Gravitation Class 9 Science — Revision Notes
This chapter covers gravitational force and Newtons law of universal gravitation. Students learn about weight, mass, and orbital motion.
TL;DR: This chapter covers gravitational force and Newtons law of universal gravitation. Students learn about weight, mass, and orbital motion.
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This chapter covers gravitational force and Newtons law of universal gravitation. Students learn about weight, mass, and orbital motion.
Newtons Law of Universal Gravitation
- Every object attracts every other object with force proportional to product of masses and inversely proportional to square of distance
- F = G m1 m2 / r^2 where G is gravitational constant (6.67 x 10^-11 N m^2 / kg^2)
- Force is mutual and equal on both objects
- Gravitational force is weakest of all forces
Weight and Mass
- Mass: amount of matter in object (constant)
- Weight: force of gravity on object (W = mg)
- g = 9.8 m/s^2 on Earth surface
- Weight varies with location, mass does not
Free Fall and g
- Free fall: motion under gravity alone
- Acceleration due to gravity is same for all objects (ignoring air resistance)
- g = 9.8 m/s^2 at Earths surface
- g decreases with altitude and increases with depth
Orbital Motion
- Gravitational force provides centripetal force for orbital motion
- Satellites orbit because gravitational pull balances outward motion
- Orbital speed: v = sqrt(GM/r) where M is central mass, r is orbital radius
- Geostationary satellites have orbital period of 24 hours
Key Terms
- Gravity: Force of attraction between all objects due to their masses
- Weight: Force exerted on object by gravity (W = mg)
- Gravitational Constant: G = 6.67 x 10^-11 N m^2 / kg^2
Frequently Asked Questions
Why do all objects fall with same acceleration?
Gravitational force F = mg is proportional to mass. From F = ma, acceleration a = F/m = mg/m = g, which is independent of mass.
What is difference between mass and weight?
Mass is amount of matter (constant everywhere). Weight is gravitational force on mass (W = mg, varies with location). On Moon, weight is 1/6 of Earth value but mass remains same.
More Class 9 Science Revision Notes
- Matter in Our Surroundings
- Is Matter Around Us Pure
- Atoms and Molecules
- Structure of the Atom
- The Fundamental Unit of Life
- Tissues
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