Introduction
Motion, Force, and Energy form the core of classical mechanics — one of the most heavily tested Physics
areas across RRB NTPC, Group D, and ALP exams. This chapter covers Newton's Laws, motion equations, and
the work-energy-power relationship.
Types of Motion
| Type | Description | Example |
|---|
| Linear Motion | Movement along a straight line | Car moving on a straight road |
| Circular Motion | Movement along a circular path | Earth around the Sun |
| Rotatory Motion | Spinning around its own axis | Earth's rotation, a spinning top |
| Oscillatory Motion | Back-and-forth motion about a fixed point | Pendulum, swing |
| Periodic Motion | Motion that repeats at regular intervals | Simple pendulum, orbiting planets |
Newton's Three Laws of Motion
| Law | Statement | Everyday Example |
|---|
| First Law (Inertia) | An object remains at rest or in uniform motion unless acted upon by an external force | Passengers jerk forward when a bus stops suddenly |
| Second Law | Force = mass × acceleration (F = ma) | A heavier ball needs more force to achieve the same acceleration |
| Third Law | Every action has an equal and opposite reaction | A rocket launches forward as gases are expelled backward |
📌 Key Formula: F = ma (Force = mass × acceleration) — the single most tested formula from this chapter, used across many numerical questions.
Equations of Motion
| Equation | Variables |
|---|
| v = u + at | v=final velocity, u=initial velocity, a=acceleration, t=time |
| s = ut + ½at² | s=distance covered |
| v² = u² + 2as | Relates velocity and distance without time |
Q. A car starts from rest and accelerates at 2 m/s² for 5 seconds. Find its final velocity.
v = u + at = 0 + (2×5) = 10 m/s
Types of Force
| Force | Description |
|---|
| Gravitational Force | Attractive force between any two masses |
| Frictional Force | Opposes relative motion between two surfaces in contact |
| Magnetic Force | Force exerted by magnets/moving charges |
| Centripetal Force | Force directed towards the centre, keeping an object in circular motion |
| Normal Force | Perpendicular contact force exerted by a surface |
Work, Energy, and Power
| Quantity | Formula | SI Unit |
|---|
| Work | W = Force × Displacement (in the direction of force) | Joule (J) |
| Kinetic Energy | KE = ½mv² | Joule (J) |
| Potential Energy | PE = mgh | Joule (J) |
| Power | Power = Work / Time | Watt (W) |
Q. A body of mass 2 kg is moving with a velocity of 5 m/s. Find its kinetic energy.
KE = ½mv² = ½ × 2 × 5² = ½ × 2 × 25 = 25 Joules
Law of Conservation of Energy
📌 Key Principle: Energy can neither be created nor destroyed — it can only be transformed from one form to another. The total energy of an isolated system remains constant.
Types of Energy
| Type | Example |
|---|
| Mechanical Energy | Sum of kinetic and potential energy of a moving object |
| Thermal Energy | Heat energy from molecular motion |
| Chemical Energy | Stored in chemical bonds (e.g., batteries, food) |
| Electrical Energy | Energy from the flow of electric charge |
| Nuclear Energy | Released from nuclear reactions (fission/fusion) |
✅ Exam Focus: Newton's three laws and real-world examples · F=ma and the three equations of motion · Types of force · Work-Energy-Power formulas (W, KE=½mv², PE=mgh, Power=W/t) · Law of Conservation of Energy.