📖 Plus One Physics Answer Key
March 2026 Examination · Full solutions & concise answers
🔹 SECTION A (5 × 1 = 5)
1 At the centre of Earth, the value of g is
✅ Zero (0 m/s²) – gravitational field cancels out.
2 Area under force–time graph represents
✅ (b) impulse (impulse = change in momentum).
3 Simple pendulum with time period 2 seconds is called
✅ Seconds pendulum.
4 Average kinetic energy of gas molecule ∝ ?
✅ (c) Temperature (KEavg = ½ mv² ∝ T).
5 Centre of mass does NOT lie on the body for
✅ (d) Bangle (CM lies at geometric centre, outside material).
6 Steel is more elastic than rubber.
✅ True (steel has higher Young’s modulus).
7 Wave relation: frequency, wavelength, velocity
✅ (b) f = ν / λ (where ν = wave velocity, f = frequency).
🔸 SECTION B (5 × 2 = 10)
8 Moment of inertia & factors
Moment of inertia (I) = Σ mi ri² — rotational inertia.
📌 Factors: mass of body, distribution of mass (shape), axis position.
📌 Factors: mass of body, distribution of mass (shape), axis position.
9 Derive S = ut + ½ at² from v–t graph
Area under v–t = rectangle (ut) + triangle (½ × t × (v−u)). Since v−u = at ⇒ S = ut + ½ at².
10 Mean free path
Average distance between collisions. λ = 1/(√2 π d² n) or λ = kT/(√2 π d² P).
d = molecular diameter, n = number density, P = pressure, T = temp.
d = molecular diameter, n = number density, P = pressure, T = temp.
11 Match the following
(a) Surface tension → N/m (r)
(b) Surface energy → J/m² (p)
(c) Pressure → N/m² (s)
(d) Capillary rise → m (q)
(b) Surface energy → J/m² (p)
(c) Pressure → N/m² (s)
(d) Capillary rise → m (q)
12 Heat engine & block diagram
Converts heat → mechanical work.
📌 Diagram: Source (hot reservoir) → Working substance → Sink (cold reservoir) + Work output.
📌 Diagram: Source (hot reservoir) → Working substance → Sink (cold reservoir) + Work output.
13 Escape speed & relation with orbital velocity
Escape speed ve = √(2GM/R). Orbital velocity vo = √(GM/R). Hence ve = √2 · vo.
14 Define power. Convert 1 kWh → joule
Power = rate of doing work (P = dW/dt).
1 kWh = 3.6 × 10⁶ J (since 1 kW × 3600 s = 1000 × 3600 = 3.6×10⁶ J).
1 kWh = 3.6 × 10⁶ J (since 1 kW × 3600 s = 1000 × 3600 = 3.6×10⁶ J).
🔹 SECTION C (6 × 3 = 18)
15 (a) Unit definition & (b) unit of c
(a) Unit = standard reference used to measure physical quantity.
(b) x = a + bt + ct² → ct² has dimension of meter ⇒ unit of c = m/s².
(b) x = a + bt + ct² → ct² has dimension of meter ⇒ unit of c = m/s².
16 Modulus of elasticity, SI unit & elastomer
Modulus = stress/strain. SI unit: N/m² (Pa). Dimensions: [M L⁻¹ T⁻²].
Elastomer: highly elastic material (e.g., rubber).
Elastomer: highly elastic material (e.g., rubber).
17 Ratio of frequencies (air column modes)
Fundamental (first mode) f₁ = v/4L ; next mode (third harmonic) f₃ = 3v/4L ⇒ ratio f₁ : f₃ = 1 : 3.
18 Open/closed systems & first law
(i) Open: exchanges energy & matter. (ii) Closed: exchanges energy only.
First law: ΔU = Q – W, obeys energy conservation (energy neither created nor destroyed).
First law: ΔU = Q – W, obeys energy conservation (energy neither created nor destroyed).
19 Vector product & resultant force
(a) A×B = AB sinθ n̂.
(b) R = √(5²+7²+2·5·7·cos60°) = √(25+49+35) = √109 ≈ 10.44 N.
(b) R = √(5²+7²+2·5·7·cos60°) = √(25+49+35) = √109 ≈ 10.44 N.
20 Conservative vs non-conservative forces
Conservative: work independent of path (eg: gravity).
Non-conservative: work depends on path (eg: friction).
Non-conservative: work depends on path (eg: friction).
21 Angular momentum conservation & ice skater
(a) If τext = 0 ⇒ L = constant (proof: τ = dL/dt).
(b) Stretching arms → I ↑ → ω ↓ ; folding arms → I ↓ → ω ↑.
(b) Stretching arms → I ↑ → ω ↓ ; folding arms → I ↓ → ω ↑.
🔸 SECTION D (4 × 4 = 16)
22 Heat vs temperature & Fahrenheit/Celsius relation
(a) Heat = energy transfer; Temperature = avg KE of molecules.
(b) F = C/2. Using (F-32)/9 = C/5 → C/2 - 32 = 9C/5 → solve: C ≈ -24.6°C.
(b) F = C/2. Using (F-32)/9 = C/5 → C/2 - 32 = 9C/5 → solve: C ≈ -24.6°C.
23 Newton's second law, 1 N definition & lift reaction
(a) F = dp/dt. 1 N = force to accelerate 1 kg at 1 m/s².
(b) Lift acceleration = 2g upward ⇒ R = m(g + 2g) = 3mg upward.
(b) Lift acceleration = 2g upward ⇒ R = m(g + 2g) = 3mg upward.
24 SHM definition, energy-position graph & wave parameters
(a) SHM: a ∝ –x.
(b) PE parabola, KE inverted parabola, TE constant.
(c) From figure: frequency = 1/T (Hz), amplitude = max displacement (m).
(b) PE parabola, KE inverted parabola, TE constant.
(c) From figure: frequency = 1/T (Hz), amplitude = max displacement (m).
25 Distance vs displacement & school trip
(a) Distance = scalar (total path); Displacement = vector (net change in position).
(b) Total distance = 4 km ; displacement = 0 km (returns home).
(b) Total distance = 4 km ; displacement = 0 km (returns home).
⭐ SECTION E (3 × 5 = 15)
26 Acceleration due to gravity (g)
(a) g = acceleration of freely falling body.
(b) g = GM/R² = 6.67×10⁻¹¹×6×10²⁴/(6.4×10⁶)² ≈ 9.8 m/s².
(c) At altitude h: gh = GM/(R+h)² ≈ g(1 – 2h/R) ⇒ decreases with height.
(b) g = GM/R² = 6.67×10⁻¹¹×6×10²⁴/(6.4×10⁶)² ≈ 9.8 m/s².
(c) At altitude h: gh = GM/(R+h)² ≈ g(1 – 2h/R) ⇒ decreases with height.
27 Projectile motion
(a) Object thrown under gravity only.
(b) Horizontal velocity constant; vertical changes linearly (parabolic).
(c) Time of flight T = 2u sinθ / g.
(b) Horizontal velocity constant; vertical changes linearly (parabolic).
(c) Time of flight T = 2u sinθ / g.
28 Capillary rise & mercury depression
(a) Rise/fall due to surface tension.
(b) h = 2S cosθ / (ρgr).
(c) For mercury (θ=140°, r=2mm, S=0.456 N/m, ρ=13.6×10³): cos140° = -0.766 → h = [2×0.456×(-0.766)] / [13.6e3×9.8×2e-3] ≈ -2.62 mm (depression).
(b) h = 2S cosθ / (ρgr).
(c) For mercury (θ=140°, r=2mm, S=0.456 N/m, ρ=13.6×10³): cos140° = -0.766 → h = [2×0.456×(-0.766)] / [13.6e3×9.8×2e-3] ≈ -2.62 mm (depression).
29 Hooke’s law & work in stretching wire
(a) Stress ∝ Strain within elastic limit.
(b) PE = ½ kx² (work done to stretch).
(c) Work = ½ × F × extension = ½ × (5×9.8) × (0.5×10⁻²) = 0.1225 J.
(b) PE = ½ kx² (work done to stretch).
(c) Work = ½ × F × extension = ½ × (5×9.8) × (0.5×10⁻²) = 0.1225 J.
📘 Note: Graphical derivations (v-t graph for eqn of motion, energy-position curves, capillary rise diagram, etc.) follow standard textbook methods.
The answers provided align with the FY Physics syllabus. For full step-by-step derivations, refer to class notes.
✅ FY Physics Answer Key (March 2026) — All sections covered | Marks distribution as per paper pattern