Gravitation, Work, Energy, Pressure and Machines
Gravitation, Work, Energy, Pressure and Machines
This topic covers the mechanics questions NTPC sets beyond motion: weight on the Moon and escape velocity, work-energy-power numericals, pressure and floating, and lever classes. Most are one-step numericals or "which principle explains this" questions.
1. Gravitation
| Fact | Value or statement |
|---|---|
| Universal law | F = G m₁m₂/r²; G = 6.67 × 10⁻¹¹ N m²/kg² |
| g on Earth | About 9.8 m/s²; greatest at the poles, least at the equator |
| g on the Moon | About one-sixth of Earth's, so weight on the Moon is one-sixth |
| Mass vs weight | Mass (kg) never changes; weight (N) = mg changes with g |
| Escape velocity from Earth | About 11.2 km/s |
| Kepler's laws | Planets move in elliptical orbits with the Sun at one focus |
| Weightlessness | Felt in a freely falling lift and in an orbiting spacecraft |
2. Work, Energy and Power
Work W = F × s × cos θ (no work if force is perpendicular to motion)
Kinetic energy = ½mv²; potential energy = mgh
Power = work ÷ time
| Situation | Key point |
|---|---|
| Doubling speed | Kinetic energy becomes four times |
| Falling body | Potential energy turns into kinetic energy; total mechanical energy stays constant (ignoring air) |
| Coolie carrying a load on a level road | Work done against gravity is zero |
| Energy conversion in a dynamo | Mechanical to electrical |
| Energy conversion in a cell | Chemical to electrical |
3. Pressure, Fluids and Machines
| Principle | Statement or use |
|---|---|
| Pressure | Force ÷ area; sharp knives and nails work because the area is small |
| Liquid pressure | p = hρg; increases with depth, so dams are thicker at the base |
| Pascal's law | Pressure applied to an enclosed liquid spreads equally; hydraulic lift and brakes |
| Archimedes' principle | Upthrust equals the weight of liquid displaced |
| Floating | A body floats if its density is less than the liquid's; ships float by displacing much water |
| Atmospheric pressure | About 101.3 kPa at sea level; measured by a barometer |
| Bernoulli's principle | Faster-moving fluid has lower pressure; aeroplane lift |
| Class I lever | Fulcrum in the middle: see-saw, scissors, pliers |
| Class II lever | Load in the middle: nutcracker, wheelbarrow, bottle opener |
| Class III lever | Effort in the middle: tongs, tweezers, fishing rod, human forearm |
✗ Mass of a body on the Moon is one-sixth of its mass on Earth | ✓ Mass stays the same; only the weight becomes one-sixth
✗ A nutcracker is a Class I lever | ✓ In a nutcracker the load is between fulcrum and effort, so it is Class II
हिंदी नोट: चंद्रमा पर भार पृथ्वी का लगभग छठा भाग होता है, पर द्रव्यमान वही रहता है। पृथ्वी से पलायन वेग लगभग 11.2 किमी/सेकंड है। आर्किमिडीज़ के सिद्धांत के अनुसार उत्प्लावन बल हटाए गए द्रव के भार के बराबर होता है। सरौता द्वितीय श्रेणी का और चिमटा तृतीय श्रेणी का उत्तोलक है।
Exam Pointer: No NTPC shift was verified for these patterns in this run; they are tagged syllabus-based. Weight-on-Moon and buoyancy questions found online were from other RRB exams, so the same type is expected in NTPC.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Weight, g and escape velocity
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. A person weighs 600 N on Earth. What will he weigh on the Moon, and what will his mass be there (take g = 10 m/s²)?
His mass is 600/10 = 60 kg everywhere. Weight on the Moon is one-sixth: 600/6 = 100 N.
Answer: 100 N; 60 kg
EXAMATLAS LEVEL
Q. Where on Earth is g greatest, and what is the escape velocity from Earth's surface? Why do astronauts in an orbiting station feel weightless?
g is greatest at the poles, where the Earth's radius is least. Escape velocity is about 11.2 km/s. The station and astronauts fall freely around the Earth together, so there is no normal force on them.
Answer: Poles; 11.2 km/s; free fall
Pattern 2: Work, energy and power numericals
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. A 2 kg ball moves at 3 m/s. Find its kinetic energy.
KE = ½ × 2 × 9 = 9 J.
Answer: 9 J
EXAMATLAS LEVEL
Q. A 50 kg boy climbs 20 stairs, each 0.2 m high, in 10 seconds. Find the work done and his power (g = 10 m/s²). By what factor does kinetic energy change if speed is doubled?
Height = 20 × 0.2 = 4 m, work = mgh = 50 × 10 × 4 = 2,000 J and power = 2,000/10 = 200 W. Kinetic energy depends on v², so it becomes four times.
Answer: 2,000 J, 200 W; four times
Pattern 3: Pressure, Pascal and Archimedes
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. On which principle does a hydraulic lift work?
Pascal's law: pressure on an enclosed liquid is passed equally in all directions.
Answer: Pascal's law
EXAMATLAS LEVEL
Q. Why does an iron nail sink while an iron ship floats? Why is a dam wall thicker at the bottom?
The ship's hollow shape displaces water whose weight equals the ship's weight, so upthrust balances it (Archimedes). A solid nail displaces little water. Liquid pressure rises with depth (p = hρg), so the base of a dam must be stronger.
Answer: Ship displaces more water; pressure increases with depth
Pattern 4: Classes of lever
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. In which class of lever is the effort between the fulcrum and the load?
Class III, as in tongs or the human forearm.
Answer: Class III
EXAMATLAS LEVEL
Q. Classify: scissors, wheelbarrow, fishing rod, see-saw, bottle opener.
Scissors and see-saw have the fulcrum in the middle (Class I). Wheelbarrow and bottle opener have the load in the middle (Class II). A fishing rod has the effort in the middle (Class III).
Answer: I: scissors, see-saw; II: wheelbarrow, bottle opener; III: fishing rod
60-Second Revision
- Weight on the Moon is one-sixth; mass unchanged; escape velocity 11.2 km/s.
- KE = ½mv² (doubling speed gives four times KE); PE = mgh; power = work ÷ time.
- Pascal: hydraulic lift; Archimedes: floating; Bernoulli: aeroplane lift.
- Lever classes: fulcrum, load, effort in the middle gives I, II, III.