Brainy Coaching Point

Gravitation Class 9 Notes

Gravitational Force : Every object in this universe attract every other object with a certain force .
The force with which two object attract each other is called gravitational force .

Gravitational force is a non contact force .

> The strength of the gravitational attraction between two object depends on two factors

How big the object are
How far apart they are

Universal law of Gravitation

Given by newton also known as newton's law of gravitation , Everybody in the universe attract every other body with a force which is directly proportional to the product of their masses and inversely proportional to the squire of the distance between them

GM₁M₂

_{F = __________}

R²

where G is the universal gravitation constant.

Value of G = 6.673*10-11Nm2Kg-2

Gravitational force

Importance of Universal law of Gravitation

It bind 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 the sea on earth .

class 9 gravitation notes

Free Fall: The falling of a body or object from height towards the earth under gravitational force of earth ( with no other force acting on it ) and such a body is called freely falling body.

Acceleration due to Gravity (g) :

The uniform acceleration produced in a freely falling body due to gravitational force of the earth is known as acceleration due to gravity.

Value of acceleration due to gravity is 9.8 m/s²

> g does not depends on the mass of the falling object of smaller mass .

Calculation of acceleration due to gravity ( g) -

F= mass * acceleration

F= ma = GM₁M₂

_{_____________________________}

R²

^{acceleration came due to the gravity of the earth}

g = GM

_________

R²

G g = 6.67x10^-11 x 6 x 10²⁴

_______________________ = 9.8 m/s²

(6.4 x 10⁶)²

The three equation of motion

Equation of motion

Motion due to gravity

· v= u +at

^·S = ut + ½ at²

· v²- u² = 2as

· v= u +gt

^·S = ut + ½ gt²

· v²- u² = 2gs

> When a body is falling vertically downwards its velocity is increasing. So the acceleration due to gravity is taken as positive

. g = + 9.8 m/s²

^>When a body is thrown vertically upwards its velocity is decreasing . So the acceleration due to gravity is taken as negative

g = - 9.8 m/s²

> If an object is just drop or fall from a height then u = 0

Difference between Mass and Weight

Mass

Weight

· Actual content present in a body

· Mass is always constant

· Mass is never zero or negative

· SI unit = Kg

· It is the force exert by the body on earth.

· Weight is variable ( change from place to place )

· Weight = mass * acceleration

· Weight can be zero.

· SI unit = N ( newton)

Gravity on moon is 1/6 less than the earth

gravity on earth

Gravity on moon = __________________

Frictional Force : The force which always opposes the motion of one body over another body is called frictional force .

Structure of the Atom

An Atoms contain three basics particles namely Proton, neutrons, and electorns .The nucleus of atoms contain Proton and neutrons, where Proton are positively charged and neutrons are neutral .The electrons located at the outermost Regine called electron shall .

Structure of Atoms

Electron : JJ Thompson discovered negatively charged sub-atomic particles electron , emmited by the Cathode towards the anode in the cathode ray experiment , these negitvaly charged particles are electrons.

Protons : Eugen Goldstein discovered that with a different condition in the same chamber Anode emitted positively charged particles known as canal ray or later name as protons.

Neutron : J. Chandwick ,discovered a sub - atomic particle with no charge and a mass equivalent to proton in the nucleus of all atoms.These nuterally charged particles are neutrons .

structure of atom class 9 notes

Thompson Model of an Atom :- According to the Thompson an atoms consist of a positively charged sphere and the electrons are embedded in it .

ii)- The negative and positive charges are equal in magnitude , so the atom as a whole is electrically neutral.
Thompson atomic model is compared to Watermelon, where he considered seeds as negatively charged particles and red part of the watermelon as positively charged.

Limitation of Thompson's model :

> Thompson structure of an atom failed to explain the arrangement of protons and electrons in its structure.

> Also failed to explain stability of atoms.

Rutherford's model of an atom : -

Rutherford conduct an experiment and Bombarding the alpha particles on a gold foil . He observed the path of the alpha particles after passing through an atom and drafted some postulate.

> Most of the space in an atom is empty as the particles passed through the gold foil without and hindrance .

Rutherford's Model of an atom

> The positively charged center is called the nucleus and the mass of an atom reside in the center.

> The particles deflected 180 degree after bombarding the nucleus.

> The size of the nucleus is small compared to the total size of the atom.

> Positive and negative charges balance with each other

Drawbacks of the Model :

> The electrons revolve in an unstable path and they undergo acceleration energy, when the electron revolve they lose energy. Soon electron would collapse into the nucleus

> The tendency would make an atom highly unstable while the atoms is highly stable .

>It also failed to explain the atomic number concept as it explained only presence of protons in the nucleus .

Bohr's Model of an Atom

To overcome of drawbacks of Rutherford Neil Bohr proposed modified model of structure of atom.

> Electrons revolve around the necleus in stable orbits without emission of radiant energy.

> Each orbit has a definite energy and is called an energy shell or energy level.

> An orbit is designated a K,L,M,N shell when the electron is in the lowest energy level it is said to be in ground state .

> An electron absorbe energy when it jumps from one orbit to another .

Orbit : orbits are the energy shell surrounding the nucleus in which electrons revolve