Force and Motion II: Revision Notes (Form 5 SPM Physics)

What this chapter covers

• 1.1 Resultant Force
• 1.2 Resolution of Forces
• 1.3 Forces in Equilibrium
• 1.4 Elasticity

Key ideas explained

• Why a ladder slips on a smooth floor, A leaning ladder stays put only when friction at the floor balances the outward push at the base. On a smooth floor there is too little friction, so the balance breaks and the ladder slides.
• How a spring balance measures weight, A spring stretches in proportion to the force pulling on it. A spring balance reads weight because the extension of its spring is directly linked to the load, following Hooke's law.
• Why a tightrope can never be perfectly horizontal, The person's weight must be balanced by the upward parts of the rope's tension. The flatter the rope, the smaller that upward part, so a truly horizontal rope would need an impossible tension.

Formulas to memorise (none are given in the exam)

• Fₓ = F cos θ, Horizontal component of a force, Fₓ = horizontal component of force (N); F = force (N); θ = angle to the horizontal (°)
• F_y = F sin θ, Vertical component of a force, F_y = vertical component of force (N); F = force (N); θ = angle to the horizontal (°)
• F = kx, Hooke's law, F = force (N); k = spring (force) constant (N m⁻¹); x = extension (m)
• E = ½Fx = ½kx², Elastic potential energy, E = elastic potential energy (J); F = force (N); x = extension (m); k = spring (force) constant (N m⁻¹)

Experiments the DSKP names

• 1.4.2 · Investigate the relationship between force F and extension x of a spring (Hooke's law)

Terms you must define precisely

• Inclined plane, A flat surface set at an angle to the horizontal, on which an object's weight can be resolved into components along and perpendicular to the surface.
• 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.
• 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.
• 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.
• Resultant force, The single force that has the same effect as two or more forces acting together on an object.

A revision order that works

1. Read the 4 standards above and tick the ones you can already explain aloud without notes.
2. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.
3. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.
4. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
5. Finish with a timed set of calculations with units on every line.

More Force and Motion II resources

• Chapter overview: Force and Motion II
• Common Mistakes
• 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)