Gravitation Class 9 Notes Science - Chapter 10

Chapter: 10

What Are Gravitation ?

Gravitation

Force between two objects or bodies which attract each other due to their masses

Effects of Gravity

Free fall

  • When an object falls down towards the earth under the gravitational force only.
  • Velocity of free falling body changes and is said to be acceleration due to gravity.
  • Acceleration due to gravity, โ€˜gโ€™. Unit is m/s2. Value of g = 9.8 m/s2.
Equation of Motion Acceleration due to gravity
  • v = u + gt
  • It is the gravitational attraction of earth
  • h = ut + gt2
  • g = 9.8 m/s2
  • v2 โ€“ u2 = 2gh
  • When a body is dropped freely from a height u = 0, g = 9.8 m/s2

Universal Law of Gravitation

  • It states that every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance only.
  • Fโˆ(Mm/d2)

F =(GMm/d2) [ โˆต G = Universal gravitational constant]

  • SI unit of G = Nm2/Kg2
  • Value of G = 6.673 ร— 10โ€“11 Nm2/Kg2

Relation Between g And G

  • g =(GM/R2)

(where G = gravitational constant, discovered by Cavendish

M = Mass of the earth

R = Radius of the earth)

g =(6.7ร—10-11Nm2Kgโ€“2 ร—6 ร— 1024Kg/(6.4 ร— 10โ€“6 m)2)โ€‹

  • As the radium of the earth increases from the poles to the equator, the value of g becomes greater at the poles than at the equator.

Mass And Weight

Mass

  • Mass of an object is the measure of its inertia. (Scaler quantity)
  • It remains the same everywhere in the Universe.
  • SI Unit : Kgย ย 

Weight

  • Weight is the force of attraction of the earth on the object (vector quantity)
  • Weight of the body changes form place to place
  • W = mg
  • SI Unit: Newton (N)

Thrust

  • Net force applied in a particular direction.
  • Depends on the area on which it acts.
  • SI Unit: Newton

Archimedesโ€™ Principle

When a body is immersed fully or partially in a field, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Applications:

  • Used in designing ships and submarines
  • Used in lactomoters to find purity of milk.
  • Used in hydrometer to determine density of liquid.
Upthrust Pressure
  • When an object is immersed in a fluid, they experience a buoyant force.
  • Force per unit area
  • Depends on the density of the fluid.
  • P =(F/A)
  • Upward force entered by liquid on a body.
  • Pressure =(Thrust/Area)
  • Density of object < density of liquid ⇒ object floats.
  • Density of object > density of liquid ⇒ object sinks.