Units, Motion and Newton's Laws
Units, Motion and Newton's Laws
Physics in NTPC starts with three basics: the SI unit of a quantity, the language of motion (speed, velocity, acceleration and their graphs) and Newton's three laws. Questions are short, but options are built from common confusions, such as mass with weight, speed with velocity, and the first law with the third.
1. SI Units
| Quantity | SI unit | Symbol |
|---|---|---|
| Length, mass, time | metre, kilogram, second | m, kg, s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
| Force | newton | N (kg m/s²) |
| Work and energy | joule | J (N m) |
| Power | watt | W (J/s) |
| Pressure | pascal | Pa (N/m²) |
| Frequency | hertz | Hz |
| Charge | coulomb | C |
| Power of a lens | dioptre | D |
1 light year = distance light travels in a year ≈ 9.46 × 10¹⁵ m (a unit of distance, not time)
1 horsepower ≈ 746 W; 1 kWh = 3.6 × 10⁶ J
2. Motion and Graphs
| Term | Meaning |
|---|---|
| Distance | Total path length; scalar |
| Displacement | Shortest straight-line change of position; vector |
| Speed | Distance ÷ time; scalar |
| Velocity | Displacement ÷ time; vector |
| Acceleration | Change in velocity ÷ time; m/s² |
| Slope of distance-time graph | Speed |
| Slope of velocity-time graph | Acceleration |
| Area under velocity-time graph | Displacement |
v = u + at
s = ut + ½at²
v² = u² + 2as
3. Newton's Laws
| Law | Statement | Example |
|---|---|---|
| First (inertia) | A body stays at rest or in uniform motion unless an unbalanced force acts | Passengers lurch forward when a bus brakes |
| Second | Force = rate of change of momentum, F = ma | A cricketer pulls hands back to reduce the force of a catch |
| Third | Every action has an equal and opposite reaction | Rocket launch, recoil of a gun |
| Momentum | p = mv, unit kg m/s | Conserved when no external force acts |
✗ A body moving with uniform velocity must have a force acting on it | ✓ Uniform velocity means zero acceleration, so the net force is zero (first law)
✗ A light year is a unit of time | ✓ A light year is a unit of distance
हिंदी नोट: बल का SI मात्रक न्यूटन, कार्य का जूल और शक्ति का वाट है। प्रकाश वर्ष दूरी का मात्रक है। न्यूटन का पहला नियम जड़त्व का नियम है; एकसमान वेग से चलती वस्तु पर कुल बल शून्य होता है। वेग-समय ग्राफ का ढलान त्वरण और उसके नीचे का क्षेत्रफल विस्थापन देता है।
Exam Pointer: Verified NTPC pattern: net force on a body moving with uniform velocity (June 2025 Graduate CBT-1). Unit and graph questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Newton's laws and inertia
[PYQ: NTPC Graduate CBT-1 5-Jun-2025 Shift-3]
EXAM LEVEL
Q. A 4 kg trolley moves in a straight line at a constant 3 m/s on a smooth floor. What is the net force on it?
Its velocity does not change, so its acceleration is zero and by the first law the net force is zero.
Answer: 0 N
EXAMATLAS LEVEL
Q. A force of 20 N acts on a 5 kg block at rest for 3 seconds. Find its final velocity and momentum. Which law explains why a gun recoils?
a = F/m = 20/5 = 4 m/s², so v = 0 + 4 × 3 = 12 m/s and momentum = 5 × 12 = 60 kg m/s. Recoil follows from the third law: the bullet is pushed forward and the gun backward.
Answer: 12 m/s, 60 kg m/s; third law
Pattern 2: SI units and derived units
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. What is the SI unit of pressure?
The pascal, equal to one newton per square metre.
Answer: Pascal
EXAMATLAS LEVEL
Q. Which pair is wrongly matched: power and watt, work and joule, luminous intensity and lux, frequency and hertz?
The SI base unit of luminous intensity is the candela; lux measures illuminance. The other pairs are correct.
Answer: Luminous intensity and lux
Pattern 3: Equations of motion
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. A car starts from rest and accelerates at 2 m/s² for 5 seconds. How far does it travel?
s = ut + ½at² = 0 + ½ × 2 × 25 = 25 m.
Answer: 25 m
EXAMATLAS LEVEL
Q. A train moving at 20 m/s is braked uniformly and stops in 100 m. Find the retardation and the time taken.
From v² = u² + 2as, 0 = 400 + 200a, so a = −2 m/s², a retardation of 2 m/s². From v = u + at, 0 = 20 − 2t, so t = 10 s.
Answer: 2 m/s²; 10 s
Pattern 4: Distance, displacement and graphs
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. An athlete runs one full round of a circular track of radius 7 m. Find the distance and the displacement.
Distance = 2πr = 2 × 22/7 × 7 = 44 m. He returns to the start, so displacement = 0.
Answer: 44 m; 0
EXAMATLAS LEVEL
Q. What does the slope of a velocity-time graph give, and what does the area under it give? What does a straight line parallel to the time axis on a distance-time graph mean?
The slope gives acceleration and the area gives displacement. A flat distance-time line means distance is not changing, so the body is at rest.
Answer: Acceleration, displacement; body at rest
60-Second Revision
- N, J, W, Pa, Hz, C, D: force, work, power, pressure, frequency, charge, lens power.
- Light year is distance; 1 kWh = 3.6 × 10⁶ J; 1 hp ≈ 746 W.
- v = u + at; s = ut + ½at²; v² = u² + 2as.
- Uniform velocity means zero net force; F = ma; action-reaction for recoil and rockets.