Force and Motion II: Common Mistakes (Form 5 SPM Physics)

The mistakes, and the fix

1. Fₓ = F cos θ, Use cos for the component adjacent to the angle (usually horizontal). Measure θ from the horizontal.
2. F_y = F sin θ, Use sin for the component opposite the angle (usually vertical). Measure θ from the horizontal.
3. F = kx, x is the extension (change in length), not the total length. Valid only up to the elastic limit.
4. E = ½Fx = ½kx², Square the extension in ½kx². x is the extension, not the total length. Valid within the elastic limit.
5. In the Relationship between force and extension of a spring (Hooke's law) practical, skipping this: Add and remove loads gently and wait for the spring to settle before reading.
6. In the Relationship between force and extension of a spring (Hooke's law) practical, skipping this: Read the pointer at eye level to avoid parallax error.
7. Mixing up Elasticity and Hooke's law. Elasticity: The property of a material that allows it to return to its original shape and size after the force deforming it is removed. Hooke's law: Within the elastic limit, the extension of a spring is directly proportional to the force applied to it.
8. Mixing up Spring constant and Elastic limit. Spring constant: The force needed to produce unit extension in a spring, a measure of its stiffness; measured in newtons per metre (N m⁻¹). Elastic limit: The maximum force or extension beyond which a spring or material no longer returns to its original length.
9. Mixing up Limit of proportionality and Elastic potential energy. Limit of proportionality: The point up to which the extension of a spring stays directly proportional to the applied force. Elastic potential energy: The energy stored in a spring or elastic object when it is stretched or compressed.
10. Mixing up Resultant force and Resolution of forces. Resultant force: The single force that has the same effect as two or more forces acting together on an object. Resolution of forces: The process of splitting a single force into two components, usually perpendicular to each other.
11. Mixing up Forces in equilibrium and Free body diagram. Forces in equilibrium: The condition in which the resultant of all forces acting on an object is zero, so it stays at rest or moves with constant velocity. Free body diagram: A diagram that shows all the forces acting on a single object, drawn as arrows from the object.
12. Dropping the unit on the final line, an answer without its SI unit is incomplete.
13. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.
14. Answering "describe" with a definition, describe means say what happens; define means say what it is.
15. Forgetting that direction matters for vector quantities in this chapter, state the sign convention before substituting.
16. Reading a graph gradient from two data points instead of two well-separated points on the best-fit line.

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 II resources

• Chapter overview: Force and Motion II
• 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)