These comprehensive RBSE Class 9 Science Notes Chapter 10 Gravitation will give a brief overview of all the concepts.

## RBSE Class 9 Science Chapter 10 Notes Gravitation

→ Gravitation : The force of attraction between two objects in the universe is known as gravitational force.

→ Centripetal Force : A force which acts upon the object that keeps it moving in a circular path, is known as centripetal force. It is exerted from the centre of path. Due to this force, velocity of the objects also changes. For example – Rotation of moon around the Earth.

→ Universal Law of Gravitation : According to universal law of attraction, ‘Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them’. This force acts along the line joining the centres of.. two masses. This las was proposed by sir Isaac Newton in 1687. It is also known as Newton’s law of gravitation.

∴ F = \(\frac{G m_{1} m_{2}}{r^{2}}\)

Hence, G is a universal gravitational constant. It is also called inverse square law.

→ G is called a universal constant because its value does not depend on the nature of the intervening medium or any other physical variable. S.I. unit of G is Nm-/kg.

→ Importance of Universal Law of Gravitation :

- The motion of Earth around the Sun.
- The force that binds us to Earth.
- The motion of Moon around the Earth.
- The tides due to the Moon and the Sun.

→ Free fall or Gravity : When an object is thrown upward, then it sarts falling down towards Earth, after reaching at a certain height. It is because the Earth’s gravitational force exerts on it. This is said to do free fall. This acceleration is called acceleration due to gravity, denoted by ‘g’. Its unit is m/s.

→ Value of ‘g’ on the surface of Earth :

g = \(\frac{G M_{\mathrm{e}}}{R^{2}}\)

If G = 6.67 × 10^{-11} Nm^{2}/kg^{2}, Me = 6 × 10^{24} kg, R^{2} = (6.37 10^{6})^{2}

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→ Variation of acceleration due to gravity “g”:

Its value changes from place to place.

- The value of g is maximum at the poles and minimum at equator.
- The value of g decreases with increase in altitude above the surface of the Earth.
- The value of g decreases with depth below the surface of Earth.

→ Mass and Weight:

S. No. | Mass (m) | Weight (W = mg) |

1. | Mass of an object is a measure of its inertia. | Weight of an object is the force which it is attracted towards the earth. |

2. | The SI unit of mass is kilogram (kg). | The SI unit of weight is Newton (N) |

3. | Mass is a scalar quantity having only a magnitude and no sense of direction. | Weight is a vector quantity having a magnitude as well as a direction of its own. |

4. | Mass of an object is constant and it does not change at all, mass can never be zero. | Weight of an object change from place to place. Weight of an object may be zero too. |

5. | It is measured by a beam balance. | It is measured by using a spring balance. |

→ Weight of an object on the Moon :

The weight of an object on the Moon is the force of attraction of the Moon on the object. The value of g on the surface of the Moon is about th of the value of g on the Earth.

W_{m} = 0.165 W_{e} = \(\frac{W_{e}}{6}\)

→ Thrust and Pressure :

The force acting in perpendicular direction on an object is called thrust. It is similar to force applied to an object, so unit of thrust is same as that of force. It is a vector quantity. Its SI unit is Newton (N). The thrust on unit surface area is called pressure, i.e. force acting per unit area of a surface in a direction perpendicular is called pressure. It is a scalar quantity.

SI unit : N/m^{2} or Pa (Pascal)

→ Pressure on solids and in fluids :

- Solids : They exert pressure on the surface because of their weight.
- Fluids : They exert pressure on the surface and the walls of the container, i.e.. fluids exert pressure in all directions.

→ Buoyancy : The property of a liquid due to which it exerts an upward force on any object immersed in the fluid, is called buoyancy, The upward force is called buoyant force or upthrust. Buoyant force is directly proportional to the volume of immersed part of the object, and to the density of the fluid. It is independent of the nature of the immersed object.

→ If the density of an object is less than the density of a liquid, it will float on liquid and if the density of an object is more than the density of a liquid, it will sink in the liquid.

→ Archimedes’ Principle : It states that when a body is immersed fully or partially in a fluid, it experiences a upward force, that is equal to the weight of the fluid displaced by it.

→ Applications of Archimedes’ Principle :

- Used in designing ships and submarines.
- Used in determining relative density of substances
- Hydrometres and lactometers are made on this principle.

→ Density : The mass per unit volume is called density of an object. If m is the mass and V is the volume, then

Density (d) = \(\frac{\operatorname{Mass}(m)}{\text { Volume }(v)}\)

SI unit is kg/m^{3}.

→ Relative density : It is the ratio of the density of a substance to that of the density of water.

It has no unit.

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