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A Level H2 Physics Practice Paper 3

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A Level H2 Physics AI Generated Generated by DeepSeek V4 Pro Updated 2026-06-03

Questions

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TuitionGoWhere Practice Paper - Physics H2 A-Level

TuitionGoWhere Practice Paper (AI)

Subject: Physics H2 (9478) Level: A-Level Paper: Practice Paper – Mechanics Version: 3 Duration: 1 hour 30 minutes Total Marks: 60

Name: _________________________ Class: _________________________ Date: _________________________


Instructions to Candidates

  1. This paper consists of 20 questions on the topic of Mechanics.
  2. Answer all questions in the spaces provided.
  3. The total mark for this paper is 60.
  4. Marks for each question or part-question are indicated in brackets [ ].
  5. You are advised to spend about 1 hour 30 minutes on this paper, including time for checking.
  6. Show all working clearly. Marks will be awarded for correct method even if the final answer is wrong.
  7. Take acceleration due to gravity, g = 9.81 m s⁻², unless otherwise stated.
  8. You may use a scientific calculator.

Section A: Kinematics and Dynamics (Questions 1–7)

Answer all questions in this section.


1. A car accelerates uniformly from rest to a speed of 25 m s⁻¹ in 8.0 s. It then maintains this speed for 12 s before decelerating uniformly to rest in a further 6.0 s.

(a) Sketch a velocity–time graph for the motion of the car. Label all key values on the axes. [3]

(b) Calculate the total distance travelled by the car. [2]

(c) Determine the average speed of the car for the entire journey. [1]

[Total: 6 marks]


2. A stone is projected horizontally from the top of a vertical cliff 45 m high with a speed of 15 m s⁻¹.

(a) Calculate the time taken for the stone to reach the sea below. [2]

(b) Determine the horizontal distance from the base of the cliff at which the stone enters the sea. [1]

(c) Calculate the magnitude and direction of the velocity of the stone just before it hits the sea. [3]

[Total: 6 marks]


3. A particle moves along a straight line such that its displacement x from a fixed point O at time t is given by:

x=4.0t22.0t3x = 4.0t^2 - 2.0t^3

where x is in metres and t is in seconds.

(a) Determine expressions for the velocity and acceleration of the particle as functions of time. [2]

(b) Calculate the displacement of the particle when its velocity is zero. [2]

(c) Find the time at which the particle returns to O after t = 0. [1]

[Total: 5 marks]


4. A block of mass 5.0 kg is pulled along a rough horizontal surface by a force of 30 N applied at an angle of 25° above the horizontal, as shown in the diagram below. The coefficient of kinetic friction between the block and the surface is 0.25.

      F = 30 N
       /|
      / |
     /  | 25°
    /   |
   /____|
   |     |
   | 5.0 kg |
   |_________|

(a) Draw a free-body diagram showing all the forces acting on the block. [2]

(b) Calculate the normal reaction force exerted by the surface on the block. [2]

(c) Determine the acceleration of the block. [3]

[Total: 7 marks]


5. Two masses, m₁ = 3.0 kg and m₂ = 5.0 kg, are connected by a light inextensible string that passes over a smooth, frictionless pulley. The 3.0 kg mass rests on a smooth horizontal table, while the 5.0 kg mass hangs vertically over the edge of the table. The system is released from rest.

   [m₁ = 3.0 kg]----[pulley]
                       |
                       |
                    [m₂ = 5.0 kg]

(a) Draw separate free-body diagrams for each mass, labelling all forces. [2]

(b) Write down the equations of motion for each mass. [2]

(c) Calculate the acceleration of the system and the tension in the string. [3]

[Total: 7 marks]


6. A ball of mass 0.15 kg is thrown vertically upwards with an initial speed of 12 m s⁻¹ from a point 2.0 m above the ground. Air resistance is negligible.

(a) Calculate the maximum height reached by the ball above the ground. [2]

(b) Determine the speed of the ball when it is 5.0 m above the ground on its way down. [2]

(c) Calculate the total time from the moment the ball is thrown until it hits the ground. [3]

[Total: 7 marks]


7. A car of mass 1200 kg travels around a circular bend of radius 80 m on a horizontal road. The coefficient of static friction between the tyres and the road is 0.60.

(a) Explain, with reference to Newton's laws, what provides the centripetal force needed for the car to travel around the bend. [2]

(b) Calculate the maximum speed at which the car can travel around the bend without skidding. [2]

(c) State and explain what would happen to the maximum safe speed if the road were wet, reducing the coefficient of friction to 0.30. [2]

[Total: 6 marks]


Section B: Work, Energy, Power, and Momentum (Questions 8–14)

Answer all questions in this section.


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


9. A trolley of mass 2.0 kg moving at 4.0 m s⁻¹ collides with a stationary trolley of mass 3.0 kg. After the collision, the trolleys move together.

(a) State the type of collision that has occurred and explain your reasoning. [2]

(b) Calculate the common velocity of the trolleys after the collision. [2]

(c) Determine the kinetic energy lost during the collision. [2]

[Total: 6 marks]


10. A bullet of mass 20 g is fired horizontally into a stationary wooden block of mass 1.98 kg resting on a smooth horizontal surface. The bullet becomes embedded in the block, and the block moves off with a speed of 2.5 m s⁻¹.

(a) Calculate the initial speed of the bullet. [2]

(b) Determine the impulse exerted by the bullet on the block. [2]

(c) Explain why the kinetic energy of the system is not conserved in this collision. [2]

[Total: 6 marks]


11. A force F acts on a particle that moves along the x-axis. The variation of the force with displacement is shown in the graph below.

F/N
^
|     _______
|    /       \
|   /         \
|  /           \
| /             \
|/_______________\_____> x/m
0   2   4   6   8

The force increases linearly from 0 to 10 N over the first 2.0 m, remains constant at 10 N for the next 2.0 m, and then decreases linearly to 0 over the final 2.0 m.

(a) Explain what the area under a force–displacement graph represents. [1]

(b) Calculate the total work done by the force as the particle moves from x = 0 to x = 6.0 m. [3]

(c) If the particle has a mass of 0.50 kg and starts from rest at x = 0, calculate its speed at x = 6.0 m, assuming no other forces act. [2]

[Total: 6 marks]


12. A pump is used to raise water from a well 25 m deep and deliver it at a rate of 15 kg s⁻¹ through a pipe. The water emerges from the pipe at a speed of 4.0 m s⁻¹.

(a) Calculate the gain in gravitational potential energy per second of the water. [2]

(b) Calculate the gain in kinetic energy per second of the water. [1]

(c) Hence determine the minimum power output required from the pump, assuming it is 100% efficient. [1]

(d) In practice, the pump has an efficiency of 70%. Calculate the actual power input required. [2]

[Total: 6 marks]


13. A ball of mass 0.50 kg is dropped from a height of 3.0 m onto a horizontal surface. It rebounds to a height of 2.0 m. The ball is in contact with the surface for 0.080 s.

(a) Calculate the speed of the ball just before it hits the surface. [1]

(b) Calculate the speed of the ball just after it leaves the surface. [1]

(c) Determine the change in momentum of the ball during the impact. [2]

(d) Calculate the average force exerted by the surface on the ball during the impact. [2]

[Total: 6 marks]


14. A spring of negligible mass and spring constant k = 200 N m⁻¹ is placed vertically on a horizontal surface. A block of mass 0.80 kg is placed on top of the spring and the spring is compressed by 0.15 m. The spring is then released, launching the block vertically upwards.

(a) Calculate the elastic potential energy stored in the compressed spring. [1]

(b) Assuming all the elastic potential energy is transferred to the block, calculate the maximum height reached by the block above the point of release. [3]

(c) Explain why, in practice, the block would not reach this calculated height. [2]

[Total: 6 marks]


Section C: Circular Motion and Gravitation (Questions 15–20)

Answer all questions in this section.


15. Define the term angular velocity and state its SI unit. [2]


16. A particle of mass 0.25 kg is attached to one end of a light inextensible string of length 0.80 m. The particle moves in a horizontal circle on a smooth table with constant angular speed. The string passes through a small hole in the centre of the table and is attached to a hanging mass of 0.60 kg, which remains stationary.

      [particle]----(string)----[hole]----[0.60 kg]
       moving in
       a circle

(a) Draw a diagram showing the forces acting on the particle and on the hanging mass. [2]

(b) Explain why the tension in the string is equal to the weight of the hanging mass. [1]

(c) Calculate the angular speed of the particle. [3]

[Total: 6 marks]


17. A satellite of mass 500 kg orbits the Earth in a circular orbit at a height of 400 km above the Earth's surface.

Given:

  • Mass of Earth, M_E = 6.0 × 10²⁴ kg
  • Radius of Earth, R_E = 6.4 × 10⁶ m
  • Gravitational constant, G = 6.67 × 10⁻¹¹ N m² kg⁻²

(a) Calculate the gravitational force acting on the satellite. [2]

(b) Show that the orbital speed of the satellite is approximately 7.7 × 10³ m s⁻¹. [2]

(c) Determine the period of the satellite's orbit. Express your answer in hours. [2]

[Total: 6 marks]


18. A conical pendulum consists of a small bob of mass 0.15 kg attached to a string of length 1.2 m. The bob moves in a horizontal circle such that the string makes an angle of 30° with the vertical.

(a) Draw a free-body diagram showing the forces acting on the bob. [1]

(b) Resolve the forces and write equations for the vertical and horizontal components. [2]

(c) Calculate the tension in the string. [1]

(d) Determine the angular speed of the bob. [2]

[Total: 6 marks]


19. Explain, using Newton's law of gravitation, why the gravitational field strength g at the Earth's surface is approximately 9.81 N kg⁻¹. Derive an expression for g in terms of G, M_E, and R_E, and calculate its value using the data provided in Question 17. [3]


20. Two stars of equal mass M are separated by a distance d. They orbit about their common centre of mass in circular orbits.

(a) Explain why the two stars must always be on opposite sides of the centre of mass. [1]

(b) Derive an expression for the period of rotation T of the stars in terms of G, M, and d. [3]

(c) State one assumption made in your derivation. [1]

[Total: 5 marks]


END OF PAPER


Check your work carefully. Ensure all answers are in the spaces provided.

Answers

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TuitionGoWhere Practice Paper – Physics H2 A-Level

Answer Key and Marking Scheme – Version 3

Paper: Mechanics Practice Paper Total Marks: 60


Section A: Kinematics and Dynamics

Question 1 [6 marks]

(a) Velocity–time graph [3 marks]

  • Correct axes labelled (velocity/m s⁻¹ vs time/s) [1]
  • Three distinct linear segments: acceleration (0 to 8 s, 0 to 25 m s⁻¹), constant velocity (8 to 20 s, 25 m s⁻¹), deceleration (20 to 26 s, 25 to 0 m s⁻¹) [1]
  • All key values labelled: (0,0), (8,25), (20,25), (26,0) [1]

(b) Total distance [2 marks]

Distance = area under v–t graph = (½ × 8.0 × 25) + (12 × 25) + (½ × 6.0 × 25) = 100 + 300 + 75 = 475 m [2]

Award [1] for correct method, [1] for correct answer with unit.

(c) Average speed [1 mark]

Average speed = total distance / total time = 475 / 26 = 18.3 m s⁻¹ [1]


Question 2 [6 marks]

(a) Time to reach sea [2 marks]

Vertical motion: s = ut + ½at² 45 = 0 + ½ × 9.81 × t² [1] t = √(2 × 45 / 9.81) = √9.17 = 3.03 s [1]

(b) Horizontal distance [1 mark]

Horizontal distance = u_x × t = 15 × 3.03 = 45.4 m [1]

(c) Velocity just before impact [3 marks]

v_x = 15 m s⁻¹ (constant) [1] v_y = u_y + at = 0 + 9.81 × 3.03 = 29.7 m s⁻¹ [1] Magnitude: v = √(15² + 29.7²) = √(225 + 882) = √1107 = 33.3 m s⁻¹ Direction: θ = tan⁻¹(29.7/15) = 63.2° below the horizontal [1]


Question 3 [5 marks]

(a) Velocity and acceleration expressions [2 marks]

v = dx/dt = 8.0t – 6.0t² [1] a = dv/dt = 8.0 – 12.0t [1]

(b) Displacement when v = 0 [2 marks]

v = 0 → 8.0t – 6.0t² = 0 → t(8.0 – 6.0t) = 0 t = 0 or t = 8.0/6.0 = 1.33 s [1] At t = 1.33 s: x = 4.0(1.33)² – 2.0(1.33)³ = 7.11 – 4.74 = 2.37 m [1]

(c) Time to return to O [1 mark]

x = 0 → 4.0t² – 2.0t³ = 0 → t²(4.0 – 2.0t) = 0 t = 0 or t = 2.0 s Time after t = 0 is 2.0 s [1]


Question 4 [7 marks]

(a) Free-body diagram [2 marks]

Forces shown:

  • Weight (mg = 49.05 N) acting downwards [0.5]
  • Normal reaction (N) acting upwards [0.5]
  • Applied force (F = 30 N) at 25° above horizontal [0.5]
  • Friction (f) acting opposite to direction of motion [0.5]

(b) Normal reaction force [2 marks]

Vertical equilibrium: N + F sin 25° = mg [1] N = mg – F sin 25° = (5.0 × 9.81) – 30 sin 25° = 49.05 – 12.68 = 36.4 N [1]

(c) Acceleration [3 marks]

Horizontal resultant force: F cos 25° – f = ma [1] f = μN = 0.25 × 36.37 = 9.09 N [1] F cos 25° = 30 cos 25° = 27.19 N a = (27.19 – 9.09) / 5.0 = 18.10 / 5.0 = 3.62 m s⁻² [1]


Question 5 [7 marks]

(a) Free-body diagrams [2 marks]

m₁ (3.0 kg on table):

  • Weight (m₁g) downwards [0.5]
  • Normal reaction (N) upwards [0.5]
  • Tension (T) to the right [0.5]

m₂ (5.0 kg hanging):

  • Weight (m₂g) downwards [0.25]
  • Tension (T) upwards [0.25]

(b) Equations of motion [2 marks]

For m₁: T = m₁a [1] For m₂: m₂g – T = m₂a [1]

(c) Acceleration and tension [3 marks]

Adding equations: m₂g = (m₁ + m₂)a [1] a = m₂g / (m₁ + m₂) = (5.0 × 9.81) / 8.0 = 49.05 / 8.0 = 6.13 m s⁻² [1] T = m₁a = 3.0 × 6.13 = 18.4 N [1]


Question 6 [7 marks]

(a) Maximum height above ground [2 marks]

v² = u² + 2as → 0 = 12² + 2(–9.81)s [1] s = 144 / 19.62 = 7.34 m (above point of release) Maximum height above ground = 7.34 + 2.0 = 9.34 m [1]

(b) Speed at 5.0 m above ground on way down [2 marks]

Height fallen from max = 9.34 – 5.0 = 4.34 m [1] v² = u² + 2as = 0 + 2 × 9.81 × 4.34 = 85.15 v = 9.23 m s⁻¹ [1]

(c) Total time until hitting ground [3 marks]

Time to max height: v = u + at → 0 = 12 – 9.81t₁ → t₁ = 1.22 s [1] Displacement from max height to ground: s = –9.34 m s = ut + ½at² → –9.34 = 0 + ½(–9.81)t₂² t₂ = √(2 × 9.34 / 9.81) = 1.38 s [1] Total time = 1.22 + 1.38 = 2.60 s [1]

Alternative method using s = ut + ½at² with s = –2.0 m also acceptable.


Question 7 [6 marks]

(a) Centripetal force explanation [2 marks]

  • The centripetal force is provided by the frictional force between the tyres and the road [1]
  • By Newton's second law, a net force towards the centre of the circle is required to produce centripetal acceleration (v²/r). The friction force acts towards the centre of the bend, preventing the car from skidding outwards [1]

(b) Maximum speed [2 marks]

f_max = μN = μmg = 0.60 × 1200 × 9.81 = 7063 N [1] mv²/r = f_max → v = √(f_max × r / m) = √(7063 × 80 / 1200) = √470.9 = 21.7 m s⁻¹ [1]

(c) Effect of wet road [2 marks]

  • Maximum safe speed would decrease [1]
  • Since v_max = √(μgr), reducing μ by half reduces v_max by a factor of √2 ≈ 0.707. The new maximum speed would be approximately 15.3 m s⁻¹. This is because the maximum frictional force available is halved, so the maximum centripetal force is also halved [1]

Section B: Work, Energy, Power, and Momentum

Question 8 [2 marks]

Principle of conservation of linear momentum:

The total linear momentum of a system remains constant (or is conserved) provided no net external force acts on the system. [2]

Award [1] for "total momentum constant/conserved" and [1] for "no net external force" or "isolated/closed system".


Question 9 [6 marks]

(a) Type of collision [2 marks]

  • Perfectly inelastic collision [1]
  • The trolleys move together (coalesce) after the collision, which is characteristic of a perfectly inelastic collision where kinetic energy is not conserved [1]

(b) Common velocity [2 marks]

By conservation of momentum: m₁u₁ + m₂u₂ = (m₁ + m₂)v [1] (2.0 × 4.0) + (3.0 × 0) = (2.0 + 3.0)v 8.0 = 5.0v → v = 1.6 m s⁻¹ [1]

(c) Kinetic energy lost [2 marks]

Initial KE = ½ × 2.0 × 4.0² = 16.0 J [1] Final KE = ½ × 5.0 × 1.6² = 6.4 J KE lost = 16.0 – 6.4 = 9.6 J [1]


Question 10 [6 marks]

(a) Initial speed of bullet [2 marks]

By conservation of momentum: m_bullet × u = (m_bullet + m_block) × v [1] 0.020 × u = (0.020 + 1.98) × 2.5 u = (2.00 × 2.5) / 0.020 = 5.0 / 0.020 = 250 m s⁻¹ [1]

(b) Impulse on block [2 marks]

Impulse = change in momentum of block [1] = m_block × v – 0 = 1.98 × 2.5 = 4.95 N s [1]

(c) Why KE is not conserved [2 marks]

  • The collision is inelastic because the bullet becomes embedded in the block [1]
  • Some of the initial kinetic energy is converted to other forms such as heat, sound, and the work done in deforming the bullet and the block [1]

Question 11 [6 marks]

(a) Area under F–x graph [1 mark]

The area under a force–displacement graph represents the work done by the force. [1]

(b) Total work done [3 marks]

Area = area of trapezium (0 to 2 m) + rectangle (2 to 4 m) + triangle (4 to 6 m) [1] = [½ × (0 + 10) × 2.0] + [10 × 2.0] + [½ × 10 × 2.0] [1] = 10 + 20 + 10 = 40 J [1]

(c) Speed at x = 6.0 m [2 marks]

Work done = change in KE = ½mv² – 0 [1] 40 = ½ × 0.50 × v² v² = 160 → v = 12.6 m s⁻¹ [1]


Question 12 [6 marks]

(a) Gain in GPE per second [2 marks]

GPE gained per kg = mgh = 1 × 9.81 × 25 = 245.25 J kg⁻¹ [1] Mass flow rate = 15 kg s⁻¹ GPE gain per second = 15 × 245.25 = 3.68 × 10³ W (or J s⁻¹) [1]

(b) Gain in KE per second [1 mark]

KE gained per kg = ½ × 1 × 4.0² = 8.0 J kg⁻¹ KE gain per second = 15 × 8.0 = 120 W [1]

(c) Minimum power output [1 mark]

Minimum power = GPE gain + KE gain = 3680 + 120 = 3.80 × 10³ W [1]

(d) Actual power input [2 marks]

Efficiency = useful power output / power input [1] 0.70 = 3800 / P_in P_in = 3800 / 0.70 = 5.43 × 10³ W [1]


Question 13 [6 marks]

(a) Speed just before impact [1 mark]

v² = u² + 2as = 0 + 2 × 9.81 × 3.0 = 58.86 v = 7.67 m s⁻¹ (downwards) [1]

(b) Speed just after impact [1 mark]

v² = u² + 2as → 0 = u² + 2(–9.81)(2.0) u = √(39.24) = 6.26 m s⁻¹ (upwards) [1]

(c) Change in momentum [2 marks]

Taking upwards as positive: Initial momentum (downwards) = –0.50 × 7.67 = –3.835 kg m s⁻¹ [1] Final momentum (upwards) = +0.50 × 6.26 = +3.13 kg m s⁻¹ Δp = 3.13 – (–3.835) = 6.97 kg m s⁻¹ (upwards) [1]

(d) Average force [2 marks]

F_avg = Δp / Δt [1] = 6.97 / 0.080 = 87.1 N (upwards) [1]


Question 14 [6 marks]

(a) Elastic potential energy [1 mark]

EPE = ½kx² = ½ × 200 × (0.15)² = 2.25 J [1]

(b) Maximum height [3 marks]

Energy conservation: EPE = GPE gained [1] 2.25 = mgh = 0.80 × 9.81 × h [1] h = 2.25 / (0.80 × 9.81) = 2.25 / 7.848 = 0.287 m (above point of release) [1]

(c) Why block would not reach this height [2 marks]

  • Some energy is lost due to air resistance as the block moves upwards [1]
  • The spring itself has mass, so some kinetic energy remains in the spring / energy is dissipated as internal energy in the spring (hysteresis) [1]
  • Accept any two valid reasons.

Section C: Circular Motion and Gravitation

Question 15 [2 marks]

Angular velocity is the rate of change of angular displacement with respect to time. [1] SI unit: radian per second (rad s⁻¹) [1]


Question 16 [6 marks]

(a) Force diagrams [2 marks]

Particle:

  • Tension (T) towards centre of circle [0.5]
  • Weight (mg) downwards [0.25]
  • Normal reaction (N) upwards [0.25]

Hanging mass:

  • Tension (T) upwards [0.5]
  • Weight (Mg) downwards [0.5]

(b) Tension equals weight of hanging mass [1 mark]

The hanging mass is stationary (in equilibrium), so the net force on it is zero. Therefore, T – Mg = 0, so T = Mg. [1]

(c) Angular speed [3 marks]

For the particle in circular motion: T = mrω² [1] T = Mg = 0.60 × 9.81 = 5.886 N [1] 5.886 = 0.25 × 0.80 × ω² ω² = 5.886 / 0.20 = 29.43 ω = 5.43 rad s⁻¹ [1]


Question 17 [6 marks]

(a) Gravitational force [2 marks]

r = R_E + h = 6.4 × 10⁶ + 4.0 × 10⁵ = 6.8 × 10⁶ m [1] F = GM_E m / r² = (6.67 × 10⁻¹¹ × 6.0 × 10²⁴ × 500) / (6.8 × 10⁶)² = (2.001 × 10¹⁷) / (4.624 × 10¹³) = 4.33 × 10³ N [1]

(b) Orbital speed [2 marks]

Gravitational force provides centripetal force: GMm/r² = mv²/r [1] v = √(GM/r) = √(6.67 × 10⁻¹¹ × 6.0 × 10²⁴ / 6.8 × 10⁶) = √(4.002 × 10¹⁴ / 6.8 × 10⁶) = √(5.885 × 10⁷) = 7.67 × 10³ m s⁻¹ ≈ 7.7 × 10³ m s⁻¹ [1]

(c) Period of orbit [2 marks]

T = 2πr / v [1] = 2π × 6.8 × 10⁶ / 7.67 × 10³ = 4.27 × 10⁷ / 7.67 × 10³ = 5.57 × 10³ s = 5570 / 3600 = 1.55 hours [1]


Question 18 [6 marks]

(a) Free-body diagram [1 mark]

Forces on bob:

  • Weight (mg) vertically downwards [0.5]
  • Tension (T) along the string at 30° to the vertical [0.5]

(b) Force resolution [2 marks]

Vertical: T cos 30° = mg [1] Horizontal: T sin 30° = mrω², where r = L sin 30° = 1.2 sin 30° = 0.60 m [1]

(c) Tension [1 mark]

T cos 30° = 0.15 × 9.81 = 1.4715 T = 1.4715 / cos 30° = 1.4715 / 0.8660 = 1.70 N [1]

(d) Angular speed [2 marks]

T sin 30° = mrω² [1] 1.699 × 0.5 = 0.15 × 0.60 × ω² 0.8495 = 0.09 ω² ω = √(0.8495 / 0.09) = √9.44 = 3.07 rad s⁻¹ [1]


Question 19 [3 marks]

Explanation and derivation:

By Newton's law of gravitation, the force on a mass m at the Earth's surface is: F = GM_E m / R_E² [1]

By definition, gravitational field strength g = F/m, so: g = GM_E / R_E² [1]

Substituting values: g = (6.67 × 10⁻¹¹ × 6.0 × 10²⁴) / (6.4 × 10⁶)² = 4.002 × 10¹⁴ / 4.096 × 10¹³ = 9.77 N kg⁻¹ ≈ 9.8 N kg⁻¹ [1]


Question 20 [5 marks]

(a) Why stars are on opposite sides [1 mark]

The centre of mass of the system must lie between the two stars. Since the stars have equal mass, the centre of mass is at the midpoint of the line joining them. For both stars to orbit the centre of mass, they must always be on opposite sides of it, maintaining the same separation d. [1]

(b) Period of rotation [3 marks]

Each star orbits at radius r = d/2 from the centre of mass. Gravitational force on each star: F = GM²/d² [1] This provides centripetal force: GM²/d² = M(d/2)ω² [1] ω² = 2GM/d³ T = 2π/ω = 2π √(d³/2GM) [1]

(c) Assumption [1 mark]

  • The stars are point masses / The orbits are circular (not elliptical) / Only gravitational forces between the two stars are considered (no other bodies) [1]
  • Accept any one valid assumption.

END OF ANSWER KEY