Physics Basics – SSC JE (General Awareness & Current Affairs)
Audience: Junior Engineer candidates who need solid grounding in the first principles of physics, but who also appreciate a deeper, MIT‑style perspective to sharpen reasoning skills and avoid common pitfalls.
Goal:
- Understand the why behind each law, not just the what.
- Learn to recognize edge cases and subtle assumptions that can trip up exam answers.
- Practice a challenging problem that forces you to synthesize concepts.
Structure
- Fundamental principles (classical mechanics, electromagnetism, thermodynamics, waves, modern physics).
- Rigorous derivations from first principles.
- Edge‑case analysis (non‑inertial frames, idealisations, limiting behaviour).
- Illustrative examples and common pitfalls.
- Challenging problem with full solution.
- Key take‑aways & quick‑reference.
Tip: While SSC JE focuses on conceptual questions, the derivations below will sharpen your intuition and help you spot subtle answer choices.
1. Classical Mechanics
1.1 Kinematics
| Quantity | Symbol | Definition | Unit |
|---|---|---|---|
| Displacement | (\Delta \mathbf{r}) | (\mathbf{r}_f-\mathbf{r}_i) | m |
| Velocity | (\mathbf{v}) | (\dfrac{d\mathbf{r}}{dt}) | m s(^{-1}) |
| Acceleration | (\mathbf{a}) | (\dfrac{d\mathbf{v}}{dt}) | m s(^{-2}) |
Equations of Motion (constant (a))
- ( \mathbf{v} = \mathbf{v}_0 + \mathbf{a}t )
- ( \Delta \mathbf{r} = \mathbf{v}_0t + \tfrac12\mathbf{a}t^2 )
- ( \mathbf{v}^2 = \mathbf{v}_0^2 + 2\mathbf{a}\cdot\Delta\mathbf{r} )
Derivation from first principles
Newton’s 2nd law ( \mathbf{F}=m\mathbf{a} ) integrates once to give velocity, twice for displacement.
The third equation follows by eliminating (t) from the first two.
1.2 Newton’s Laws
| Law | Statement | Implication |
|---|---|---|
| 1 | In an inertial frame, a body remains at rest or in uniform motion unless acted upon by a net external force. | Sets the reference frame. |
| 2 | ( \mathbf{F} = m\mathbf{a} ) | Defines force as the cause of acceleration. |
| 3 | For every action, there is an equal and opposite reaction. | Conservation of momentum in isolated systems. |
Edge Case – Non‑inertial frames: Add fictitious forces ( \mathbf{F}f = -m\mathbf{a}{\text{frame}}).
Common Pitfall – Treating friction as a force rather than an interaction; remember it depends on normal force and coefficient of friction.
1.3 Work, Energy, Power
| Symbol | Definition | Formula |
|---|---|---|
| Work | (W = \int \mathbf{F}\cdot d\mathbf{r}) | (W = \Delta K) (Work‑Energy Theorem) |
| Kinetic Energy | (K = \tfrac12 m v^2) | |
| Potential Energy | (U = mgh) (uniform (g)) | |
| Power | (P = \dfrac{dW}{dt}) | (P = \mathbf{F}\cdot \mathbf{v}) |
Derivation
Start from Newton’s 2nd law: (\mathbf{F}\cdot d\mathbf{r} = m\mathbf{a}\cdot d\mathbf{r} = m,d(\tfrac12 v^2)). Integrate to get (W=K_f-K_i).
1.4 Conservation Laws
-
Momentum ( \mathbf{p} = m\mathbf{v} )
(\sum \mathbf{F}_{\text{ext}}=0 ;\Rightarrow; \sum \mathbf{p} = \text{constant}). -
Energy (in absence of non‑conservative forces)
(K