W11_2024
Official Answer Key
|
Q |
Ans |
Q |
Ans |
Q |
Ans |
Q |
Ans |
|
1 |
B |
11 |
B |
21 |
A |
31 |
C |
|
2 |
A |
12 |
D |
22 |
C |
32 |
C |
|
3 |
C |
13 |
A |
23 |
C |
33 |
C |
|
4 |
B |
14 |
C |
24 |
B |
34 |
B |
|
5 |
C |
15 |
B |
25 |
D |
35 |
C |
|
6 |
D |
16 |
C |
26 |
A |
36 |
B |
|
7 |
B |
17 |
B |
27 |
D |
37 |
A |
|
8 |
B |
18 |
B |
28 |
A |
38 |
C |
|
9 |
A |
19 |
C |
29 |
C |
39 |
C |
|
10 |
A |
20 |
C |
30 |
B |
40 |
C |
Detailed Explanations
Q1 — Correct answer: B
Power is the rate of energy transfer.
The SI unit of power is watt, W. For a crane motor lifting a heavy load, the power is likely large, so kW is suitable.
Why others are wrong:
- A, kN: unit of force, not power.
- C, kN/s: force per time, not power.
- D, kWh: unit of energy, not power.
Q2 — Correct answer: A
Mass is a scalar quantity because it has magnitude only, no direction.
Its SI unit is the kilogram, kg.
Why others are wrong:
- B: newton is the unit of weight/force, not mass.
- C: mass is not a vector.
- D: both vector and newton are wrong for mass.
Q3 — Correct answer: C
The bob is pulled horizontally by a force equal to its weight W.
So:
horizontal force = W
vertical force = W
The pendulum thread aligns with the resultant of these two equal perpendicular forces.
tan θ = horizontal force / vertical force
tan θ = W / W
tan θ = 1
So:
θ = 45°
Answer C.
Why others are wrong:
- A, 15°: horizontal force would be much smaller than weight.
- B, 30°: horizontal force would still be smaller than weight.
- D, 60°: horizontal force would be larger than weight.
Q4 — Correct answer: B
Average speed:
average speed = total distance / total time
First part:
speed = 3.0 km/h
time = 20 min = 1/3 h
distance = 3.0 × 1/3 = 1.0 km
Second part:
speed = 4.0 km/h
time = 15 min = 1/4 h
distance = 4.0 × 1/4 = 1.0 km
Total distance = 2.0 km
Total time = 35 min = 35/60 h
Average speed = 2.0 / (35/60)
Average speed = 3.43 km/h
≈ 3.4 km/h
Answer B.
Why others are wrong:
- A: too low; likely uses displacement or wrong time.
- C: close, but not the calculated value.
- D: uses displacement idea incorrectly or adds speeds.
Q5 — Correct answer: C
The car starts from rest and has constant acceleration.
For constant acceleration from rest:
distance increases with time in a curved way.
So the graph is a distance-time graph, with:
- x-axis = time
- y-axis = distance
Answer C.
Why others are wrong:
- A: swaps the axes.
- B: speed-time graph for constant acceleration would be a straight line, not this curve.
- D: if y-axis were speed and x-axis time, the graph should be straight for constant acceleration.
Q6 — Correct answer: D
An object falling without air resistance accelerates uniformly.
This means its speed increases every second.
For equal time intervals, the distance between dots must get larger and larger in the direction of motion.
Answer D.
Why others are wrong:
- A: spacing decreases, showing slowing down.
- B: spacing appears constant, showing constant speed.
- C: does not correctly show increasing distance intervals during free fall.
Q7 — Correct answer: B
The gas is trapped in a container, so no gas escapes.
Therefore, the mass of gas does not change.
Why others are wrong:
- A: pressure increases when the gas is compressed.
- C: collision frequency increases because particles hit the walls more often.
- D: density increases because the same mass occupies a smaller volume.
Q8 — Correct answer: B
Weight = mass × gravitational field strength
W = mg
8000 = m × 9.8
m = 8000 / 9.8
m = 816 kg approximately
Closest answer = 820 kg
Answer B.
Why others are wrong:
- A, 810 kg: close, but 820 kg is the better rounded answer.
- C, 8000 kg: treats weight as mass.
- D, 78 000 kg: multiplies instead of dividing.
Q9 — Correct answer: A
Driving force = 80 kN
Resistive force = 20 kN
Resultant force:
80 – 20 = 60 kN forwards
Since the resultant force is forwards, the lorry accelerates.
Answer A.
Why others are wrong:
- B: adds the forces instead of subtracting.
- C: says deceleration, but driving force is greater than resistive force.
- D: both deceleration and 100 kN are wrong.
Q10 — Correct answer: A
When brakes are used, friction acts between the brake pads and wheel/rim/disc.
This friction transfers energy from the kinetic store to the thermal store.
Answer A.
Why others are wrong:
- B: mass does not cause the energy transfer.
- C: thrust is a forward driving force, not braking.
- D: weight acts vertically and does not directly cause braking energy transfer.
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
The limit of proportionality is the point where the graph stops being a straight line.
Before this point:
load is proportional to extension.
At point B, the spring stops obeying Hooke’s law.
Why others are wrong:
- A: still on the straight-line proportional region.
- C and D: beyond the limit; proportionality has already failed.
Q12 — Correct answer: D
A satellite moving in a circular orbit needs centripetal force.
Centripetal force always acts towards the centre of the circular path.
For a satellite orbiting Earth, this force is gravitational attraction towards the centre of Earth.
Answer D.
Why others are wrong:
- A: away from Earth would make the satellite move outwards.
- B: force in the direction of motion would speed it up.
- C: force opposite motion would slow it down.
Q13 — Correct answer: A
Take moments about the pivot.
The 3.0 N weight is 5.0 cm to the left of the pivot.
The 2.0 N weight on the right is 30 cm to the right of the pivot.
The left 2.0 N weight is x + 5.0 cm from the pivot.
Clockwise moment:
2.0 × 30 = 60 N cm
Anticlockwise moments:
3.0 × 5.0 + 2.0 × (x + 5.0)
= 15 + 2x + 10
= 25 + 2x
For equilibrium:
clockwise moment = anticlockwise moment
60 = 25 + 2x
2x = 35
x = 17.5 cm
Answer A.
Why others are wrong:
- B: gives too much anticlockwise moment.
- C and D: much too large; beam would rotate anticlockwise.
Q14 — Correct answer: C
When an object is suspended freely, its centre of gravity lies vertically below the point of suspension.
The dashed vertical line shows where the centre of gravity must lie.
Since the card is wider/heavier lower down, its centre of gravity is below the middle but not right at the bottom.
So the correct cross is C.
Why others are wrong:
- A: at the pin, too high.
- B: on the vertical line, but too high for this shape.
- D: too low, near the edge of the card.
Q15 — Correct answer: B
Take right as positive.
Initial momentum:
trolley X: 2.0 × 2.0 = 4.0 kg m/s
trolley Y: 1.0 × (-1.0) = -1.0 kg m/s
Total initial momentum:
4.0 – 1.0 = 3.0 kg m/s
After collision, they stick together.
Total mass = 2.0 + 1.0 = 3.0 kg
Final velocity:
v = 3.0 / 3.0 = 1.0 m/s to the right
For trolley X:
initial velocity = 2.0 m/s
final velocity = 1.0 m/s
Change in momentum of X:
Δp = mΔv
Δp = 2.0 × (1.0 – 2.0)
Δp = -2.0 kg m/s
Magnitude = 2.0 kg m/s
Force = change in momentum / time
F = 2.0 / 0.20
F = 10 N
Answer B.
Why others are wrong:
- A: too small.
- C: likely uses total momentum change incorrectly.
- D: too large; uses wrong velocity change.
Q16 — Correct answer: C
Kinetic energy:
KE = 1/2 mv²
Given:
KE = 10 J
m = 5.0 kg
10 = 1/2 × 5.0 × v²
10 = 2.5v²
v² = 4
v = 2.0 m/s
Answer C.
Why others are wrong:
- A and B: too small.
- D: gives KE = 40 J, not 10 J.
Q17 — Correct answer: B
The radio needs useful energy transfers such as:
- chemical energy from the person → elastic potential energy in the spring
- elastic potential energy → kinetic energy of rotating parts
- kinetic energy/electrical energy → sound energy
But:
kinetic energy → thermal energy
is wasted energy, not useful.
Answer B.
Why others are wrong:
- A: useful because the spring unwinds and drives rotation.
- C: useful because the person winds the spring.
- D: useful because the radio produces sound.
Q18 — Correct answer: B
Total power input = 60 W
Wasted power = 46 W
Useful power:
60 – 46 = 14 W
Efficiency:
efficiency = useful output / total input
Efficiency ratio = 14 / 60
Answer B.
Why others are wrong:
- A: uses 74, which is not the input power.
- C: divides useful power by wasted power.
- D: uses wasted power over total input.
Q19 — Correct answer: C
Pressure:
pressure = force / area
Force from the liquid:
F = mg
The base is circular.
Diameter = d
Radius = d/2
Area:
A = πr²
A = π(d/2)²
A = πd² / 4
Pressure:
p = mg / (πd²/4)
p = 4mg / πd²
Answer C.
Why others are wrong:
- A: misses the factor 4.
- B and D: include h unnecessarily. If mass m is already given, the force is mg, so h is not needed.
Q20 — Correct answer: C
This is a mercury barometer.
Atmospheric pressure is:
p = ρgh
The relevant mercury height is 760 mm = 0.760 m.
ρ = 1.4 × 10⁴ kg/m³
g = 9.8 N/kg
h = 0.760 m
p = 1.4 × 10⁴ × 9.8 × 0.760
p = 104 272 Pa
≈ 104 kPa
Answer C.
Why others are wrong:
- A and B: far too small.
- D: uses the wrong height.
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.
Q21 — Correct answer: A
When the first 1.0 kg mass is added, volume decreases from 100 cm³ to 90 cm³.
But gas pressure and volume are not linearly related. For a gas:
pressure × volume is approximately constant, if temperature is constant.
So adding another equal mass causes a smaller volume decrease than the first 10 cm³ decrease.
Answer A.
Why others are wrong:
- B: volume cannot reduce to zero under ordinary compression.
- C: adding mass increases pressure above atmospheric pressure.
- D: compressing the gas would not make its temperature decrease; if anything, rapid compression tends to warm it.
Q22 — Correct answer: C
Melting temperature = -17 °C
Boiling temperature = 117 °C
At -10 °C:
- temperature is above melting point
- below boiling point
- so it is liquid
At 110 °C:
- still below boiling point
- so it is still liquid
Answer C.
Why others are wrong:
- A and B: at -10 °C it is not solid.
- D: at 110 °C it has not reached its boiling temperature, so it is not gas.
Q23 — Correct answer: C
Latent heat is absorbed during a change of state.
On a heating curve, change of state occurs during flat sections, where temperature stays constant.
The flat sections are:
- X: melting
- Z: boiling
Answer C.
Why others are wrong:
- A: W is heating the solid, not latent heat.
- B: W and Y are sloping sections, where temperature rises.
- D: Y is heating the liquid, not latent heat.
Q24 — Correct answer: B
The paper says:
- average kinetic energy decreases
- spacing between molecules decreases
- they move downwards
This describes cooler, denser fluid sinking.
That is convection.
Answer B.
Why others are wrong:
- A: conduction is energy transfer through particles/electrons without bulk movement.
- C: evaporation involves more energetic molecules escaping from the surface.
- D: radiation transfers energy by infrared waves, not by downward movement of molecules.
Q25 — Correct answer: D
The ray passes from glass to air.
When light goes from a denser medium to a less dense medium, it bends away from the normal.
Arrow D is the ray in air bending furthest away from the normal.
Why others are wrong:
- A: along the normal; would require normal incidence.
- B and C: not bent far enough away from the normal for the shown incident ray.
Q26 — Correct answer: A
A short-sighted eye focuses distant objects in front of the retina.
To correct this, a diverging lens is used. It spreads rays out before they enter the eye so the image forms on the retina.
Answer A.
Why others are wrong:
- B: behind the retina is long-sightedness, corrected by converging lens.
- C: if the image is on the retina, no correction is needed.
- D: behind retina is not short-sightedness.
Q27 — Correct answer: D
Infrared waves:
- cannot be seen by the human eye
- travel at about 3.0 × 10⁸ m/s in air/vacuum, not 330 m/s
- are transverse electromagnetic waves
- are used in intruder alarms
So statements 3 and 4 are correct.
Answer D.
Why others are wrong:
- A: includes statement 1, which is false.
- B: includes statement 2, which is false.
- C: statement 2 only is false.
Q28 — Correct answer: A
Sound waves in water are longitudinal.
Longitudinal waves have vibrations parallel to the direction of wave travel.
Why others are wrong:
- B: ultraviolet waves are electromagnetic and transverse.
- C: surface water waves are treated as transverse/up-down at this level.
- D: X-rays are electromagnetic and transverse.
Q29 — Correct answer: C
An echo is caused when sound reflects from a surface, such as a cliff face.
Answer C.
Why others are wrong:
- A: absorption would reduce the sound, not return it.
- B: dispersion is splitting into different wavelengths/frequencies.
- D: refraction is bending due to speed change, not echo formation.
Q30 — Correct answer: B
The plastic becomes negatively charged.
A negative charge means the plastic has gained electrons.
So electrons must have transferred from the cloth to the plastic.
Answer B.
Why others are wrong:
- A: electrons are not transferred from Earth in this rubbing situation.
- C: moving electrons within the plastic would not give it a net charge.
- D: protons do not transfer in electrostatic charging by rubbing.
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.
Q31 — Correct answer: C
Electric field lines go from positive to negative.
Between parallel plates, the field is uniform, so the lines should be:
- straight
- parallel
- equally spaced
- directed from the positive upper plate to the negative lower plate
That means the arrows point downward.
Answer C.
Why others are wrong:
- A and B: field lines are horizontal, which is wrong between horizontal plates.
- D: lines are vertical but point from negative to positive, wrong direction.
Q32 — Correct answer: C
Electromotive force, e.m.f., is defined as:
electrical work done by a source in moving a unit charge around a complete circuit.
Answer C.
Why others are wrong:
- A: power is energy transferred per unit time.
- B: potential difference is energy transferred per unit charge between two points/components.
- D: energy is not energy per unit charge.
Q33 — Correct answer: C
There are four 1.5 V cells, but one is connected in the opposite direction to the others.
So three cells aid and one opposes.
Net voltage:
3 × 1.5 – 1 × 1.5
= 4.5 – 1.5
= 3.0 V
Answer C.
Why others are wrong:
- A: would mean the cells cancel completely.
- B: would mean only one net cell.
- D: would mean three cells only, but the opposing cell must be considered.
Q34 — Correct answer: B
The resistors are in series, so the same current flows everywhere.
Therefore:
I0 = I1 = I2
So I0 is equal to I2.
The battery potential difference is shared across the two resistors.
Therefore:
V0 = V1 + V2
Answer B.
Why others are wrong:
- A: V0 is not equal to only V2.
- C: currents in series do not add.
- D: V0 part is correct, but the current part is wrong.
Q35 — Correct answer: C
The current varies between 0.40 A and 1.2 A.
A fuse should have a rating just above the normal operating current, so it does not melt during normal use but still protects the appliance.
The closest suitable fuse above 1.2 A is:
1.5 A
Answer C.
Why others are wrong:
- A: 0.5 A would blow during normal use.
- B: 1.2 A is equal to the maximum normal current, so it may blow unnecessarily.
- D: 13 A is much too high and gives poor protection.
Q36 — Correct answer: B
The magnetic field around a straight current-carrying wire forms concentric circles.
The direction can be clockwise or anticlockwise depending on the direction of current.
Since the current direction is unknown, both possible circular directions are allowed.
Diagrams 1 and 3 show possible circular field patterns and directions.
Answer B.
Why others are wrong:
- A: includes diagram 2, which is not a consistent field direction for one current direction.
- C: misses diagram 3.
- D: diagram 2 is not possible.
Q37 — Correct answer: A
In an atom:
- proton is positive
- electron is negative
- neutron is neutral
Answer A.
Why others are wrong:
- B: reverses proton and electron.
- C: neutron is neutral, not negative.
- D: neutron is not positive, and proton is not negative.
Q38 — Correct answer: C
For nuclide notation:
R = nucleon number = protons + neutrons
Z = proton number
A neutral atom has:
electrons = protons = Z
Total number of protons, neutrons and electrons:
protons + neutrons + electrons
= R + Z
Answer C.
Why others are wrong:
- A: Z is only the number of protons.
- B: R is only protons + neutrons.
- D: 2Z + R double-counts incorrectly.
Q39 — Correct answer: C
The order of planets from the Sun is:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Answer C.
Why others are wrong:
- A: Venus and Mercury are reversed; Saturn and Jupiter are reversed.
- B: Mars, Earth and Venus are in the wrong order.
- D: several outer planets are in the wrong order.
Q40 — Correct answer: C
Redshift is:
an increase in the observed wavelength of electromagnetic radiation from receding stars and galaxies.
So:
1 = an increase
2 = wavelength
3 = receding
Answer C.
Why others are wrong:
- A: redshift is not an increase in frequency.
- B: redshift is not a decrease in frequency from approaching galaxies in this definition.
- D: approaching objects show blueshift, not redshift.
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.