Secondary 3 Physics Semestral Assessment 2 (End of Year) Paper 2

SA2 Practice Paper — Physics Secondary 3

Answer Key (Version 2 of 5)

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Section A — Short Answer Questions

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Question 1 [2]

Answer: Newton's First Law states that an object will remain at rest or continue to move with uniform velocity in a straight line unless acted upon by a resultant (net) force.

Marking notes:

• 1 mark for stating the object remains at rest or moves with constant velocity.
• 1 mark for stating "unless acted upon by a resultant force" (or equivalent wording).
• Accept "unbalanced force" in place of "resultant force".

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Question 2 [2]

Answer:
Given: u = 0 m/s, v = 24 m/s, t = 8.0 s
Using a = (v − u) / t
a = (24 − 0) / 8.0
a = 3.0 m/s²

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

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Question 3 [2]

Answer: The resultant force is the single force that has the same effect as all the individual forces acting on an object combined (i.e., the vector sum of all forces).

Marking notes:

• 1 mark for "single force" or "one force".
• 1 mark for "same effect as all forces combined" or equivalent.

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Question 4 [2]

Answer:
Two forces:

• Weight (W) acting downwards = mg = 5.0 × 10 = 50 N
• Normal reaction force (R) acting upwards = 50 N

Free-body diagram: A dot (box) with a downward arrow labelled "Weight = 50 N" and an upward arrow labelled "Normal reaction = 50 N", both arrows equal in length.

Marking notes:

• 1 mark for correct diagram with two clearly labelled, equal and opposite arrows.
• 1 mark for correct magnitudes (50 N each).
• Accept g = 9.8 m/s² giving 49 N.

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Question 5 [2]

Answer:
(a) Force — newton (N)
(b) Momentum — kilogram metre per second (kg·m/s)

Marking notes:

• 1 mark each for correct SI unit.
• Accept "N" and "kg m/s" (or equivalent notation).

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Question 6 [2]

Answer: The acceleration is 10 m/s² downwards (or g = 10 m/s², directed towards the Earth).

Marking notes:

• 1 mark for the magnitude (10 m/s² or 9.8 m/s²).
• 1 mark for the correct direction (downwards/towards Earth).
• Common mistake: students say acceleration is zero at the highest point — this is incorrect because velocity is zero but acceleration is still g.

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Question 7 [2]

Answer:
Work done = Force × distance (in the direction of the force)
W = 30 × 4.0
W = 120 J

Marking notes:

• 1 mark for correct formula or substitution.
• 1 mark for correct answer with unit (J).

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Question 8 [2]

Answer:

• Mass is the amount of matter in an object. It is a scalar quantity measured in kilograms (kg). It does not change with location.
• Weight is the gravitational force acting on an object. It is a vector quantity measured in newtons (N). It depends on the gravitational field strength and changes with location.

Marking notes:

• 1 mark for a correct distinction regarding what each quantity represents (amount of matter vs. gravitational force).
• 1 mark for a correct distinction regarding units or scalar/vector nature or variation with location.

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Question 9 [2]

Answer:
Centripetal force: F = mv² / r
F = (0.50 × 6.0²) / 2.0
F = (0.50 × 36) / 2.0
F = 18 / 2.0
F = 9.0 N

Marking notes:

• 1 mark for correct substitution into F = mv²/r.
• 1 mark for correct answer with unit.

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Question 10 [2]

Answer: Newton's Third Law states that if object A exerts a force on object B, then object B exerts an equal and opposite force on object A (action and reaction are equal in magnitude, opposite in direction, and act on different bodies).

Example: A person standing on the ground pushes down on the Earth (action); the ground pushes up on the person with an equal normal reaction force (reaction).

Marking notes:

• 1 mark for correct statement of the law (must include "equal and opposite" and imply different bodies).
• 1 mark for a valid, clearly explained example.

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Section B — Structured Questions

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Question 11 [6]

(a) [2]
Using F = ma:
12 = 2.0 × a
a = 12 / 2.0
a = 6.0 m/s²

Marking notes:

• 1 mark for correct substitution into F = ma.
• 1 mark for correct answer with unit.

(b) [2]
Using v = u + at:
v = 0 + 6.0 × 3.0
v = 18 m/s

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

(c) [2]
Resultant force needed for a = 3.0 m/s²:
F_resultant = ma = 2.0 × 3.0 = 6.0 N
Friction = Applied force − F_resultant
Friction = 12 − 6.0
Friction = 6.0 N (opposing motion)

Marking notes:

• 1 mark for calculating resultant force correctly.
• 1 mark for subtracting to find friction force with correct unit.

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Question 12 [5]

(a) [2]
Using s = ut + ½at² (u = 0, a = g = 10 m/s²):
s = 0 + ½ × 10 × 3.0²
s = 5 × 9.0
s = 45 m

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

(b) [2]
Using v = u + at:
v = 0 + 10 × 3.0
v = 30 m/s

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

(c) [1]
Assumption: Air resistance is negligible (ignored).

Marking notes:

• 1 mark for stating air resistance is negligible / no air resistance / free fall conditions.
• Accept "g is constant" or "no air friction".

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Question 13 [5]

(a) [1]
Weight = mg = 200 × 10
Weight = 2000 N

Marking notes:

• 1 mark for correct answer with unit.

(b) [2]
Tension = 2000 N
Explanation: Since the load moves at constant speed, the resultant force is zero (Newton's First Law). Therefore, the upward tension in the cable must exactly balance the downward weight of the load.

Marking notes:

• 1 mark for correct tension value.
• 1 mark for explanation referencing constant speed → zero resultant force → tension = weight.

(c) [2]
Power = Force × velocity
P = 2000 × 1.5
P = 3000 W (or 3.0 kW)

Marking notes:

• 1 mark for correct formula or substitution.
• 1 mark for correct answer with unit.

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Question 14 [4]

(a) [1]
Newton's Law of Gravitation states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Marking notes:

• 1 mark for a complete, correct statement.
• Must include both "directly proportional to product of masses" and "inversely proportional to square of distance".

(b) [3]
F = Gm₁m₂ / r²
F = (6.67 × 10⁻¹¹ × 4.0 × 6.0) / 3.0²
F = (6.67 × 10⁻¹¹ × 24) / 9.0
F = (1.6008 × 10⁻⁹) / 9.0
F = 1.78 × 10⁻¹⁰ N (or 1.8 × 10⁻¹⁰ N to 2 s.f.)

Marking notes:

• 1 mark for correct formula.
• 1 mark for correct substitution.
• 1 mark for correct answer with unit (accept answers in range 1.7 × 10⁻¹⁰ to 1.8 × 10⁻¹⁰ N).

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Section C — Application Question

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Question 15 [10]

(a)(i) [2]
Free-body diagram: A dot representing the student with:

• A downward arrow labelled "Weight (W) = 700 N"
• An upward arrow labelled "Normal reaction (R) from scale"
  Both arrows equal in length (since the lift is stationary).

Marking notes:

• 1 mark for correct diagram with two forces shown and labelled.
• 1 mark for arrows of equal length (indicating equilibrium).

(a)(ii) [2]
Reading = 700 N
Explanation: When the lift is stationary, the resultant force on the student is zero. The scale reads the normal reaction force, which equals the student's weight (R = mg = 70 × 10 = 700 N).

Marking notes:

• 1 mark for correct reading (700 N).
• 1 mark for explanation linking stationary → equilibrium → R = W.

(b)(i) [1]
F_resultant = ma = 70 × 2.0
F_resultant = 140 N (upwards)

Marking notes:

• 1 mark for correct answer with unit and implied direction.

(b)(ii) [2]
Using Newton's Second Law (upward positive):
R − W = ma
R − 700 = 70 × 2.0
R − 700 = 140
R = 840 N

The scale reads the normal reaction force, so the reading is 840 N.

Marking notes:

• 1 mark for correct equation (R − mg = ma).
• 1 mark for correct answer (840 N).

(c) [2]
Reading = 700 N
Explanation: When the lift moves at constant velocity, the acceleration is zero, so the resultant force is again zero. The normal reaction force once again equals the student's weight (R = mg = 700 N). The scale reading is the same as when the lift was stationary.

Marking notes:

• 1 mark for correct reading (700 N).
• 1 mark for explanation referencing constant velocity → zero acceleration → equilibrium → R = W.

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End of Answer Key

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