Physics Basics – Study Notes
For Punjab Police Constable – General Awareness & Current Affairs
1️⃣ What is Physics?
Physics is the science that deals with the fundamental principles governing matter, energy, motion, and the forces that act upon them. It explains how things move, why they move, and how energy changes form.
2️⃣ Key Concepts & Their Simple Explanations
| Concept | Simple Explanation | Important Formulae |
|---|---|---|
| Distance & Displacement | Distance = total path travelled (scalar). Displacement = straight‑line change in position with direction (vector). |
None |
| Speed & Velocity | Speed = distance ÷ time (scalar). Velocity = displacement ÷ time (vector). |
(v = \frac{s}{t}) (\vec{v} = \frac{\Delta \vec{s}}{t}) |
| Acceleration | Change in velocity per unit time. | (a = \frac{\Delta v}{t}) |
| Newton’s First Law | An object stays at rest or moves uniformly unless acted on by an external force. | None |
| Newton’s Second Law | Force equals mass times acceleration. | (\boxed{F = m a}) |
| Newton’s Third Law | For every action there is an equal and opposite reaction. | None |
| Work | Energy transferred by a force acting over a displacement. | (W = F \cdot d \cdot \cos\theta) |
| Kinetic Energy (KE) | Energy of motion. | (KE = \frac{1}{2} m v^2) |
| Potential Energy (PE) | Energy stored due to position. | (PE = m g h) |
| Power | Work done per unit time. | (P = \frac{W}{t}) |
| Conservation of Energy | Total energy in an isolated system remains constant. | (KE_i + PE_i = KE_f + PE_f) |
| Simple Harmonic Motion (SHM) | Oscillatory motion where restoring force ∝ displacement. | (x(t) = A\cos(\omega t + \phi)) |
| Waves | Disturbance that transfers energy without transferring matter. | Wave speed (v = f \lambda) |
| Sound | Longitudinal compression waves in a medium. | Frequency (f) in Hz, speed in m/s |
| Light | Electromagnetic waves that travel at (c = 3.0\times10^8,\text{m/s}). | (E = h f) |
| Electricity (Ohm’s Law) | Relationship between voltage, current, and resistance. | (V = I R) |
| Magnetism | Force on moving charges; magnetic fields interact with electric currents. | Lorentz force ( \vec{F} = q(\vec{E} + \vec{v} \times \vec{B})) |
3️⃣ Worked Example – Calculating Work Done by a Police Baton
Scenario:
A constable swings a baton of mass (m = 0.5,\text{kg}) with a force (F = 200,\text{N}) that acts over a displacement of (d = 1.2,\text{m}). The force is applied in the same direction as the displacement (θ = 0°).
Step‑by‑Step:
-
Identify the relevant formula
Work: (W = F \cdot d \cdot \cos\theta) -
Plug in the values
(W = 200,\text{N} \times 1.2,\text{m} \times \cos 0^\circ) -
Compute
(\cos 0^\circ = 1)
(W = 200 \times 1.2 = 240,\text{J}) -
Result
The constable does 240 Joules of work on the baton.
Takeaway:
Work depends on the magnitude of the force, the distance over which it acts, and the angle between them. For maximum work, the force should be applied in the same direction as the displacement.
4️⃣ Quick Revision Summary
| Topic | Key Point | Formula (if any) |
|---|---|---|
| Distance / Displacement | Distance is scalar, displacement is vector | – |
| Speed / Velocity | Speed = distance/time; Velocity = displacement/time | (v=s/t) (\vec{v}=\Delta\vec{s}/t) |
| Acceleration | Rate of change of velocity | (a=\Delta v / t) |
| Newton’s Laws | 1st: inertia; 2nd: (F=ma); 3rd: action‑reaction | – |
| Work | Force × displacement × cosθ | (W=F d \cos\theta) |
| Energy | KE = ½mv²; PE = mgh | – |
| Power | Work/time | (P=W/t) |
| Conservation of Energy | KE+PE constant | – |