It is said that when Newton was sitting under a tree, an apple fell on him. The fall of the apple inspired Newton to think in a new way.
》He thought that if the Earth can attract the apple then can't it attract the Moon?
》Is the same force acting in both cases?
Ultimately he hypothesized that the same kind of force was responsible in both situations.
Gravitation: Class 9 Science Notes
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in this chapter-
We will study about gravity and universal law of gravitation
We will know in detail what kind of changes occur in the motion of objects under the influence of gravitational force on the Earth?
We will study how the weight of an object changes from one place to another?
Universal Law of Gravitation
Every object in the universe attracts every other object with a force, this force is called gravitational force.
》This force is proportional to the product of the masses of both object and inversely proportional to the square of the distance between them.
》This force is applied in the direction of the line joining the two objects.
Or,
》Where G is a proportionality constant and is called the universal gravitational constant.
➡The SI unit of G is Nm 2 kg –2 .
➡A scientist named Henry Cavendish found the value of G.
➡ The current accepted value of G is 6.673 × 10 –11 Nm 2 kg –2 .
Importance of Universal Law of Gravitation
The universal law of gravitation explains many phenomena that were not previously thought to be connected to each other:
(i) The force that binds us to the earth;
(ii) Motion of the Moon around the Earth;
(iii) the motion of the planets around the Sun; And
(iv) Tides and ebbs due to Moon and Sun.
Centripetal Force
When an object is kept moving in a circular path, the force which acts towards the center is called centripetal force.
》The motion of the Moon around the Earth is due to centripetal force.
》Centripetal force is achieved due to the force of attraction of the Earth. If there is no such force then the Moon will continue to move on a straight line path at a uniform speed.
Free fall
When objects fall towards the earth only due to the force of gravity, then we call this situation the state of free fall of the object.
》There is no change in the direction of motion of objects while falling.
》Due to Earth's attraction there is a change in the magnitude of velocity (acceleration).
Gravitational Acceleration(g)
In free fall, the change in velocity/acceleration of objects while falling is due to the gravitational force of the Earth. Therefore it is called gravitational acceleration.
》It is shown by the symbol 'g'.
》The units of g are ms –2 .
Value of Gravitational acceleration (g)
》The Earth is not a perfect sphere. The radius of the Earth increases as we move from the poles to the equator, hence the value of g is greater at the poles than at the equator.
》For most calculations the value of g at or near the Earth's surface is assumed to be approximately constant,
》Thus the value of gravitational acceleration g = 9.8 ms –2 It happens.
》 In a state of free fall -
F = mg
Three equations of motion Motion of objects under the influence of Earth's gravitational force
》 All objects, hollow or solid, big or small, fall at the same rate from any height
》 Near the Earth, the value of g is constant, hence all the equations of uniformly accelerated motion will remain valid even if g is replaced by acceleration a.
1. v = u + at
2. s = ut + ½at 2
3. v 2 = u 2 + 2as
Here: u=initial velocity. v=final velocity. s = distance traveled. t= time
Mass
The mass of an object is a measure of its inertia.
》The greater the mass of an object, the greater will be its inertia.
》The mass of an object remains the same whether the object is on Earth, on the Moon or in outer space.
》Thus the mass of the object remains constant and does not change from one place to another.
Weight
The weight of an object is the force with which it is attracted towards the Earth.
》The force of attraction of the Earth on an object is called the weight of the object.
》It is shown with the symbol W.
W = m × g
》 The SI unit of weight is Newton (N).
》Weight is a force that acts vertically downwards, so it has both magnitude and direction.
》The weight of an object may vary at different places, but the mass remains constant.
Weight of an object on the moon
》 Weight of the object on the Moon = (1/6) × Its weight on the Earth
Thrust and Pressure
The force applied perpendicular to the surface of an object is called thrust.
Pressure= Thrust/Area
》SI unit of pressure is obtained. This unit is N/m 2 or Nm –2 .
》In honor of the scientist Blaise Pascal, the SI unit of pressure is called Pascal, which is expressed as Pa.
Buoyancy
The force which pushes an object partially or completely immersed in a liquid upwards by that liquid is called buoyant force and this tendency of that liquid is called buoyancy.
》 The magnitude of buoyancy force depends on the density of the liquid.
Why do objects float or sink when placed on the surface of water?
》 This happens due to the difference in their densities.
》The density of a substance is called its mass per unit volume.
》Objects of density less than the density of the liquid float on the liquid. Objects whose density is greater than the density of the liquid sink in the liquid.
Archimedes' Principle
When an object is completely or partially immersed in a liquid, it experiences a force in the upward direction which is equal to the weight of the liquid displaced by the object.
Applications of Archimedes' Principle -
》It is used in designing ships and submarines.
》Lactometer, which is used to check the purity of a milk sample.
》Hydrometers, which are used to measure the density of liquids, are based on this principle.
Read more: Atoms and Molecules class 9 science notes
We hope that Gravitation: Class 9 Science Notes would have been helpful for your better and deep understanding on various topics related to this chapter.
