Gravitation
“Gravitation is the force by which every object in the universe attracts every other object.”
Newton’s Law of Universal Gravitation
Every object in the universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Where,
- F = gravitational force
- G = gravitational constant = 6.674 × 10⁻¹¹ Nm²/kg²
- m₁ and m₂ = masses of the two objects
- r = distance between the centers of the two objects
Acceleration Due to Gravity (g)
It is the acceleration produced in a body due to the gravitational force of Earth.
Where M is the mass of Earth and R is the radius of Earth. Standard value of g = 9.8 m/s².
Free Fall
When an object falls under the influence of gravity alone, it is said to be in free fall. In this case, acceleration is equal to g.
Mass and Weight
- Mass: Quantity of matter in a body. It is constant everywhere.
- Weight: The force with which Earth attracts a body. W = m × g
Mass is in kilograms (kg), weight is in Newtons (N).
Thrust and Pressure
- Thrust: Force acting on a surface in a perpendicular direction.
- Pressure = Thrust / Area
SI unit of pressure is Pascal (Pa).
Buoyancy and Archimedes’ Principle
Buoyancy is the upward force exerted by a fluid on a submerged object. Archimedes’ Principle states:
“When a body is immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced.”
Relative Density
Relative density is the ratio of the density of a substance to the density of water.
- Relative Density = Density of substance / Density of water
It has no units.
Previous Year Questions
- SSC CGL 2020: What is the SI unit of gravitational constant (G)?
- Nm²/kg²
- HSSC JE 2019: A body has a mass of 10 kg. What is its weight on Earth?
- W = m × g = 10 × 9.8 = 98 N
- RRB NTPC 2021: Who discovered the law of universal gravitation?
- Sir Isaac Newton
Important Points
- Gravitational force is the weakest among the four fundamental forces.
- Weight changes with location, mass does not.
- Gravitational force acts along the line joining the centers of masses.
Note: At the poles, gravity is slightly more (≈9.83 m/s²) and at the equator it is slightly less (≈9.78 m/s²).
Conclusion
Gravitation governs motion of celestial bodies, keeps us grounded, and influences many everyday phenomena. Understanding this force helps in space science and mechanical applications.