Force and Motion I: Paper 2 Answering Guide (Form 4 SPM Physics)

Paper 2 at a glance

• Section C: 1 item worth 20 marks

"Define" items from this chapter

One precise sentence each; the unit is part of the definition where there is one.

• Mass, The amount of matter in an object; a scalar quantity measured in kilograms (kg), and a measure of its inertia.
• Momentum, The product of the mass and velocity of an object; a vector quantity (momentum = mass × velocity), measured in kg m s⁻¹.
• Principle of conservation of momentum, The principle that the total momentum of a system remains constant in the absence of an external force, so total momentum before a collision equals total momentum after.
• Force, A push or pull acting on an object that can change its shape, or its state of rest or motion; a vector quantity measured in newtons (N).
• Impulse, The product of a force and the time for which it acts, equal to the change in momentum it produces; measured in newton seconds (N s).
• Impulsive force, The large force acting over a very short time during a collision or explosion (impulsive force = change in momentum ÷ time).

"Explain" items from this chapter

Give the physics reason, not a description. Model answers in one breath:

• What inertia really is, Inertia is an object's natural resistance to any change in its state of motion. It depends only on mass, so heavier objects are harder to start and to stop.
• Why a seatbelt works, In a crash a seatbelt makes you stop over a longer time. The same change in momentum spread over more time means a smaller, safer force on your body.
• Why heavier objects do not fall faster, In free fall every object gains speed at the same rate, because a heavier object also has more inertia to move. Air resistance, not weight, is what usually makes some things fall slower.

"Calculate" items from this chapter

Formula, substitution with units, answer with unit, and the trap examiners set for each one.

• v = u + at, First equation of motion, v = final velocity (m s⁻¹); u = initial velocity (m s⁻¹); a = acceleration (m s⁻²); t = time (s) · Valid only for uniform (constant) acceleration. Treat deceleration as a negative a and keep direction signs consistent.
• s = ut + ½at², Second equation of motion, s = displacement (m); u = initial velocity (m s⁻¹); a = acceleration (m s⁻²); t = time (s) · Square the time (t²) before multiplying, and do not forget the factor ½. Use the same sign for a as the direction of u.
• v² = u² + 2as, Third equation of motion, v = final velocity (m s⁻¹); u = initial velocity (m s⁻¹); a = acceleration (m s⁻²); s = displacement (m) · This gives v², so take the square root at the end. Use it when time is unknown (no t term).
• a = (v − u) / t, Acceleration, a = acceleration (m s⁻²); v = final velocity (m s⁻¹); u = initial velocity (m s⁻¹); t = time (s) · Use the change in velocity (v − u), not v alone. A negative answer means deceleration.
• p = mv, Momentum, p = momentum (kg m s⁻¹); m = mass (kg); v = velocity (m s⁻¹) · Momentum is a vector, include direction. Its unit is kg m s⁻¹, not N.
• F = ma, Newton's second law, F = resultant force (N); m = mass (kg); a = acceleration (m s⁻²) · F must be the net (resultant) force, not a single applied force. The acceleration is in the same direction as F.
• Impulse = Ft = mv − mu, Impulse, F = force (N); t = time of contact (s); m = mass (kg); v = final velocity (m s⁻¹); u = initial velocity (m s⁻¹) · Impulse equals the change in momentum. Its unit N s is the same as kg m s⁻¹. Watch the sign when the velocity reverses.
• F = (mv − mu) / t, Impulsive force, F = impulsive force (N); m = mass (kg); v = final velocity (m s⁻¹); u = initial velocity (m s⁻¹); t = time of contact (s) · The numerator is the change in momentum. A shorter contact time gives a larger impulsive force.
• W = mg, Weight, W = weight (N); m = mass (kg); g = gravitational acceleration (m s⁻²) · Weight is a force in newtons, not mass in kilograms. It changes with location because g changes.

Other command words

• Sketch, axes labelled with units, the shape that matters (curve vs straight line, intercept, origin), no plotted numbers needed.
• Compare, same feature, both cases, one line each, then the physics that links them.

Where the marks are in this chapter

The 8 content standards of Force and Motion I can each be asked as a definition, an explanation, a calculation or a diagram. We do not predict which will appear, prepare all of them.

• 2.1 Linear Motion
• 2.2 Linear Motion Graphs
• 2.3 Free Fall Motion
• 2.4 Inertia
• 2.5 Momentum
• 2.6 Force
• 2.7 Impulse and Impulsive Force
• 2.8 Weight

A three-step answer routine

1. Decide the answer type first: definition, reason, number or drawing.
2. For numbers, write the rearranged formula on its own line before substituting.
3. Check the magnitude is sensible for the situation before moving on.

More Force and Motion I resources

• Chapter overview: Force and Motion I
• Revision Notes
• Common Mistakes
• Practice Questions
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)