Mechanics and Motion
Mechanics and Motion
Mechanics and Motion is Section A of CSEC Physics. It tests how quantities are measured and represented, how objects move, and why motion changes under forces. Expect questions on scalars and vectors, kinematics graphs, Newton's laws, moments, pressure, work, energy and power.
Core rules and concepts
- Scalars and vectors. Distance, speed, mass and energy are scalars; displacement, velocity, acceleration, force and momentum are vectors. Average speed = total distance ÷ total time; velocity includes direction.
- Uniform acceleration. For constant acceleration: v = u + at, s = ut + ½at², v² = u² + 2as. CSEC commonly takes g = 10 N kg⁻¹, equivalent to 10 m s⁻².
- Forces. Newton's first law: no resultant force means no change in motion. Second law: F = ma. Third law: forces come in equal, opposite pairs acting on different bodies. Weight W = mg, not mass.
- Equilibrium and moments. Resultant force and resultant moment must both be zero. Principle of moments: sum of clockwise moments = sum of anticlockwise moments, using perpendicular distance from the pivot.
- Pressure and energy. P = F/A; liquid pressure p = ρgh. Work W = Fs, power P = W/t, kinetic energy ½mv², gravitational potential energy mgh; energy is conserved.
Worked example
A car starts from rest and accelerates uniformly at 2 m s⁻² for 5 s. Find its final velocity and distance travelled. Reasoning: list u = 0, a = 2, t = 5. Use v = u + at = 0 + (2)(5) = 10 m s⁻¹. Then s = ut + ½at² = 0 + ½(2)(25) = 25 m. The distance is 25 m; the final velocity is 10 m/s in the direction of motion.
Traps that cost marks
- Giving speed when velocity is asked, or omitting direction.
- Mixing mass in kg and weight in N; using g = 9.8 when the question expects 10.
- Using a slanted distance instead of the perpendicular distance in a moment.
- Forgetting to convert cm to m, g to kg, or minutes to seconds.
- Treating an action–reaction pair as if both forces act on the same body.