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CBSE Class 9 Science, Gravitation: Chapter notes (Part-I)

Nov 29, 2017 09:54 IST
    Class 9 Science Chapter Notes
    Class 9 Science Chapter Notes

    et the CBSE Class 9 Science notes on chapter 10 ‘Gravitation’ (Part-I) from this article. These chapter notes are prepared by the subject experts to give you a crisp and brief explanation of all the important topics related to the gravitational force of the Earth. At the end of the notes you can try the questions asked from the discussed set of topics. These questions will help you to assess your preparation level and get a hold on the subject.

    CBSE Class 9 Science Syllabus 2017-2018

    Main topics covered in this part of CBSE Class 9 Science,  Gravitation: Chapter Notes, are:

    • Gravitation
    • Universal Law of Gravitation
    • Free Fall
    • Acceleration Due to Gravity (g)
    • Difference between G and  g
    • Mass and Weight
    • Weight of an Object on the Surface of Moon

    Key notes for Chapter - The Fundamental Unit of Life, are:


    Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitational pull.

    Universal Law of Gravitation

    Every object in the universe attracts other object by a force of attraction, called gravitation, which is directly proportional to the product of masses of the objects and inversely proportional to the square of distance between them. This is called Law of Gravitation or Universal Law of Gravitation.

    Let masses (M) and (m) of two objects are distance (d) apart. Let F  be the attractional force between two masses.

     Universal Law of Gravitation

    Importance of The Universal Law of Gravitation

    • It binds us to the earth.
    • It is responsible for the motion of the moon around the earth.
    • It is responsible for the motion of planets around the Sun.
    • Gravitational force of moon causes tides in seas on earth.

    Free Fall

    When an object falls from any height under the influence of gravitational force only, it is known as free fall.

    Acceleration Due to Gravity

    When an object falls towards the earth there is a change in its acceleration due to the gravitational force of the earth. So this acceleration is called acceleration due to gravity.

    The acceleration due to gravity is denoted by g.

    The unit of g  is same as the unit of acceleration, i.e., ms−2

    Mathematical Expression for g

    From the second law of motion, force is the product of mass and acceleration.    

                F = ma

    For free fall, acceleration is replaced by acceleration due to gravity. 

    Therefore, force becomes:                      

                F = mg            ….(i)

    But from Universal Law of Gravitation,

    Mathematical Expression for g, Acceleration Due to Gravity

    Factors Affecting the Value of g

    • As the radius of the earth increases from the poles to the equator, the value of g becomes greater at the poles than at the equator.
    • As we go at large heights, value of g decreases.

    To Calculate the Value of g

    Value of universal gravitational constant, G = 6.7 × 10–11 N m2/ kg2,

    Mass of the earth, M = 6 × 1024 kg, and

    Radius of the earth, R = 6.4 × 106 m

    Putting all these values in equation (iii), we get:

     value of acceleration due to gravity

    Thus, the value of acceleration due to gravity of the earth, g = 9.8 m/s2.

    Difference between Gravitation Constant (G) and  Gravitational Acceleration (g)

    S. No.

    Gravitation Constant (G)

    Gravitational acceleration (g)


    Its value is 6.67×10-11Nm2/kg2.

    Its value is 9.8 m/s2.


    It is a scalar quantity.

    It is a vactor quantity.


    Its value remains constant always and everywhere.

    Its value varies at various places.


    Its unit is Nm2/kg2

    Its unit is m/s2.

    Motion of Objects Under the Influence of Gravitational Force of the Earth

    Let an object is falling towards earth with initial velocity u. Let its velocity, under the effect of gravitational acceleration g, changes to v after covering the height h in time t.

    Then the three equations of motion can be represented as:
    Velocity (v) after t seconds, v = u + ght
    Height covered in t seconds, h = ut + ½gt2
    Relation between v and u excluding t, v2 = u2 + 2gh
    The value of g is taken as positive in the case of object moving towards earth and taken as negative in the case of obyect is thrown in opposite direction of earth.

    Mass & weight
    Mass (m)

    • The mass of a body is the quantity of matter contained in it. 
    • Mass is a scalar quantity which has only magnitude but no direction.
    • Mass of a body always remains constant and does not change from place to place.
    • SI unit of mass is kilogram (kg).
    • Mass of a body can never be zero.

    Weight (W)

    • The force with which an object is attracted towards the centre of the earth, is called the weight of the object.
      Now, Force = m × a
      But in case of earth,  a = g
      ∴ F = m × g
      But the force of attraction of earth on an object is called its weight (W).
      ∴ W = mg
    • As weight always acts vertically downwards, therefore, weight has both magnitude and direction and thus it is a vector quantity.
    • The weight of a body changes from place to place, depending on mass of object.
    • The SI unit of weight is Newton.
    • Weight of the object becomes zero if g is zero.

    Weight of an Object on the Surface of Moon
    Mass of an object is same on earth as well as onmoon. But weight is different.

    Weight of an object is given as,

    weight of a body on moon

    Hence, weight of the object on the moon = (1/6) × its weight on the earth.

    Try the following questions:

    Q1. State the universal law of gravitation.

    Q2. When we move from the poles to the equator, the value of g decreases. Why?

    Q3. If two stones of 150 gm and 500 gm are dropped from a height, which stone will reach the surface of earth first and why ? 

    Q4. Differentiate between weight and mass.

    Q5. Why is the weight of an object on the moon 1/6th its weight on the earth??

    CBSE Class 9 Science Solved Practice Paper 2017-2018: Set-I

    CBSE Class 9th Science (Physics) Practice Paper


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