angles are measured from the normal.

Diagram D shows the ray reflecting so that the incident and reflected rays make equal angles with the normal.

Why others are wrong:

• A: the ray continues through the mirror instead of reflecting properly.
• B: the reflected angle is not equal to the incident angle.
• C: the ray passes through the mirror rather than reflecting from it.

Q29 — Correct answer: D

A plane mirror forms an image that is:

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

So the answer is D.

Why others are wrong:

• A: plane mirror images are not real and not smaller.
• B: same size is correct, but the image is not real.
• C: virtual is correct, but the image is not smaller.

Q30 — Correct answer: C

The diagram shows:

• at 20°, light refracts out of glass into air
• at 40°, light still refracts out
• at 50°, total internal reflection occurs

The critical angle must be greater than 40° but less than 50°.

Only 45° fits.

Answer C.

Why others are wrong:

• A, 15°: if critical angle were 15°, total internal reflection would occur at 20°, 40° and 50°.
• B, 30°: if critical angle were 30°, total internal reflection would occur at 40° and 50°.
• D, 60°: if critical angle were 60°, total internal reflection would not occur at 50°.

Q31 — Correct answer: B

Magnification = image length / object length

A:

3.5 / 0.80 = 4.375

B:

4.7 / 0.60 = 7.83

C:

4.8 / 0.70 = 6.86

D:

5.2 / 0.90 = 5.78

Greatest magnification is B.

Why others are wrong:

• A: smaller ratio.
• C: large image, but object is also larger than B’s object.
• D: largest image, but object is much larger, so magnification is not greatest.

Q32 — Correct answer: B

Iron balls become induced magnets. The pole induced near a magnet pole is always the opposite pole.

From the given poles on W and X:

• the lower end of magnet M1 near Y must induce a north pole in Y
• the lower end of magnet M3 near Y must also induce a north pole in Y

So ball Y has N and N at the two contact regions.

Answer B.

Why others are wrong:

• A: gives one N and one S, but both induced poles at Y should be N.
• C: also gives one S and one N, wrong arrangement.
• D: gives two S poles, the opposite of what is required.

Q33 — Correct answer: C

Total resistance inside the tube:

R = V / I

R = 9.0 / 0.75
R = 12 Ω

Check option C:

20 Ω and 30 Ω in parallel:

1/R = 1/20 + 1/30
1/R = 3/60 + 2/60
1/R = 5/60
R = 12 Ω

So the answer is C.

Why others are wrong:

• A: 0.75 Ω and 6.0 Ω in parallel gives less than 0.75 Ω, not 12 Ω.
• B: 0.75 Ω + 6.0 Ω = 6.75 Ω.
• D: 20 Ω + 30 Ω = 50 Ω.

Q34 — Correct answer: C

The voltmeter is connected across the variable resistor.

The fixed resistor R and the variable resistor are in series.

Voltage across variable resistor:

V = E × variable resistance / total resistance

When variable resistor = 0:

V = 0

When variable resistor = 2R:

V = E × 2R / (R + 2R)
V = 2E / 3

So the reading increases from 0 to 2/3 E.

Answer C.

Why others are wrong:

• A: 1/3 E would be the voltage across R when the variable resistor is 2R.
• B: 1/2 E would happen if the two resistances were equal.
• D: it cannot reach E because the fixed resistor R still takes some voltage.

Q35 — Correct answer: D

A transformer works due to electromagnetic induction.

Changing current in the primary coil produces a changing magnetic field, which induces a voltage in the secondary coil.

Answer D.

Why others are wrong:

• A: a battery works by chemical energy.
• B: an electromagnet works because current produces a magnetic field, not electromagnetic induction.
• C: a motor works mainly due to the motor effect, not induction.

Q36 — Correct answer: A

A beta-particle is negatively charged.

It travels to the right. A magnetic field out of the page would cause a positive charge to be deflected downward. Since a beta-particle is negative, the force is reversed, so it is deflected upward.

Therefore, the field must be a magnetic field directed out of the page.

Answer A.

Why others are wrong:

• B: into the page would deflect the beta-particle downward.
• C: electric field to the left would push a negative beta-particle to the right, not upward.
• D: electric field to the right would push a negative beta-particle left, not upward.

Q37 — Correct answer: A

Thorium-223 emits an alpha-particle.

An alpha-particle is:

mass number 4
proton number 2

So:

Mass number: 223 – 4 = 219
Proton number: 90 – 2 = 88

Element with proton number 88 is radium, Ra.

So the nuclide is:

²¹⁹₈₈Ra

Answer A.

Why others are wrong:

• B: proton number increases to 92, but alpha emission decreases proton number.
• C: mass number increases to 227, impossible in alpha emission.
• D: both mass and proton number are wrong.

Q38 — Correct answer: B

Paper reduces the count rate but count remains above background.

That means alpha was stopped by paper, but another radiation still passed through.

Aluminium then reduces the count rate to background.

That means the remaining radiation was beta, because beta is stopped by aluminium.

There is no gamma radiation, because gamma would still pass through aluminium and keep the count above background.

So the source emits alpha and beta only.

Answer B.

Why others are wrong:

• A: includes gamma, but gamma would not be stopped by aluminium.
• C: includes gamma and misses beta.
• D: misses alpha, but paper clearly stops some radiation.

Q39 — Correct answer: A

Earth has an elliptical orbit around the Sun.

Answer A.

Why others are wrong:

• B: Earth is a planet, not a star.
• C: Earth rotates once in about 24 hours, not 28 hours.
• D: the Moon orbits Earth; Earth does not orbit the Moon once a year.

Q40 — Correct answer: C

Redshift means the observed wavelength of light from distant stars/galaxies increases.

Student 1 says wavelength decreases, so student 1 is wrong.

Student 2 says it occurs because stars and galaxies move away from Earth, which is correct in this O Level context.

So only student 2 is correct.

Answer C.

Why others are wrong:

• A: student 1 is not correct.
• B: student 1 only is wrong.
• D: student 2 is correct, so “neither” is wrong.

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.