A Level H2 Physics Mechanics Quiz

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A-Level Physics H2 Quiz - Mechanics

Name: ___________________________
Class: ___________________________
Date: ___________________________
Score: ___________ / 40

Duration: 60 minutes
Total Marks: 40
Topic: Mechanics (Projectile Motion, Collisions, Circular Motion, Gravitational Fields, Oscillations)

Instructions:

• Answer all 20 questions.
• Show all working clearly where calculation is required.
• Use the data sheet values: $g = 9.81\ \text{m s}^{-2}$, $G = 6.67 \times 10^{-11}\ \text{N m}^2\ \text{kg}^{-2}$, $M_E = 5.97 \times 10^{24}\ \text{kg}$, $R_E = 6.37 \times 10^{6}\ \text{m}$.
• Write answers in the spaces provided.

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Section A: Foundations of Mechanics (Questions 1–5)

1. State the principle of conservation of linear momentum. [2]

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2. A ball of mass $0.20\ \text{kg}$ is attached to a spring and oscillates with simple harmonic motion of amplitude $0.040\ \text{m}$ and angular frequency $3.14\ \text{rad s}^{-1}$. Calculate the maximum acceleration of the ball. [2]

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3. A projectile is launched horizontally from a cliff of height $45\ \text{m}$ with speed $20\ \text{m s}^{-1}$. Calculate the time taken to reach the ground. [2]

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4. Define centripetal force. [1]

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5. A car of mass $1200\ \text{kg}$ moves around a circular track of radius $80\ \text{m}$ at constant speed $25\ \text{m s}^{-1}$. Calculate the centripetal force acting on the car. [2]

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Section B: Motion, Collisions and Circular Motion (Questions 6–10)

6. A body moves with uniformly accelerated motion. Given $u = 5\ \text{m s}^{-1}$, $a = 2\ \text{m s}^{-2}$, $t = 4\ \text{s}$, calculate the final velocity $v$. [2]

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7. Two trolleys collide on a smooth track. Trolley A ($m_A = 1.0\ \text{kg}$) moves at $3.0\ \text{m s}^{-1}$ and trolley B ($m_B = 2.0\ \text{kg}$) is at rest. After collision they stick together. Calculate their common final velocity. [3]

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8. State two precautions that improve accuracy in a collision experiment using trolleys and a linear track. [2]

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9. A stone is tied to a string of length $0.50\ \text{m}$ and whirled in a vertical circle. At the lowest point its speed is $4.0\ \text{m s}^{-1}$. Calculate the centripetal acceleration. [2]

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10.

Image pending generation: graph for Q10.

Using the graph in Q10-fig1, determine the acceleration of the particle. [2]

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Section C: Gravitational Fields and Oscillations (Questions 11–15)

11. Write Newton's law of gravitation in equation form and state the meaning of each symbol. [3]

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12. Calculate the gravitational field strength at the surface of Earth using $g = \frac{GM}{R^2}$. [2]

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13. A satellite orbits Earth at radius $2R_E$. Calculate the gravitational field strength at this orbit. [2]

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14. A mass on a spring performs SHM with period $T = 1.2\ \text{s}$ and amplitude $0.030\ \text{m}$. Calculate the maximum velocity. [3]

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15. State the condition for resonance to occur in a forced oscillation system. [1]

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Section D: Data Interpretation and Synthesis (Questions 16–20)

16.

Image pending generation: graph for Q16.

From Q16-fig1, determine (a) the amplitude, (b) the period of oscillation. [2]

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17. A projectile is fired at $30^\circ$ above horizontal with initial speed $50\ \text{m s}^{-1}$. Calculate the maximum height reached. [3]

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18. A ball of mass $0.15\ \text{kg}$ falls from rest and hits the ground at $10\ \text{m s}^{-1}$. The impact lasts $0.020\ \text{s}$. Calculate the average force exerted by the ground. [3]

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19. Explain why a geostationary satellite must orbit above the Equator. [2]

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20. A pendulum of length $1.0\ \text{m}$ has a bob of mass $0.50\ \text{kg}$. Calculate the centripetal force at the lowest point if speed there is $2.0\ \text{m s}^{-1}$. [3]

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</stage3_quiz_answers_md>

A-Level Physics H2 Quiz - Mechanics: Answer Key

Total Marks: 40
Topic: Mechanics

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Section A: Foundations of Mechanics

Q1. [2 marks]
Principle: In a closed (isolated) system, the total linear momentum before an event equals the total linear momentum after the event, provided no net external force acts.
Marking: 1 mark for "total momentum constant / conserved"; 1 mark for condition "no external force / closed system".
Teaching note: Momentum $p = mv$ is conserved vectorially. Do not confuse with energy conservation.

Q2. [2 marks]
For SHM, $a_{\max} = \omega^2 X_0$.
Substitute: $\omega = 3.14\ \text{rad s}^{-1}$, $X_0 = 0.040\ \text{m}$.
$a_{\max} = (3.14)^2 \times 0.040 = 9.86 \times 0.040 = 0.394\ \text{m s}^{-2}$.
Marking: 1 mark for correct formula, 1 mark for answer with unit.
Common mistake: Using $a = \omega X_0$ (forgets square).

Q3. [2 marks]
Vertical motion: $s = \frac{1}{2}gt^2$ (initial vertical velocity = 0).
$45 = \frac{1}{2}(9.81)t^2 \Rightarrow t^2 = \frac{90}{9.81} = 9.17 \Rightarrow t = 3.03\ \text{s}$.
Marking: 1 mark for use of equation, 1 mark for answer.

Q4. [1 mark]
Centripetal force is the resultant force directed towards the centre of a circular path that keeps a body in circular motion.
Marking: 1 mark for "towards centre" + "maintains circular motion".

Q5. [2 marks]
$F = \frac{mv^2}{r} = \frac{1200 \times 25^2}{80} = \frac{1200 \times 625}{80} = 9375\ \text{N}$.
Marking: 1 mark formula, 1 mark answer.

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Section B: Motion, Collisions and Circular Motion

Q6. [2 marks]
$v = u + at = 5 + (2)(4) = 13\ \text{m s}^{-1}$.
Marking: 1 mark substitution, 1 mark answer.

Q7. [3 marks]
Conservation of momentum: $m_A u_A + m_B u_B = (m_A+m_B)v$.
$1.0 \times 3.0 + 2.0 \times 0 = 3.0 = 3.0 v \Rightarrow v = 1.0\ \text{m s}^{-1}$.
Marking: 1 mark momentum equation, 1 mark substitution, 1 mark answer.

Q8. [2 marks]
Examples: (1) Use a smooth track / lubricate to reduce friction (systematic error). (2) Use light gates / tickertape at eye level to reduce parallax.
Marking: 1 mark each valid precaution linked to accuracy.

Q9. [2 marks]
$a_c = \frac{v^2}{r} = \frac{4.0^2}{0.50} = \frac{16}{0.50} = 32\ \text{m s}^{-2}$.
Marking: 1 mark formula, 1 mark answer.

Q10. [2 marks]
From graph: slope = $\frac{16-4}{6-0} = \frac{12}{6} = 2.0\ \text{m s}^{-2}$.
Marking: 1 mark gradient calculation, 1 mark answer.
Visual needed: straight line through (0,4) and (6,16).

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Section C: Gravitational Fields and Oscillations

Q11. [3 marks]
$F = \frac{G m_1 m_2}{r^2}$.
$G$ = gravitational constant, $m_1, m_2$ = masses, $r$ = separation.
Marking: 1 mark equation, 2 marks symbol meanings (any 2).

Q12. [2 marks]
$g = \frac{GM_E}{R_E^2} = \frac{(6.67\times10^{-11})(5.97\times10^{24})}{(6.37\times10^6)^2} = \frac{3.98\times10^{14}}{4.06\times10^{13}} = 9.80\ \text{N kg}^{-1}$.
Marking: 1 mark substitution, 1 mark answer.

Q13. [2 marks]
At $r = 2R_E$, $g' = \frac{GM}{(2R_E)^2} = \frac{1}{4}g = \frac{9.80}{4} = 2.45\ \text{N kg}^{-1}$.
Marking: 1 mark factor 1/4, 1 mark answer.

Q14. [3 marks]
$v_{\max} = \omega X_0 = \frac{2\pi}{T}X_0 = \frac{2\pi}{1.2}(0.030) = 5.24 \times 0.030 = 0.157\ \text{m s}^{-1}$.
Marking: 1 mark $\omega$, 1 mark formula, 1 mark answer.

Q15. [1 mark]
Resonance occurs when the driving frequency equals the natural frequency of the system.
Marking: 1 mark for equality of frequencies.

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Section D: Data Interpretation and Synthesis

Q16. [2 marks]
(a) Amplitude = $5\ \text{cm}$ (from peak). (b) Period = $4\ \text{s}$ (repeat interval).
Marking: 1 mark each.
Visual: sinusoidal with peak 5 cm, period 4 s.

Q17. [3 marks]
Vertical component $u_y = 50\sin30^\circ = 25\ \text{m s}^{-1}$.
At max height $v_y=0$: $h = \frac{u_y^2}{2g} = \frac{25^2}{2\times9.81} = \frac{625}{19.62} = 31.9\ \text{m}$.
Marking: 1 mark component, 1 mark formula, 1 mark answer.

Q18. [3 marks]
Change in momentum $\Delta p = mv = 0.15\times10 = 1.5\ \text{kg m s}^{-1}$.
Average force $F = \frac{\Delta p}{t} = \frac{1.5}{0.020} = 75\ \text{N}$ (upward).
Marking: 1 mark $\Delta p$, 1 mark division, 1 mark answer.

Q19. [2 marks]
A geostationary satellite has period = Earth's rotation (24 h) and must stay above same point; only above Equator does the centripetal force direction align with gravity to give zero inclination orbit.
Marking: 1 mark period match, 1 mark Equator/alignment reason.

Q20. [3 marks]
$F_c = \frac{mv^2}{r} = \frac{0.50 \times 2.0^2}{1.0} = \frac{0.50\times4.0}{1.0} = 2.0\ \text{N}$.
Marking: 1 mark identify r = L, 1 mark formula, 1 mark answer. </stage3_quiz_answers_md>

<stage3_exam_md>

TuitionGoWhere Practice Paper - Physics H2 A-Level

TuitionGoWhere Exam Practice (AI)
Subject: Physics H2
Level: A-Level
Paper: Practice Paper (Mechanics) Version 1
Duration: 60 minutes
Total Marks: 40
Name: ___________________________
Class: ___________________________
Date: ___________________________

Instructions:

• This paper contains 20 questions on Mechanics.
• Answer all questions in the spaces provided.
• Show all working. Use $g = 9.81\ \text{m s}^{-2}$, $G = 6.67 \times 10^{-11}\ \text{N m}^2\ \text{kg}^{-2}$.
• Section A (Q1–5): 10 marks, Section B (Q6–10): 10 marks, Section C (Q11–15): 10 marks, Section D (Q16–20): 10 marks.

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Section A: Foundations (10 marks)

1. State the principle of conservation of linear momentum. [2]

2. A spring oscillator has amplitude $0.050\ \text{m}$, $\omega = 2.00\ \text{rad s}^{-1}$. Find max acceleration. [2]

3. A body dropped from rest falls $20\ \text{m}$. Find time taken. [2]

4. Define centripetal force. [1]

5. Calculate centripetal force for $800\ \text{kg}$ car at $20\ \text{m s}^{-1}$ on $50\ \text{m}$ radius bend. [3]

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Section B: Motion and Collisions (10 marks)

6. Use $v = u + at$ for $u=2$, $a=3$, $t=5$ to find $v$. [2]

7. Trolley X ($1.5\ \text{kg}$, $4\ \text{m s}^{-1}$) hits stationary Y ($1.5\ \text{kg}$), they stick. Find $v$. [3]

8. Give two accuracy precautions for trolley collision exp. [2]

9. $0.40\ \text{m}$ string, speed $3\ \text{m s}^{-1}$ at bottom: find $a_c$. [1]

10.

Image pending generation: graph for Q10.

Find acceleration from Q10-fig1. [2]

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Section C: Gravity and Oscillation (10 marks)

11. State Newton's gravitation law equation and symbols. [3]

12. Compute $g$ at Earth surface. [1]

13. Field strength at $3R_E$? [2]

14. $T=2.0\ \text{s}$, $X_0=0.10\ \text{m}$: max velocity? [3]

15. Condition for resonance. [1]

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Section D: Synthesis (10 marks)

16.

Image pending generation: graph for Q16.

From Q16-fig1 give amplitude & period. [2]

17. Launch $40\ \text{m s}^{-1}$ at $45^\circ$: max height. [2]

18. $0.20\ \text{kg}$ ball stops from $15\ \text{m s}^{-1}$ in $0.030\ \text{s}$: avg force. [2]

19. Why geostationary satellite above Equator? [2]

20. Pendulum $2.0\ \text{m}$, bob $0.40\ \text{kg}$, speed low point $3\ \text{m s}^{-1}$: $F_c$. [2] </stage3_exam_md>

<stage3_exam_answers_md>

TuitionGoWhere Practice Paper - Physics H2 A-Level: Answer Key (Version 1)

Total Marks: 40

Section A (10 marks)

Q1 [2] Total momentum conserved in closed system, no external net force. (1+1) Q2 [2] $a_{\max}=\omega^2 X_0 = 4\times0.05=0.20\ \text{m s}^{-2}$. (1+1) Q3 [2] $20=\frac{1}{2}(9.81)t^2 \Rightarrow t=2.02\ \text{s}$. (1+1) Q4 [1] Resultant force towards centre for circular motion. Q5 [3] $F=800\times20^2/50=6400\ \text{N}$. (1 formula,2 ans)

Section B (10 marks)

Q6 [2] $v=2+3\times5=17\ \text{m s}^{-1}$. Q7 [3] $1.5\times4 = 3v \Rightarrow v=2.0\ \text{m s}^{-1}$. Q8 [2] e.g. smooth track, eye-level reading. Q9 [1] $a_c=9/0.4=22.5\ \text{m s}^{-2}$. Q10 [2] slope=$(12-2)/5=2.0\ \text{m s}^{-2}$.

Section C (10 marks)

Q11 [3] $F=Gm_1m_2/r^2$; G const, m masses, r dist. Q12 [1] $9.81\ \text{N kg}^{-1}$. Q13 [2] $g/9=1.09\ \text{N kg}^{-1}$. Q14 [3] $v_{\max}=2\pi/2.0 \times0.10=0.314\ \text{m s}^{-1}$. Q15 [1] driving freq = natural freq.

Section D (10 marks)

Q16 [2] amp 4 cm, period 2 s. Q17 [2] $u_y=40\sin45=28.3$, $h=28.3^2/(2\times9.81)=40.8\ \text{m}$. Q18 [2] $\Delta p=0.2\times15=3$, $F=3/0.03=100\ \text{N}$. Q19 [2] period match + equatorial alignment. Q20 [2] $F=0.4\times3^2/2=1.8\ \text{N}$. </stage3_exam_answers_md>

<stage3_quiz_md>

A-Level Physics H2 Quiz - Mechanics

Name: ___________________________
Class: ___________________________
Date: ___________________________
Score: ___________ / 40

Duration: 60 minutes
Total Marks: 40
Topic: Mechanics (Projectile Motion, Collisions, Circular Motion, Gravitational Fields, Oscillations)

Instructions:

• Answer all 20 questions.
• Show all working clearly where calculation is required.
• Use the data sheet values: $g = 9.81\ \text{m s}^{-2}$, $G = 6.67 \times 10^{-11}\ \text{N m}^2\ \text{kg}^{-2}$, $M_E = 5.97 \times 10^{24}\ \text{kg}$, $R_E = 6.37 \times 10^{6}\ \text{m}$.
• Write answers in the spaces provided.

---

Section A: Foundations of Mechanics (Questions 1–5)

1. State the principle of conservation of linear momentum. [2]

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---

2. A ball of mass $0.20\ \text{kg}$ is attached to a spring and oscillates with simple harmonic motion of amplitude $0.040\ \text{m}$ and angular frequency $3.14\ \text{rad s}^{-1}$. Calculate the maximum acceleration of the ball. [2]

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3. A projectile is launched horizontally from a cliff of height $45\ \text{m}$ with speed $20\ \text{m s}^{-1}$. Calculate the time taken to reach the ground. [2]

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---

4. Define centripetal force. [1]

---

5. A car of mass $1200\ \text{kg}$ moves around a circular track of radius $80\ \text{m}$ at constant speed $25\ \text{m s}^{-1}$. Calculate the centripetal force acting on the car. [2]

---

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Section B: Motion, Collisions and Circular Motion (Questions 6–10)

6. A body moves with uniformly accelerated motion. Given $u = 5\ \text{m s}^{-1}$, $a = 2\ \text{m s}^{-2}$, $t = 4\ \text{s}$, calculate the final velocity $v$. [2]

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7. Two trolleys collide on a smooth track. Trolley A ($m_A = 1.0\ \text{kg}$) moves at $3.0\ \text{m s}^{-1}$ and trolley B ($m_B = 2.0\ \text{kg}$) is at rest. After collision they stick together. Calculate their common final velocity. [3]

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8. State two precautions that improve accuracy in a collision experiment using trolleys and a linear track. [2]

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9. A stone is tied to a string of length $0.50\ \text{m}$ and whirled in a vertical circle. At the lowest point its speed is $4.0\ \text{m s}^{-1}$. Calculate the centripetal acceleration. [2]

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10.

Image pending generation: graph for Q10.

Using the graph in Q10-fig1, determine the acceleration of the particle. [2]

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Section C: Gravitational Fields and Oscillations (Questions 11–15)

11. Write Newton's law of gravitation in equation form and state the meaning of each symbol. [3]

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12. Calculate the gravitational field strength at the surface of Earth using $g = \frac{GM}{R^2}$. [2]

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13. A satellite orbits Earth at radius $2R_E$. Calculate the gravitational field strength at this orbit. [2]

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14. A mass on a spring performs SHM with period $T = 1.2\ \text{s}$ and amplitude $0.030\ \text{m}$. Calculate the maximum velocity. [3]

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15. State the condition for resonance to occur in a forced oscillation system. [1]

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Section D: Data Interpretation and Synthesis (Questions 16–20)

16.

Image pending generation: graph for Q16.

From Q16-fig1, determine (a) the amplitude, (b) the period of oscillation. [2]

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17. A projectile is fired at $30^\circ$ above horizontal with initial speed $50\ \text{m s}^{-1}$. Calculate the maximum height reached. [3]

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18. A ball of mass $0.15\ \text{kg}$ falls from rest and hits the ground at $10\ \text{m s}^{-1}$. The impact lasts $0.020\ \text{s}$. Calculate the average force exerted by the ground. [3]

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19. Explain why a geostationary satellite must orbit above the Equator. [2]

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20. A pendulum of length $1.0\ \text{m}$ has a bob of mass $0.50\ \text{kg}$. Calculate the centripetal force at the lowest point if speed there is $2.0\ \text{m s}^{-1}$. [3]

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<!-- TuitionGoWhere generation metadata: stage=3-0; model=tencent/hy3:free; model_label=Tencent HY3 Free; generated=2026-07-09; Sources: Stage 2-1 real exam-derived templates and Stage 2-2 exam-enriched syllabus. -->

A-Level Physics H2 Quiz - Mechanics: Answer Key

Total Marks: 40
Topic: Mechanics

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Section A: Foundations of Mechanics

Q1. [2 marks]
Principle: In a closed (isolated) system, the total linear momentum before an event equals the total linear momentum after the event, provided no net external force acts.
Marking: 1 mark for "total momentum constant / conserved"; 1 mark for condition "no external force / closed system".
Teaching note: Momentum $p = mv$ is conserved vectorially. Do not confuse with energy conservation.

Q2. [2 marks]
For SHM, $a_{\max} = \omega^2 X_0$.
Substitute: $\omega = 3.14\ \text{rad s}^{-1}$, $X_0 = 0.040\ \text{m}$.
$a_{\max} = (3.14)^2 \times 0.040 = 9.86 \times 0.040 = 0.394\ \text{m s}^{-2}$.
Marking: 1 mark for correct formula, 1 mark for answer with unit.
Common mistake: Using $a = \omega X_0$ (forgets square).

Q3. [2 marks]
Vertical motion: $s = \frac{1}{2}gt^2$ (initial vertical velocity = 0).
$45 = \frac{1}{2}(9.81)t^2 \Rightarrow t^2 = \frac{90}{9.81} = 9.17 \Rightarrow t = 3.03\ \text{s}$.
Marking: 1 mark for use of equation, 1 mark for answer.

Q4. [1 mark]
Centripetal force is the resultant force directed towards the centre of a circular path that keeps a body in circular motion.
Marking: 1 mark for "towards centre" + "maintains circular motion".

Q5. [2 marks]
$F = \frac{mv^2}{r} = \frac{1200 \times 25^2}{80} = \frac{1200 \times 625}{80} = 9375\ \text{N}$.
Marking: 1 mark formula, 1 mark answer.

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Section B: Motion, Collisions and Circular Motion

Q6. [2 marks]
$v = u + at = 5 + (2)(4) = 13\ \text{m s}^{-1}$.
Marking: 1 mark substitution, 1 mark answer.

Q7. [3 marks]
Conservation of momentum: $m_A u_A + m_B u_B = (m_A+m_B)v$.
$1.0 \times 3.0 + 2.0 \times 0 = 3.0 = 3.0 v \Rightarrow v = 1.0\ \text{m s}^{-1}$.
Marking: 1 mark momentum equation, 1 mark substitution, 1 mark answer.

Q8. [2 marks]
Examples: (1) Use a smooth track / lubricate to reduce friction (systematic error). (2) Use light gates / tickertape at eye level to reduce parallax.
Marking: 1 mark each valid precaution linked to accuracy.

Q9. [2 marks]
$a_c = \frac{v^2}{r} = \frac{4.0^2}{0.50} = \frac{16}{0.50} = 32\ \text{m s}^{-2}$.
Marking: 1 mark formula, 1 mark answer.

Q10. [2 marks]
From graph: slope = $\frac{16-4}{6-0} = \frac{12}{6} = 2.0\ \text{m s}^{-2}$.
Marking: 1 mark gradient calculation, 1 mark answer.
Visual needed: straight line through (0,4) and (6,16).

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Section C: Gravitational Fields and Oscillations

Q11. [3 marks]
$F = \frac{G m_1 m_2}{r^2}$.
$G$ = gravitational constant, $m_1, m_2$ = masses, $r$ = separation.
Marking: 1 mark equation, 2 marks symbol meanings (any 2).

Q12. [2 marks]
$g = \frac{GM_E}{R_E^2} = \frac{(6.67\times10^{-11})(5.97\times10^{24})}{(6.37\times10^6)^2} = \frac{3.98\times10^{14}}{4.06\times10^{13}} = 9.80\ \text{N kg}^{-1}$.
Marking: 1 mark substitution, 1 mark answer.

Q13. [2 marks]
At $r = 2R_E$, $g' = \frac{GM}{(2R_E)^2} = \frac{1}{4}g = \frac{9.80}{4} = 2.45\ \text{N kg}^{-1}$.
Marking: 1 mark factor 1/4, 1 mark answer.

Q14. [3 marks]
$v_{\max} = \omega X_0 = \frac{2\pi}{T}X_0 = \frac{2\pi}{1.2}(0.030) = 5.24 \times 0.030 = 0.157\ \text{m s}^{-1}$.
Marking: 1 mark $\omega$, 1 mark formula, 1 mark answer.

Q15. [1 mark]
Resonance occurs when the driving frequency equals the natural frequency of the system.
Marking: 1 mark for equality of frequencies.

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Section D: Data Interpretation and Synthesis

Q16. [2 marks]
(a) Amplitude = $5\ \text{cm}$ (from peak). (b) Period = $4\ \text{s}$ (repeat interval).
Marking: 1 mark each.
Visual: sinusoidal with peak 5 cm, period 4 s.

Q17. [3 marks]
Vertical component $u_y = 50\sin30^\circ = 25\ \text{m s}^{-1}$.
At max height $v_y=0$: $h = \frac{u_y^2}{2g} = \frac{25^2}{2\times9.81} = \frac{625}{19.62} = 31.9\ \text{m}$.
Marking: 1 mark component, 1 mark formula, 1 mark answer.

Q18. [3 marks]
Change in momentum $\Delta p = mv = 0.15\times10 = 1.5\ \text{kg m s}^{-1}$.
Average force $F = \frac{\Delta p}{t} = \frac{1.5}{0.020} = 75\ \text{N}$ (upward).
Marking: 1 mark $\Delta p$, 1 mark division, 1 mark answer.

Q19. [2 marks]
A geostationary satellite has period = Earth's rotation (24 h) and must stay above same point; only above Equator does the centripetal force direction align with gravity to give zero inclination orbit.
Marking: 1 mark period match, 1 mark Equator/alignment reason.

Q20. [3 marks]
$F_c = \frac{mv^2}{r} = \frac{0.50 \times 2.0^2}{1.0} = \frac{0.50\times4.0}{1.0} = 2.0\ \text{N}$.
Marking: 1 mark identify r = L, 1 mark formula, 1 mark answer.