GRAVITATION NOTES

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FORCE – Any push or pull on an object is called force.
                                              OR
It is the physical quantity that changes or tends to change the
state of rest or of motion.

  • EFFECTS OF FORCE ON AN OBJECT –1. Moves a body at rest
    2. Cause s moving object to change its speed.
    3. Cause moving object to change its direction
    4. Change shape and size of an object.

    TYPES OF FORCES –

    A)Contact force –

    When a force acts on two objects when they are in direct
    contact with each other, it is said to contact force.
    Ex.

  •  Muscular force (when we use our muscles on any object)
    (Ex. Kicking a ball, pushing an object
  •  Frictional force (force exerted by a surface when an object
    moves across it)
    (Ex. A vehicle stops when we apply brakes)
  •  Elastic force (force exerted by an elastic object)
    (Ex. Force by rubber)
  •  Applied force (when a person or an object exerts a force
    on another object)(Ex. Book kept on table)
  •  Spring force (when a stretched or compressed spring exerts a force)

  B) Non-contact force –
When a force is applied between two objects even if the two
objects are not in contact, such force is non-contact force.
Ex.

  •  Gravitational force (force of attraction exerted by any
    object on the other object)
    (Ex. Apple falling from a tree)
  • Electrical force (Force due to electric field)
    (Ex. Device running on electricity, when an electric
    current is passed through iron converting it into an
    electromagnet, it attracts iron and particles towards it.)
  •  Magnetic force (Force due to magnets)
    (Ex. Iron gets attracted towards magnet)
  •  Static force / Electrostatic force (the force between two
    objects when their surfaces are rubbed on each other)
    (Ex. The charging of the hair and attraction of paper bits
    towards it

    GRAVITATIONAL FORCE –

  •  It is the force of attraction between any two objects in the
    universe.
  •  Discovered by Sir Isaac Newton (Explained in book
    PRINCIPIA)

CENTRIPETAL FORCE –

  • The force acting on any object in a circle and directed
    towards the centre of the circle is called centripetal force.
  • In short, it is centre seeking force. i.e., due to this force, the
    object tries to go towards the centre of the circle.
    Ex. Motion of moon around the earth.

KEPLER’S LAWS –
      Johannes Kepler studied about planetary positions and              motions and stated three laws.
Ellipse –
      An ellipse is the curve obtained when a cone is cut by an
      inclined plane. It has 2 foci. The sum of distances of the 2          foci  from every point on the curve is constant

         Here, AF1 + AF2 = BF1 + BF2 = CF1 + CF2 = Constant
         (When the distance between the planet and the sun is
         minimum, then the planet moves comparatively faster               and when the distance between the planet and the sun is          maximum, then the planet moves comparatively slower.)
First law – The orbit of a planet is an ellipse with the Sun at one
        of the foci.
Second law – The line joining the planet and the Sun sweeps
        (Covers) equal areas in equal intervals of time.
       AB and CD are distances covered by the planet in equal             time.
        Here, area ASB and CSD are equal.

Third law – The square of period of revolution of the planet
       around the Sun is directly proportional to the cube of the         mean distance of a planet from the Sun.

 

If T is the period of revolution of the planet and r is the average
distance of planet from the sun.
Then, T2 α r3
i.e., T2 / r3 = constant
If the area ESF in figure 1.4 is equal to area ASB, what will you
infer about EF? (Page number 4)
According to Kepler’s second law, if area of ESF = area of ASB,
then, AB and EF are distances covered by the planet in same
time.

  • NEWTON’S UNIVERSAL LAW OF GRAVITATION –
    Every object in the universe attracts every other object with a
    definite force.
    This force is directly proportional to the product of the masses
    of the two objects and inversely proportional to the square of
    the distance between them.When two objects m1 and m2 are kept at a distance ‘d’ from
    each other,
    According to law,
    F α m1xm2 ……………. 1
    F α 1/d² ……………. 2
    Combining 1 & 2
    F α m1xm2
    d²
    ֒F = Gm1 m2
    d2
    Here, G is proportionality constant and is called universal
    gravitation constant.
    Gravitational constant:
    It is the force of attraction between any two-unit masses
    separated by a unit distance.
    SI unit of G – Nm²/kg²
    Its value was calculated by Henry Cavendish as 6.673 ×10
    ˉ¹¹Nm²/kg²
    If the mass of one object is doubled, the force between the 2
    objects also doubles.
    If the distance is doubled, th

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