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Gravitation CBSE Class 9



Gravitation is a kind of force is what keeps your body on the ground.

Any object with mass exerts a gravitational force or pull on all other objects having mass. For example, there is a gravitational force between you and every object around you.

The gravitational force between any two objects is larger when the masses of the objects are bigger. That’s why you can feel the gravitational force between earth and you beause mass of earth is huge.

The gravitational force between any two objects depends on the distance between their centers. The further objects exert weaker force on it.

Universal Law of gravitation

Univarsal law of Gravitation states that:

Every obect in the universe attract each other with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Let two object A and B of masses M and m are placed at a distance d from each other. Let the force of attraction between two object be F.

According to univarsal law of gravitation, The force between two object is proportional to the product of their masses.

F ∝ M x m ----(eqn 1)

And, The force between two object is inversely proportional to the square of the distance between them.

F ∝ 1/d² ----(eqn 2)

Combining (eqn 1) and (eqn 2), we get

F ∝ M x m/d²

To remove the proportionality in the bove equation we need a constant G which is called Universal gravitational constant.

F = G(M x m)/d²

Value of G found experimentally as 6.7 * 10^-11 N.m² / kg².

Importance of universal Law of gravitation

The universal la of gravitation successfully explained several phenomena which were belived to be unconnected:

  1. The force that binds us to the Earth.
  2. The motion of the Moon around the Earth.
  3. The motion of planets around the Sun.
  4. The tides due to the Moon and the Sun.

Free Fall

We known that earth attracts every object towards the center of the earth by a force called gravitational force. If any object is falling only due to graviational force, is termed as free fall.

Derrivation of g

We known that

=> f = mg

So, we can say that

=> mg = G(M x m)/d²

Therefore, g = G(M)/d²

Calculating the value of 'g'

To calculate the value of g, we should put the value of G (Universal constant, G = 6.7 x 10^-11 N m²/kg²), M (Mass of Earth, M = 6 x 10^24 kg) and R (Radius of Earth, R = 6.4 x 10^6 m).

Therefore, We known that

g = G(M)/d²

=> g = 6.7 x 10^-11 x 6 x 10^24 / (6.4 x 10^6)²

=> g = 9.8 m/s²

Motion of object under influence of gravitational force of the Earth

The motion of any object is under the influences of gravitational force of the earth is called free fall.

Free fall means a body falling freely because of the gravitational pull of our earth. And the motion of objwct is called free fall motion.

Weight & Mass

Mass of any object is the measure of its inertia and is constant throughout the universe. Weight of any object varies from place to place as the value of g changes. Weight is nothing but a force of attraction of the Earth on an object. Weight of an object on the Moon is 1/6 times the weight on Earth.

Weight of the object on moon

The weight of any object on the moon is about one-sixth of its weight on the earth. This is because the value of acceleration due to gravity (g=1.633)on the moon is about one-sixth of that on the earth.

Thrust and Pressure

Force acting on any object is perpendicular to the surface, it is known as thrust. Magnitude of thrust depends on the area of contact of the object. The pressure is thrust per unit area. SI unit is the pascal (Pa). Force acting on a smaller area creats more pressure than the same force acting on a larger area.

Pressure in Fluids

The pressure exerted by any fluid in any container is transmitted undiminished in all directions on the walls of the container.

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When a substance is immersed in water or any fluid ,it experiences an upward force which tends to bring the object or a body towards the surface of the water or fulid .This force is known as Buoyancy.

Why objects float or sink when placed on the surface of water?

An object floats on the surface of water beacuse it displaces more weight of water than its own weight and resultant bouyant force acting upwards is more than the weight of object .

On the other hand, an object would sink in water when if wight of displaced water is less weight is less than weight of of object , and as the buoyant force is less than weight of object.

Archimedes’ Principle

Archimedes’ Principle - The upward force exerted by any fluid on any object, that is known as upthrust or buoyant force. The amount of buoyancy depends on the density of the fluid. The density of the object is lesser than the fluid, it will float up. If the density of the object is greater than the fluid, it will sink down. According to the Archimedes’ principle, when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Relative Density

Relative density = Density of a substance/ Density of water

  1. Relative densityis also called specific gravity.
  2. It is the ratio of the density of a substance to the density of a given reference material.
  3. It is a unitless quantity.
  4. General refernce material is water and density is 1000 kg/m^3.


Q1:What is cause of motion?

Answer: Force

Q2:Which physical quantity is required change the speed or the direction of motion of an object.?

Answer: Force

Q3: Due to which reason an object dropped from a height falls towards the earth.?

Answer: Graviataional force

Q4: Due to which reason planets go around the Sun?

Answer:The sun's gravity pulls the planet toward the sun.

Q5:Why moon goes around the earth.?

Answer: Gravitational force

Q6:What is the direction of force that keeps a body moving along the circular path?

Answer: Towards the centre

Q7:A stone is tied up with string & rotating circular path in horizontal plane . What happens when string snapped ?

Answer: straight line along tangent to the circular path

Q8:What the name of force suggested by Newton for attraction between objects in universe ?

Answer: gravitational force.

Q9: What is name of scientist who determined the value of G (GRAVIATAIONAL CONSTANT)?

Answer: Henry Cavendish

Q10: What is the term used for an objects falling towards the earth due graviational force only ?

Answer: Free

Q11: What is the nature of acceleration to gravity of an object under free fall?

Answer: Constant and independent from mass of object

Q12: Two objects A & B are under free fall and mass of A is greater than mass of B ? What is ratio of their acceleration?

Answer: one

Q13: Theearth attracts every object with a certain force and this force depends on the mass (m) of the object and the acceleration due to the gravity (g). Is it True ?


Q14: The mass of an object remains the same everywhere. Is it true or false ?

Answer: true

Q15: Weight and force have not unit of measurement. Is is true or Flase ?

Answer: False

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