Physics Basics – Study Notes
Madhya Pradesh Police Sub‑Inspector (SI) – General Awareness & Current Affairs
(Prepared by an experienced teacher for quick, effective revision)
1. Simple Explanation
Physics is the science that studies matter, energy, and the interactions between them. It helps us understand how objects move, why things fall, how electricity powers our devices, and how the universe behaves. For police work, a basic grasp of physics is useful for:
- Interpreting forensic evidence (e.g., trajectory of a bullet, impact forces).
- Understanding vehicle dynamics in accident investigations.
- Explaining emergency medical conditions (e.g., heat‑stroke, hypothermia).
2. Key Points (Bullet‑Form)
| Topic | Essential Take‑aways |
|---|---|
| Kinematics | • Displacement (s), velocity (v), acceleration (a) • Key equations: (v = u + at), (s = ut + \frac{1}{2}at^2) (for constant (a)) |
| Newton’s Laws | 1. First Law – Inertia: An object stays at rest or moves uniformly unless acted on. 2. Second Law – (F = ma): Force equals mass times acceleration. 3. Third Law – Action–reaction pairs: forces are equal and opposite. |
| Work & Energy | • Work (W = F \cdot d \cdot \cos\theta) • Kinetic Energy (KE = \frac{1}{2}mv^2) • Potential Energy (gravitational) (PE = mgh) • Work–Energy Principle: Net work = change in kinetic energy. |
| Power | • (P = \frac{W}{t}) (average power) • (P = Fv) (instantaneous power). |
| Electricity & Magnetism | • Current (I = \frac{Q}{t}) • Ohm’s Law: (V = IR) • Power in circuits: (P = IV = I^2R = \frac{V^2}{R}) |
| Heat & Thermodynamics | • Heat transfer: conduction, convection, radiation • First Law of Thermodynamics: (\Delta U = Q - W) (energy conservation). |
| Modern Physics | • Light: Wave‑particle duality; photoelectric effect (E = hf). • Relativity: (E = mc^2) (mass–energy equivalence). • Atomic structure: electrons in shells, spectral lines. |
3. Worked Example
Problem
A 120 kg police officer jumps from a rooftop 10 m high.
- What is his velocity just before impact?
- What is the kinetic energy at impact?
- If the arresting rope can absorb 150 kJ of energy before breaking, will the rope hold?
Solution
Assumptions:
- Neglect air resistance.
- Acceleration due to gravity (g = 9.8\ \text{m/s}^2).
1. Velocity at Impact
Use the energy conservation formula: [ mgh = \frac{1}{2}mv^2 ] Solve for (v): [ v = \sqrt{2gh} = \sqrt{2 \times 9.8 \times 10} = \sqrt{196} \approx 14\ \text{m/s} ]
2. Kinetic Energy
[ KE = \frac{1}{2}mv^2 = \frac{1}{2}\times120\times(14)^2 = 60 \times 196 = 11{,}760\ \text{J} \approx 11.8\ \text{kJ} ]
3. Rope Capacity
- Rope can absorb 150 kJ before breaking.
- Required absorption = 11.8 kJ ≪ 150 kJ.
Conclusion: The rope will safely hold the officer; the absorbed energy is well below its breaking threshold.
4. Quick Revision Summary
- Motion: (s, v, a) – use kinematic equations for constant acceleration.
- Forces: Inertia, (F = ma), action–reaction.
- Work & Energy: Work = force × distance; kinetic + potential energies; conservation of energy.
- Power: Work per unit time; useful for measuring device output.
- Electricity: Current, voltage, resistance; Ohm’s law; power in circuits.
- Heat: Modes of transfer; first law of thermodynamics.
- Modern Physics: Light as particle/wave; energy–mass equivalence; atomic spectra.
Tip for Quick Recall:
- “F = ma” is your go‑to for any force‑acceleration problem.
- “Work = Force × Distance” (with angle) for energy calculations.
- “Power = Work / Time” (or (P = IV)) for all devices.
Final Thought
Physics provides the language and tools to analyze everyday incidents—whether it’s a traffic collision or a sub‑station fire. Master these basics and you’ll be better equipped to interpret evidence, make decisions, and communicate findings clearly in your duties as a Sub‑Inspector. Happy studying!