Force and Motion I: Revision Notes (Form 4 SPM Physics)

What this chapter covers

• 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

Key ideas explained

• 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.

Formulas to memorise (none are given in the exam)

• 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)
• s = ut + ½at², Second equation of motion, s = displacement (m); u = initial velocity (m s⁻¹); a = acceleration (m s⁻²); t = time (s)
• 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)
• 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)
• p = mv, Momentum, p = momentum (kg m s⁻¹); m = mass (kg); v = velocity (m s⁻¹)
• F = ma, Newton's second law, F = resultant force (N); m = mass (kg); a = acceleration (m s⁻²)
• 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)
• 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)
• W = mg, Weight, W = weight (N); m = mass (kg); g = gravitational acceleration (m s⁻²)

Experiments the DSKP names

• 2.3.2 · Determine the value of gravitational acceleration
• 2.4.2 · Relationship between inertia and mass

Terms you must define precisely

• Gravitational acceleration, The acceleration of an object in free fall due to gravity, about 9.81 m s⁻² near the Earth's surface.
• Inertia, The tendency of an object to resist any change to its state of rest or uniform motion.
• 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).

A revision order that works

1. Finish with a timed set of calculations with units on every line.
2. Read the 8 standards above and tick the ones you can already explain aloud without notes.
3. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.
4. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.
5. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.

More Force and Motion I resources

• Chapter overview: Force and Motion I
• 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)