Forces, Motion and Energy
What this domain tests
Forces, Motion and Energy tests kinematics, Newton's laws, momentum, work, energy, power, efficiency, pressure and elasticity.
Core rules
- v = u + at; s = ut + ½at²; v² = u² + 2as. Acceleration is change in velocity divided by time; deceleration is negative acceleration.
- Newton's first law: with no resultant force, a body stays at rest or moves at constant velocity. Second law: F = ma. Third law: equal and opposite forces act on different bodies.
- Momentum p = mv; total momentum is conserved in collisions and explosions.
- Work W = Fs; kinetic energy KE = ½mv²; gravitational potential energy PE = mgh; power P = W/t; efficiency = useful output ÷ input × 100%.
- Hooke's law F = kx up to the elastic limit.
- Pressure P = F/A; liquid pressure P = hρg; upthrust equals the weight of fluid displaced.
Worked example
A 1000 kg car accelerates uniformly from rest to 20 m/s in 5 s. a = (20 − 0) ÷ 5 = 4 m/s². Resultant force F = ma = 1000 × 4 = 4000 N. Distance s = ut + ½at² = 0 + ½ × 4 × 25 = 50 m. Work done by the resultant force = 4000 × 50 = 200 000 J = 200 kJ. Final KE = ½ × 1000 × 20² = 200 000 J, agreeing with the work-energy principle.
Traps
Convert to metres, kilograms and seconds first. Do not add vectors as scalars. Do not say energy is 'used up'; it is transferred to surroundings by friction and heating. Distinguish mass from weight using W = mg, with g approximately 9.8 N kg⁻¹.