Gravitation and Weight
NCERT Class 9 • Science • Chapter "Gravitation"
Gravitation and Weight
Every object in the universe attracts every other object with a force. This idea, the universal law of gravitation, explains falling apples, the orbit of the moon, the motion of planets and the rise and fall of tides with a single rule.
The Law, Free Fall and Weight
- The universal law of gravitation states that the force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
- Because of the inverse square relation, the force falls off rapidly as distance increases.
- Free fall is motion under gravity alone, and the acceleration in free fall is denoted by g.
- The value of g is about 9.8 metre per second squared near the earth's surface.
- g is slightly greater at the poles than at the equator because the earth is not a perfect sphere.
- Mass is the amount of matter in a body; it is constant everywhere and is measured in kilogram.
- Weight is the force with which the earth attracts a body, equals mass times g, and is measured in newton.
Weight on the Moon, Thrust and Pressure
- The moon's gravity is about one sixth of the earth's, so an object weighs about one sixth there while its mass stays the same.
- Thrust is the force acting perpendicular to a surface, and pressure is thrust per unit area.
- Pressure is measured in pascal, which is one newton per square metre.
- The same force over a smaller area gives a larger pressure, which is why a sharp knife cuts better and a camel's broad feet do not sink in sand.
- Buoyancy is the upward force a fluid exerts on an object immersed in it.
- Archimedes' principle states that the upward buoyant force equals the weight of the fluid displaced.
- An object floats if its density is less than that of the fluid and sinks if it is greater.
| Quantity | Meaning | Unit |
|---|---|---|
| Mass | Amount of matter, constant | kilogram |
| Weight | Force of gravity on a body | newton |
| Pressure | Thrust per unit area | pascal |
| Buoyant force | Upward push of fluid | newton |
Archimedes' principle — a body immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces.
Hindi me samjhein
द्रव्यमान हर जगह समान रहता है, पर भार बदलता है क्योंकि भार गुरुत्वीय बल है। चंद्रमा पर भार लगभग छठा भाग रह जाता है जबकि द्रव्यमान वही रहता है।
Exam me kaise aata hai
- The value of g near the earth's surface is about — 9.8 m/s²
- Weight on the moon is about what fraction of earth — one sixth
- The SI unit of pressure is the — pascal
- Upward force of a fluid on an immersed body is — buoyant force
UPSC/State PSC ke liye
- The value of g is greater at the poles because the earth is flattened there, so the surface is nearer the centre and the radius is smaller.
- All bodies fall with the same acceleration in the absence of air because g does not depend on the mass of the falling body.
Yahan confuse hote hain
✗ Mass and weight are the same | Mass is constant; weight changes with gravity | ✓
✗ A heavier body falls faster in vacuum | In vacuum all bodies fall at the same rate | ✓
✗ g is the same everywhere on earth | g is slightly greater at the poles than the equator | ✓
Ek nazar me
- Gravitational force varies with the product of masses and inversely with the square of distance.
- g is about 9.8 m/s² and is slightly greater at the poles.
- Mass is constant in kilogram; weight is a force in newton.
- Pressure is thrust per area; buoyancy equals the weight of fluid displaced.