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, ‘g’. Unit is m/s2. Value of g = 9.8 m/s2.

Equation of Motion

• v = u + gt

• h = ut + gt2
• v2 – u2 = 2gh
• When a body is dropped freely from a height u = 0, g = 9.8 m/s2
  • When a body is projected vertically upward with an initial velocity, u, than g = –9.8 m/s2

Acceleration due to gravity

• It is the gravitational attraction of earth

• 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.
  • F∝(Mm/d2)
F =(GMm/d2) [ ∵ G = Universal gravitational constant]
• SI unit of G = Nm2/Kg2
• Value of G = 6.673 × 10–11 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 =(6.7×10-11Nm2Kg–2 ×6 × 1024Kg/(6.4 × 10–6 m)2)

• 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 : Kg

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

Archimedes’ 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

• When an object is immersed in a fluid, they experience a buoyant force.

• Depends on the density of the fluid.

• Upward force entered by liquid on a body.

• Density of object < density of liquid ⇒ object floats.

• Density of object > density of liquid ⇒ object sinks.

Pressure

• Force per unit area

• P =(F/A)

• Pressure =(Thrust/Area)

• SI Unit: N/m2 or Pascal