Force and Motion I: Common Mistakes (Form 4 SPM Physics)

The mistakes, and the fix

1. v = u + at, Valid only for uniform (constant) acceleration. Treat deceleration as a negative a and keep direction signs consistent.
2. s = ut + ½at², Square the time (t²) before multiplying, and do not forget the factor ½. Use the same sign for a as the direction of u.
3. v² = u² + 2as, This gives v², so take the square root at the end. Use it when time is unknown (no t term).
4. a = (v − u) / t, Use the change in velocity (v − u), not v alone. A negative answer means deceleration.
5. p = mv, Momentum is a vector, include direction. Its unit is kg m s⁻¹, not N.
6. F = ma, F must be the net (resultant) force, not a single applied force. The acceleration is in the same direction as F.
7. Impulse = Ft = mv − mu, 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.
8. F = (mv − mu) / t, The numerator is the change in momentum. A shorter contact time gives a larger impulsive force.
9. W = mg, Weight is a force in newtons, not mass in kilograms. It changes with location because g changes.
10. In the Determining gravitational acceleration g by free fall practical, skipping this: Measure h precisely from the bottom of the ball to the trapdoor.
11. In the Determining gravitational acceleration g by free fall practical, skipping this: Make sure the ball is released cleanly and falls straight onto the trapdoor.
12. In the Relationship between inertia and mass using an inertial balance practical, skipping this: Use the same amplitude each time so only the mass is changed.
13. In the Relationship between inertia and mass using an inertial balance practical, skipping this: Make sure the clamp holds the balance firmly so no energy is lost to the bench.
14. Mixing up Principle of conservation of momentum and Force. 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).
15. Dropping the unit on the final line, an answer without its SI unit is incomplete.
16. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Force and Motion I resources

• Chapter overview: Force and Motion I
• Revision Notes
• Practice Questions
• Paper 2 Answering Guide
• 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)