W25_11

Official Answer Key

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

Detailed Explanations

Q1 — Correct answer: C

The ruler has an end error. From the correction diagram, the zero mark of the ruler is not exactly at the physical end. The object appears to end at about 1.1 cm on the ruler, but the missing end correction is about 0.2 cm.

So:

1.1 cm + 0.2 cm = 1.3 cm

Therefore, the object’s true length is 1.3 cm.

Why others are wrong:

• A, 0.9 cm: subtracts too much or uses the correction in the wrong direction.
• B, 1.1 cm: ignores the end correction.
• D, 1.5 cm: adds too much correction.

Q2 — Correct answer: D

Scalars have magnitude only. Vectors have magnitude and direction.

Scalars:

• distance
• mass
• temperature

Vectors:

• displacement
• force
• weight

So the correct row is D.

Why others are wrong:

• A: displacement and force are vectors, not scalars; distance and mass are scalars, not vectors.
• B: displacement and force are wrongly placed as scalars.
• C: force and weight are wrongly placed as scalars; mass and temperature are wrongly placed as vectors.

Q3 — Correct answer: C

For a speed-time graph:

Distance = area under graph
Acceleration = gradient

For object P:

Distance = 1/2 × 4.0 × 6.0 = 12 m
Acceleration = 6.0 / 4.0 = 1.5 m/s²

For object Q:

Distance = 1/2 × 5.0 × 10.0 = 25 m
Acceleration = 10.0 / 5.0 = 2.0 m/s²

Check conclusion 1:

Difference in distance = 25 – 12 = 13 m

It is not 26 m, so conclusion 1 is false.

Check conclusion 2:

Difference in acceleration = 2.0 – 1.5 = 0.50 m/s²

So conclusion 2 is true.

Therefore, only conclusion 2 is correct.

Why others are wrong:

• A: says neither is correct, but conclusion 2 is correct.
• B: says conclusion 1 only, but conclusion 1 is false.
• D: says both are correct, but conclusion 1 is false.

Q4 — Correct answer: C

A train making a short stop must:

• slow down
• reach zero speed
• remain at zero speed briefly
• then speed up again

Graph C shows exactly this.

Why others are wrong:

• A: the train slows down but does not reach zero speed.
• B: the train speeds up; it does not stop.
• D: the train slows down but does not actually reach zero speed.

Q5 — Correct answer: B

Take moments about the toes.

Let the distance from toes to shoulders/hands be L.

The weight W acts at a distance:

3L/4 from the toes.

Let the total upward force from both hands be F.

Using principle of moments:

F × L = W × 3L/4

F = 3W/4

This is the total force on both hands. Since there are two hands:

Force on each hand = 3W/8

Pressure = force / area

Area of each hand = S

Pressure on each palm = 3W / 8S

So the answer is B.

Why others are wrong:

• A, W/8S: too small; ignores the correct moment ratio.
• C, W/4S: does not account correctly for the 3/4 distance and sharing between two hands.
• D, 3W/4S: this is close to the total hand force divided by one area, but the force is shared by two hands.

Q6 — Correct answer: B

The parachutist falls at constant speed.

Constant speed means:

resultant force = 0

So the upward air resistance/drag force equals the downward weight.

Therefore, the upward force on the parachute is equal to the weight of the parachutist.

Why others are wrong:

• A: forces are still acting; weight and air resistance are present.
• C: if upward force were greater than weight, the parachutist would accelerate upwards or slow down.
• D: if upward force were less than weight, the parachutist would accelerate downwards.

Q7 — Correct answer: A

Density = mass / volume

For an irregular stone:

• use a balance to measure mass
• use a measuring cylinder with water to find volume by displacement

So the correct equipment is a balance and a measuring cylinder containing water.

Why others are wrong:

• B: a ruler cannot accurately measure the volume of an irregular stone.
• C: no balance, so mass cannot be measured.
• D: no balance, so density cannot be calculated.

Q8 — Correct answer: C

The spring obeys Hooke’s law while extension is proportional to load.

From 0 N to 10 N:

• 2 N gives 3 cm
• 4 N gives 6 cm
• 6 N gives 9 cm
• 8 N gives 12 cm
• 10 N gives 15 cm

This is proportional: every 2 N adds 3 cm.

At 12 N, expected extension would be 18 cm, but actual extension is 20 cm.

So proportionality breaks between 10 N and 12 N.

Why others are wrong:

• A: 0 to 2 N is still proportional.
• B: 8 to 10 N is still proportional.
• D: 14 to 16 N is well beyond the point where proportionality has already failed.

Q9 — Correct answer: C

The man’s weight is the gravitational force of the Earth pulling the man down.

By Newton’s third law:

If Earth pulls the man down, the man pulls the Earth up.

So the other force in the pair is a force upwards on the Earth.

Why others are wrong:

• A: the force on Earth is not downward; it is upward towards the man.
• B: this is another force on the man, not the third-law pair.
• D: this is the normal contact force from the ground on the man, not the gravitational third-law pair of weight.

Q10 — Correct answer: A

At constant temperature, Boyle’s law applies:

P1V1 = P2V2

From the graph:

P1 = 30 kPa
V1 = 5 × 10⁻⁵ m³
P2 = 12.5 kPa

So:

30 × 5 × 10⁻⁵ = 12.5 × V2

V2 = 150 × 10⁻⁵ / 12.5

V2 = 12 × 10⁻⁵

V2 = 1.2 × 10⁻⁴ m³

So the answer is A.

Why others are wrong:

• B: too large by a factor of 4.
• C: incorrect power of ten.
• D: completely unrealistic power of ten.

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

Density is calculated using:

density = mass / volume

So the correct equation is B.

Why others are wrong:

• A: mass × volume is not density.
• C: weight / area gives pressure, not density.
• D: weight / volume is related to weight density, not ordinary density.

Q12 — Correct answer: D

The object is balanced on the pivot. A weight is hung on the left side, producing an anticlockwise moment.

For balance, the object’s own weight must produce a clockwise moment.

That means the centre of gravity of the object must be to the right of the pivot.

Position D is to the right of the pivot, so it is correct.

Why others are wrong:

• A and B: they are on the left side, so they would add more anticlockwise moment instead of balancing it.
• C: it is almost above the pivot, so it would produce little or no moment.

Q13 — Correct answer: C

The renewable energy sources are:

• hydroelectric
• solar
• geothermal
• wind
• tides

That makes 5 renewable sources.

Fossil fuel is non-renewable. Nuclear is also treated as non-renewable because nuclear fuels such as uranium are finite.

Why others are wrong:

• A, 3: misses some renewable sources.
• B, 4: still misses one renewable source.
• D, 6: wrongly counts either fossil fuel or nuclear as renewable.

Q14 — Correct answer: D

Particles in fixed positions that vibrate describe a solid.

After heating, the particles start to flow past each other but still have strong forces between them. That describes a liquid.

So the substance has changed from solid to liquid.

This is melting.

Why others are wrong:

• A, condensed: gas to liquid.
• B, evaporated: liquid to gas.
• C, frozen: liquid to solid.

Q15 — Correct answer: A

At constant pressure, the volume of a gas increases as temperature increases.

This is Charles’s law:

V is directly proportional to absolute temperature.

So the graph should be a straight line with positive gradient.

That is graph A.

Why others are wrong:

• B: shows volume decreasing in a curved way, more like an inverse relationship.
• C: shows volume decreasing as temperature increases.
• D: shows volume staying constant, which is wrong if pressure is constant and temperature changes.

Q16 — Correct answer: B

During melting, the temperature of a pure substance remains constant.

The energy supplied is used to overcome forces between particles, increasing potential energy, not temperature.

Why others are wrong:

• A: melting does not have to involve a change from negative to positive temperature.
• C: forces between particles become weaker, not stronger.
• D: kinetic energy does not increase during melting because temperature remains constant.

Q17 — Correct answer: A

A black, rough surface is the best emitter of thermal radiation.

Better emission means faster cooling.

So the black, rough can cools most rapidly.

Why others are wrong:

• B: black is good, but shiny surfaces are poor emitters.
• C: rough helps, but white surfaces emit less thermal radiation than black surfaces.
• D: white and shiny is the worst combination for emitting radiation.

Q18 — Correct answer: A

Sound waves are longitudinal waves.

They contain:

• compressions: regions of higher pressure
• rarefactions: regions of lower pressure

An echo happens when sound is reflected.

So the correct row is:

compressions, rarefactions, reflected

Why others are wrong:

• B: refraction is bending of waves, not echo formation.
• C: reverses compressions and rarefactions, and uses refraction.
• D: reverses compressions and rarefactions.

Q19 — Correct answer: D

Initial angle of incidence = 20°

It is increased by 5°:

New angle of incidence = 25°

For a plane mirror:

angle of incidence = angle of reflection

So angle of reflection = 25°.

The angle between the incident ray and reflected ray is:

25° + 25° = 50°

So the answer is D.

Why others are wrong:

• A, 10°: only doubles the change, not the new angle.
• B, 25°: this is only the angle of incidence/reflection.
• C, 45°: incorrect addition.

Q20 — Correct answer: D

In the visible spectrum, frequency increases in this order:

red → orange → yellow → green → blue → violet

So the correct answer is D.

Why others are wrong:

• A: starts with high-frequency colours, then goes back to low-frequency colours.
• B: starts in the middle and then jumps back to red.
• C: places violet too early.

Q21 — Correct answer: A

The rays actually meet on the right side of the converging lens, so the image is real.

A ray parallel to the principal axis refracts through the principal focus on the far side of the lens. In the diagram, that focus is at R.

So the image is real and the principal focus is R.

Why others are wrong:

• B: Q is around the lens/optical centre, not the principal focus.
• C: the image is not virtual because the rays really meet.
• D: S is where the image is formed, not the principal focus, and the image is not virtual.

Q22 — Correct answer: D

Diagram D is the incorrect path.

In D, the ray is shown passing through the glass/air curved surfaces without the correct bending expected from refraction. The path does not properly obey the refraction rules at the curved glass surfaces.

Why others are correct:

• A: water to air; the ray bends away from the normal, which is correct.
• B: the ray enters and exits the semicircular block normally, so there is no bending at those curved surfaces; the internal path is acceptable.
• C: the ray enters normally at the first surface, so no bending occurs there; then it bends correctly at the second surface.

Why other answers are wrong:

• A, B and C are not the incorrect diagrams because their ray paths can be explained by correct refraction/reflection behaviour.

Q23 — Correct answer: C

The ray is inside an optical fibre and reaches point Y with angle of incidence greater than the critical angle.

This causes total internal reflection.

The ray must stay inside the fibre and reflect so that angle of incidence equals angle of reflection.

Direction C shows the correct reflected path inside the fibre.

Why others are wrong:

• A and B: show the ray leaving the fibre, which cannot happen during total internal reflection.
• D: stays inside but has the wrong reflected direction/angle.

Q24 — Correct answer: A

Distance = 360 m
Time = 1.8 s

Speed = distance / time

Speed = 360 / 1.8

Speed = 200 m/s

So the answer is A.

Why others are wrong:

• B: uses 360 as if it were the speed directly.
• C: would come from using 0.9 s instead of 1.8 s.
• D: much too high for the given distance and time.

Q25 — Correct answer: B

Amplitude affects loudness.

Frequency affects pitch.

The amplitude is increased but frequency remains 50 Hz.

So the sound becomes louder, but the pitch stays the same.

Why others are wrong:

• A: pitch does not increase because frequency does not increase.
• C: loudness changes, not pitch.
• D: loudness does not stay the same because amplitude increases.

Q26 — Correct answer: B

The field-line arrows show the left end of the bar magnet acts as a north pole and the right end as a south pole.

The soft-iron bar is placed near the left end of the magnet. By magnetic induction, the end nearest the magnet becomes the opposite pole.

So:

• end Z, nearest the north pole, becomes S
• end W, farther away, becomes N

Therefore:

W = N, Z = S

Answer B.

Why others are wrong:

• A: both ends cannot be north poles.
• C: reverses the induced poles.
• D: both ends cannot be south poles.

Q27 — Correct answer: A

Aluminium is a metal, and metals are electrical conductors at room temperature.

Why others are wrong:

• B, glass: insulator.
• C, plastic: insulator.
• D, rubber: insulator.

Q28 — Correct answer: A

The balloon gains a negative charge of magnitude Q. That means electrons have moved onto the balloon.

Charge is conserved, so the cloth loses the same amount of negative charge and becomes positive with equal magnitude Q.

Opposite charges attract.

So the cloth has:

• charge equal to Q
• positive sign
• attractive force with the balloon

Answer A.

Why others are wrong:

• B: says the cloth is negative and repels, but it becomes positive.
• C: says greater than Q and repulsion; charge should be equal and attraction occurs.
• D: says greater than Q and negative; both are 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.

Q29 — Correct answer: B

The motor rotates fastest when the current through it is largest.

To get the largest current:

• cells should be in series to give maximum voltage
• resistors should be in parallel to give minimum resistance

Circuit B has the cells in series and the two resistors in parallel, so it gives the greatest current through the motor.

Why others are wrong:

• A: cells are in series, but resistors are also in series, giving higher resistance.
• C: cells are in parallel, so voltage is not increased; resistors are in series, so resistance is high.
• D: resistors are in parallel, but cells are also in parallel, so voltage is lower than in B.

Q30 — Correct answer: A

Charge is calculated using:

Q = It

Current = 2.8 × 10⁴ A
Time = 52 ms = 0.052 s

Q = 2.8 × 10⁴ × 0.052

Q = 1456 C

This rounds to about:

1500 C

So the answer is A.

Why others are wrong:

• B, 15 000 C: about 10 times too large.
• C, 24 000 C: incorrect multiplication/conversion.
• D, 54 000 C: far too large.

Q31 — Correct answer: D

Two parallel conductors carrying current in the same direction attract each other.

The forces must be in opposite directions because each conductor is pulled towards the other.

So:

• forces are in opposite directions
• conductors are forced towards each other

Answer D.

Why others are wrong:

• A: says same direction and forced apart; both wrong.
• B: says opposite directions but forced apart; that is for currents in opposite directions.
• C: says same direction; interaction forces on two separate conductors are not in the same direction.

Q32 — Correct answer: C

A simple d.c. motor needs:

• battery or power supply
• brushes
• split-ring commutator
• coil in magnetic field

A diode is not needed for a simple d.c. motor.

So the answer is C.

Why others are wrong:

• A: battery is needed to supply current.
• B: brushes are needed to maintain electrical contact with the rotating coil.
• D: split-ring commutator is needed to reverse current every half-turn, keeping rotation in the same direction.

Q33 — Correct answer: B

The component used to detect changes in temperature is a thermistor.

Symbol B represents a thermistor.

Why others are wrong:

• A: fixed resistor.
• C: variable resistor/rheostat.
• D: not the temperature-detecting component.

Q34 — Correct answer: C

Power = 2.0 kW = 2000 W
Voltage = 230 V

Current:

I = P / V

I = 2000 / 230

I = 8.7 A approximately

The fuse should be slightly above the normal operating current.

The next suitable fuse rating is 13 A.

Why others are wrong:

• A, 1 A: would blow immediately.
• B, 5 A: also too low; would blow in normal use.
• D, 20 A: too high; offers less protection than necessary.

Q35 — Correct answer: C

For a transformer:

Vs / Vp = Ns / Np

Given:

Np = 250
Ns = 200
Vp = 12 V

Vs / 12 = 200 / 250

Vs = 12 × 200 / 250

Vs = 9.6 V

So the output voltage is 9.6 V.

Why others are wrong:

• A and B: far too small; they misuse the turns ratio.
• D: 15 V would be for a step-up transformer, but this has fewer secondary turns, so voltage decreases.

Q36 — Correct answer: C

Check each statement:

1. False. Electrons are negatively charged, not positively charged.
2. True. Most of the atom is empty space.
3. True. The nucleus is positively charged because it contains protons.
4. False. The nucleus is tiny compared with the whole atom.

So the correct statements are 2 and 3.

Answer C.

Why others are wrong:

• A: includes statement 1, which is false.
• B: includes statements 1 and 4, both false.
• D: includes statement 4, which is false.

Q37 — Correct answer: B

The useful radiation is blocked by a few centimetres of air.

Alpha radiation is stopped by a few centimetres of air.

Beta travels further than alpha, and gamma is highly penetrating.

So the useful radiation must be alpha only.

Answer B.

Why others are wrong:

• A: gamma would not be blocked by a few centimetres of air.
• C: beta and gamma are too penetrating for the description.
• D: beta is not stopped by only a few centimetres of air as easily as alpha.

Q38 — Correct answer: D

Planets orbit the Sun because the Sun’s gravitational force provides the centripetal force.

A centripetal force acts towards the centre of the circular/orbital path.

So the force on each planet is towards the Sun.

Answer D.

Why others are wrong:

• A: away from the Sun would push the planet outwards, not keep it in orbit.
• B: a force in the direction of travel would speed the planet up.
• C: a force opposite to travel would slow the planet down.

Q39 — Correct answer: D

1 light-year is approximately:

9.5 × 10¹⁵ m

For 4.2 light-years:

4.2 × 9.5 × 10¹⁵

= 39.9 × 10¹⁵

= 3.99 × 10¹⁶ m

≈ 4.0 × 10¹⁶ m

So the answer is D.

Why others are wrong:

• A: much too small.
• B: too small.
• C: about one light-year, not 4.2 light-years.

Q40 — Correct answer: B

A protostar is formed when a cloud of gas and dust collapses under gravity.

As it collapses, gravitational potential energy is converted into thermal energy, so the temperature increases.

So a protostar is:

a cloud of gas that is collapsing and increasing in temperature.

Answer B.

Why others are wrong:

• A: temperature increases, not decreases.
• C: a protostar is not a star expanding and cooling.
• D: it is not a star expanding and heating; it is a collapsing gas cloud.

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.