This PSEB 9th Class Science Notes Chapter 10 Gravitation will help you in revision during exams.

## PSEB 9th Class Science Notes Chapter 10 Gravitation

→ Everybody in this universe attracts every other body towards it with a force called Force of Gravitation.

→ The revolution of the moon around the earth and the falling down of a body projected upward is due to the force of gravitation.

→ Gravitation is a weak force unless it may not involve bodies having greater masses. The attractive force due to earth is called the force of gravity.

→ The law of gravitation is a universal law which means that this law applies to all big and small objects.

→ The magnitude of the force acting between the earth and the objects lying on the surface of the earth is given by formula F = \(\frac{\mathrm{GM} m}{d^{2}}\) where G = 6.673 × 10^{-11} N-m^{2}/kg^{2}

→ The acceleration produced in the body due to the force of gravity is called acceleration due to gravity. It is denoted by ‘g’.

→ The value of ‘g’ is more on the poles than on the equatorial line.

→ The Force of gravity decreases with the increase of height above the surface of the earth.

→ The quantity of matter present in a body is called the mass of the body.

→ Mass is the measure of inertia of the body.

→ The mass of a body remains constant at all places.

→ The weight of a body is the force with which the body is attracted towards the centre of the earth.

→ The weight of an object depends upon the product of its mass (m) and the acceleration due to gravity (g). i.e. Weight (W) = mass (m) × acceleration due to gravity (g)

→ Force acting perpendicular to the surface of an object is called thrust.

→ Thrust acting per unit area is called pressure.

∴ Pressure = \(\frac {Thrust}{Area}\)

→ SI unit of pressure is Nm^{-2} (or N/m^{2}) or pascal (Pa).

→ The pressure exerted on an enclosed liquid is transmitted equally in all directions.

→ All bodies when immersed in liquid experience an upward force called upthrust or buoyant force.

→ The value of Buoyant force depends upon the density of the liquid.

→ If the weight of an immersed body is more than the upthrust then the body sinks in the liquid.

→ Those bodies float which has a density less than the density of the liquid in which they are immersed.

→ Those bodies, which have a density more than the density of the liquid in which it is immersed sink.

→ Archimedes’ principle states that when a solid body is wholly or partly immersed in a liquid then the body experiences an upward thrust due to liquid displaced by the immersed part of the body.

→ Relative density of a substance is defined as the ratio of the density of the substance to the density of the Same volume of water.

∴ Relative density = \(\frac {Density of substance}{Density of water}\)

→ Relative density has no unit.

→ According to Archimedes’ principle when a solid body is completely or partially immersed in a liquid and the weight of the body is equal to the weight of the liquid displaced by the immersed part of the body then the body floats.

→ Kepler gave the following three laws which govern the motion of the planets:

- The planetary path of each planet is elliptical at the centre of which the sun is situated.
- The line joining sun and the planet sweeps equal areas in equal intervals of time.
- The cube of the average distance ‘r’ of a planet from the sun is inversely proportional to the square of the orbital period ‘T’ of that planet.

→ Law of gravitation states that the force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. This force always acts along the line joining the centres of two objects.

→ Force of Gravitation: It is that force with which two bodies attract each other but they are lying apart.

→ Acceleration due to Gravity: Acceleration produced in the bodies falling freely under the action of gravity is called acceleration due to gravity.

→ Force of Gravity: It is the force with which earth attracts all bodies towards its centre.

→ Weight: The force with which the earth attracts bodies towards it is called weight.

→ Mass: The quantity of matter contained in the body is called it’s mass.

→ Inertial Mass: This mass measures the resistance which is produced due to a change in the position of rest or motion.

→ Universal Gravitational Constant: It is the force that acts between two bodies each of unit mass and lying unit distance apart.

→ Kepler’s first law: All planets move around the sun in elliptical orbits.

→ Kepler’s second law: The line joining planet and sun sweeps out equal area in equal intervals of time.

→ Kepler’s third law: Cube of mean distance ‘r’ of the planet from the sun is proportional to the square of its orbital period.

→ Thrust: The total force acting perpendicular to the surface of the body is called thrust.

→ Buoyant Force: The upward thrust applied by the liquid on the immersed body is called Buoyant force.

→ Pressure: Thrust acting per unit area of the surface in contact is called pressure.

→ Density: Mass per unit volume of the substance is called the density of the substance or it is termed as the ratio of the mass of a body to its volume.

- Density = \(\frac {Mass of the body}{Volume of the body}\)
- SI unit of density is kg/m
^{3}.

→ Relative Density: Relative density of a substance is defined as the ratio of the density of the substance to the density of the same volume of water. Since it is the ratio of two similar quantities, therefore it is only a number. It is also called the specific gravity of the substance.

→ Archimedes Principle: This principle states that whenever a solid body is immersed in a liquid either wholly or partly then it experiences an upward thrust due to the displaced liquid.

→ The universal law of Gravitation: The mutual force of attraction between every two bodies in this universe is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them. This force acts along the line joining the centres of the two bodies.