Why physics matters for RRB NTPC
Ever wondered why a train can pull a heavy carriage or how a lamp glows? Those everyday wonders are built on the same ideas you’ll see in the RRB NTPC physics section.
💡 In Simple Words: Physics is the study of how things move and interact. If you picture a ball rolling down a hill, physics tells you why it speeds up, how far it goes, and what stops it.
Core concepts that keep popping up
Force and Newton’s laws
Force is a push or pull that can change an object’s motion. Think of it like a gentle nudge that makes a swing start moving.
Newton’s first law (also called the law of inertia) says an object stays still or keeps moving straight unless a force steps in. Imagine a skateboard on a smooth floor; it won’t stop unless friction or a hand slows it.
Newton’s second law links force, mass, and acceleration with the formula F = m·a. Here, F is force, m is how much stuff is in the object (mass), and a is how quickly its speed changes.
Newton’s third law reminds us that forces always come in pairs: if you push a wall, the wall pushes back with the same strength.
Energy, work, and power
Energy is the ability to do work, like the fuel in a car that lets it move. It shows up as kinetic (motion) energy or potential (stored) energy.
Work happens when a force moves something over a distance. The math looks like W = F·d (force times distance). Imagine lifting a backpack; the effort you use over the height you lift equals the work done.
Power tells you how fast work gets done, using P = W/t (work divided by time). If two people lift the same box, the one who does it quicker has higher power.
Momentum and impulse
Momentum measures how hard it is to stop a moving object. It’s the product of mass and velocity: p = m·v. A bowling ball rolling fast is harder to halt than a feather.
Impulse is the change in momentum caused by a force over a short time, expressed as J = F·Δt. Think of a bat hitting a ball; the brief punch changes the ball’s momentum.
Electric basics
Current is the flow of electric charge, similar to water moving through a pipe. Measured in amperes (A), it tells how many electrons pass a point each second.
Voltage (or potential difference) pushes the current, like pressure pushes water. Measured in volts (V).
Resistance opposes current, just as a narrow pipe slows water. Measured in ohms (Ω). The relationship among them is V = I·R (Ohm’s law).
Key formulas you must memorize
| Concept | Formula | Unit |
|---|---|---|
| Force | F = m·a | Newton (N) |
| Work | W = F·d | Joule (J) |
| Power | P = W/t | Watt (W) |
| Kinetic Energy | KE = ½·m·v² | Joule (J) |
| Potential Energy (gravity) | PE = m·g·h | Joule (J) |
| Momentum | p = m·v | kg·m/s |
| Impulse | J = F·Δt | N·s |
| Ohm’s Law | V = I·R | Volt (V) |
Quick bullet summary for fast revision
- Force = mass × acceleration (push = how heavy × how fast speed changes).
- Work = force × distance (effort × how far you move).
- Power = work ÷ time (how quickly you finish the work).
- Kinetic energy depends on speed squared – double the speed, quadruple the energy.
- Potential energy grows with height – lift something higher, store more energy.
- Momentum = mass × velocity – harder to stop a heavy fast object.
- Impulse = force × contact time – a longer push changes momentum more.
- Ohm’s law links voltage, current, resistance – like water pressure, flow, and pipe width.
📝 Likely Exam Questions
1. A 2 kg block is pulled with a force of 10 N on a frictionless surface. What is its acceleration?
Answer: Using F = m·a → a = F/m = 10 N / 2 kg = 5 m/s².
2. How much work is done when the block in Q1 moves 3 m?
Answer: Work W = F·d = 10 N × 3 m = 30 J.
3. A 60 W bulb is switched on for 2 hours. Calculate the energy consumed in kilowatt‑hours.
Answer: Energy E = Power × time = 60 W × 2 h = 120 Wh = 0.12 kWh.
4. A 0.5 kg ball strikes a wall and rebounds with a speed of 4 m/s after being initially moving at 2 m/s toward the wall. Find the impulse imparted by the wall.
Answer: Initial momentum p₁ = 0.5 kg × 2 m/s = 1 kg·m/s (toward wall). Final momentum p₂ = 0.5 kg × (‑4 m/s) = –2 kg·m/s (away). Impulse J = Δp = p₂ – p₁ = –2 – 1 = –3 kg·m/s. Magnitude = 3 N·s.
5. A circuit has a voltage of 12 V and a resistance of 4 Ω. What is the current?
Answer: From Ohm’s law, I = V/R = 12 V / 4 Ω = 3 A.