Force, Work, Energy and Newton's Laws

Newton's laws of motion

1. First law (inertia) – a body stays at rest or in uniform motion unless an external force acts. Example: passengers lurch forward when a bus brakes.
2. Second law – F = m × a. SI unit of force: newton (N).
3. Third law – every action has an equal and opposite reaction. Example: rocket propulsion, recoil of a gun.

Momentum

p = m × v (kg·m/s). Total momentum is conserved when no external force acts.

Work, energy and power

• Work W = F × d × cosθ; unit joule (J). No work is done if displacement is zero or perpendicular to force.
• Kinetic energy = ½mv²; Potential energy = mgh.
• Power = work / time; unit watt (W); 1 horsepower ≈ 746 W.
• Energy is neither created nor destroyed (conservation of energy).

Exam tips

1. Doubling speed makes KE four times.
2. 1 kWh = 3.6 × 10⁶ J (the 'unit' on electricity bills).

Analogy

Newton's laws are like pushing a shopping trolley. A still trolley won't move until you push (inertia). A heavier trolley needs a harder push for the same speed-up (F = ma). And when you push it, it pushes back on your hands (action–reaction).