Physics Basics: SI Units, Motion and Newton's Laws

Physics questions in Bihar exams are conceptual and short. This note covers the core ideas with formulas you can use directly.

SI base units

• Length – metre (m); Mass – kilogram (kg); Time – second (s).
• Electric current – ampere (A); Temperature – kelvin (K).
• Amount of substance – mole (mol); Luminous intensity – candela (cd).

Speed, velocity and acceleration

• Speed = distance ÷ time (scalar).
• Velocity = displacement ÷ time (vector).
• Acceleration = change in velocity ÷ time.

Equations of motion (uniform acceleration)

v = u + at  |  s = ut + ½at²  |  v² = u² + 2as

Newton's laws of motion

1. First law (inertia): a body stays at rest or in uniform motion unless a net force acts on it.
2. Second law: force = mass × acceleration (F = ma).
3. Third law: every action has an equal and opposite reaction.

Work, energy and power

• Work = force × displacement; unit joule (J).
• Power = work ÷ time; unit watt (W).
• Kinetic energy = ½mv²; potential energy = mgh.

Example

A car starts from rest and reaches 20 m/s in 10 seconds. Acceleration = (20 − 0) ÷ 10 = 2 m/s². Distance = ½ × 2 × 10² = 100 m.