GRAVITATION

Class 9

                        Physics

Chapter 3

GRAVITATION

The Force with which the earth pulls the objects toward it is called the gravitational force of earth.

Universal law of Gravitational

1.    The force between two bodies directly
      proportional to the product of their 
       masses.{F proportional to m₁m₂}

2.    The force between two bodies is inversely 
       proportional to  the square of the distance 
       between them. .{F proportional to 1/r²}

3.      Gravitational force , F= G*m₁m₂/r²

Gravitational Constant , G

1.      G=F*r²/ m₁m₂

2.      Unit = Nm²/kg²  (Nm²kg^-2)

3.  Value of gravitational constant G = 6.67*10^-11 Nm²/Kg²

Newton’s  Third Law of Motion and Gravitation

The Newton’s third law of motion also holds good for the force of gravitation. This means that when earth exerts a force of attraction on an object , then the object also exerts an equal force on the earth, in the opposite direction.

Free Fall

The Falling of a body (or object) form a height towards the earth under the gravitational force of earth (with no other forces acting on it) is called free fall.

NOTE::: The acceleration of the object falling freely towards the earth does not depend of The Mass of the object.

Acceleration due to Gravity

The uniform acceleration produced in a freely falling body due to the gravitational force of the earth is knows as Acceleration due to Gravity.

The acceleration due to gravity ,g =9.8 m/s² .

 g= G*M/R²   (R= radius of earth G= constant M= mass of the earth)

EQUATI0N OF MOTION FOR FREELY FALLING BODIES

1)    v= u +gt

2)    h=ut+1/2gt²       (h= height )

3)    v²=u²+2gh

Note :

    a)    when a body is falling vertically downward, its velocity is increasing , so the acceleration due to gravity, g, is taken as positive (+).

     b)    When a body is thrown vertically upward, its velocity is decreasing, so the acceleration due to gravity, g, is taken as negative (-).

     c)     When a body dropped freely from a height, its initial velocity (u) is zero.

     d)    When a body is thrown vertically upward, its final velocity (v)  is zero.

MASS

     a)      The mass an object is the quantity of matter contained in it.

     b)      The SI unit of mass is (kg).

     c)       The mass of an object is constant.

     d)      The mass an object can never be zero.

WEIGHT   {w=mg}

1)    The weight of an object is the force with which it is attracted towards center of the earth.

2)    The SI unit of the weight is Newton.

3)    The weight of the object is not constant. It change with the change in acceleration due to gravity

4)    The weight of an object can be zero.

Ex. Interplanetary space, where g=0 , the weight of an object  become zero.

{Weight of an object on the moon is 1/6 of its weight o the earth.}

PRESSURE

Pressure is the force acting perpendicularly on the unit area of the object.

Pressure = force/ Area.

Its SI unit is   PASCAL (Pa) OR (N/M²)  .

PRESSURE IN FLUIDS

Those Substance which can flow easily are called fluids.

The fluid exerts pressure in all direction – even upward.

BUOYANCY (UP THRUST)

The tendency of an liquid to exert an upward force on an object placed in it , Is called Buoyancy.

Buoyant  force

The upward force acting on an object immersed in liquid is called buoyant force.

FACTORS   EFFECTING  BUOYANT FORCE

1)    Volume of object immersed in the liquid, and

2)    Density of the liquid.

·       As the volume of object immersed inside the liquid  increases, the upward “ buoyant force ”also increase.

·       The density of a liquid increases, the buoyant force exerted by it also increases.

ARCHIMEDES PRINCIPLE

 When an object is partially immersed in a liquid, it experiences    a buoyant force which is equal to the weight of liquid displaced by object.

·       Buoyant force acting on an obj. = weight of liquid displaced by object.

Application of Archimedes Principle

1)    Archimedes’ principle is used in determining the density the relative density of a Substance  .

2)    The hydrometers used for determining the density of liquids are based on Archimedes’ principle

3)    The lactometers used for determining the purity of milk are based on Archimedes’ principle.

4)    Archimedes’ principle is used in designing ships and submarines.

Why Objects Float or Sink in a Liquid

     ·       An object will float in a liquid  if its density (or average density) is less then  that of the liquid.

     ·        An object will also float in a liquid if its density is equal to that of the liquid.

     ·       An object will sink in a liquid if its density is more then that of the liquid.

 DENSITY

The density of a substance is defined as mass of the substance per unit volume.

·       Density= mass of the substance / volume of the substance

·       Density= M/VOLUME

 RELATIVE DENSITY

The relative of a substance is the ratio of its density to that of water.

·       Relative density of the substance = density of the substance /density of water.