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will you please give some detailed class notes based on the chapter Gravitataion.

1.State the Universal law of Gravitation. Why do we say that the law is universal?

   The Universal law of Gravitation was given by Newton. So it is also known as Newton's Law of  gravitation.

According to Universal Law of Gravitation : Every body in the Universe attracts every other body with a force which is directly proportional to the product of their masses and inversely  proportional to the square of the distance between them.

The law is universal because it is applicable to all objects; whether the objects are big or small,whether they are celestial or terrestrial.

2.What is the importance of Universal Law of Gravitation?

     1. It is the gravitational force between the sun and the earth, which makes the earth to move around the sun with a uniform speed. Similarly, it is the gravitational force between the earth and the moon, which makes the moon to move around the earth with uniform speed. In fact, it is the gravitational force which is responsible for the existence of our solar system.
2. The tides (rising and falling of water level in sea) are formed in sea due to the gravitational pull exerted by the sun and the moon on the surface of water.
3. It is the gravitational pull of the earth, which holds our atmosphere in place. It is the gravitational pull of the earth, due to which the rain drops fall towards the earth and the rivers flow towards the sea.
4. It is the gravitational pull of earth, which keeps us and other bodies firmly on the ground.
5. The gravitational pull of the earth, which keeps the enormous amount of water in oceans in place and prevents it from flowing.

3. Derive the formula to find the magnitude of the gravitational force between two objects on the surface of the earth.

                 


Consider  two bodies A and B of masses m1 and m2 are lying at a distance r from each other. Let the force of attraction between these two bodies be F.  Now, according to the Universal law of Gravitation.


(i). The force between two bodies is directly proportional to the product of  their masses.That is ,
                F ? m1m2   ___________________ (1)
(ii) The force between two bodies is inversely proportional to the square of the distance between them.That is,


                F ?   1/ r2 ___________________ (2)
Combining (1) and (2), we get
                F ? m1m2 /  r2
Gravitational force, F = Gm1m2/ r2


where, G is a constant known as "Universal Gravitational Constant".

4. Distinguish between Geocentric Theory and Heliocentric Theory.

Sl.no Geocentric Theory Heliocentric Theory
1 This theory was proposed by Ptolemy. This theory was proposed by Copernicus.
2

According to this theory earth is stationary and

is at the centre of the universe.

According to this theory sun is stationary and is at

the centre of the universe.
3 All the  planets including sun revolve around the earth . All the  planets including the earth, revolve around the sun.
4 This theory was also known as Ptolemaic Theory. This theory was also known as Copernican Theory.

 5. What will happen to the gravitational force,

       a. When the distance between the two bodies is doubled?

       b. When the  distance between the two bodies is halved?

(a)   F = Gm1m2/r2

       F1= Gm1m2 / (2r)2

           = F/ 4

The gravitational force becomes one fourth.

(b)     F= Gm1m2/ r2

      F2 = Gm1m2 / (r/2)2

           = 4F

The gravitational force becomes  four times.

6. What is the force of attraction due to a child of mass 20 kg and his mother of mass 100 kg. If the distance between them is 20cm?  Take G = 6.67 x 10-11 Nm2/ kg2.

Given m = 20kg 

           M = 100kg

           F = GmM/r2

              = 6.67 x 10-11x 20 x 100 / (20/100)2

             = 6.67 x 10-11 x 2000 x 25 / 1

             = 3.335 x 10-6 N

7. The mass of the earth is 6 x 1024 kg and that of the moon is 7.4 x 1022 kg.  If the distance between  the earth and the moon be 3.84 x 105 km. Calculate the force exerted by the earth on the moon. (G = 6.67 x 10-11 Nm2/kg2).

The force exerted by one body on another body is given by the Newton's formula:

 F = Gm1m2/r2

Here, mass of earth, m1= 6 x 1024 kg

  mass of the moon, m2 = 7.4 x 1022 kg

And, Distance between the earth and moon, r = 3.84 x 105 km

                                                                              = 3.84 x 105 x 1000 m

                                                                              = 3.84 x 108m

Putting these values in the above formula, we get

                  F = 6.67 x 10-11 x 6 x 1024 x 7.4 x 1022 / (3.84 x 108)2

                     = 2.01 x 1020 N

8. Define acceleration due to gravity.

The uniform acceleration produced in a freely falling body due to the gravitational fore of the earth is known as acceleration due to gravity and it is denoted by the letter g.The value of g does not depend on the mass of the body.  The value of g changes slightly from place to place but for the most of the purposes it is taken as 9.8m/s2. Thus, the acceleration due to gravity, g = 9.8m/s2.

9. Difference between mass and  weight.

Sl. No Mass Weight
1

The mass of an object is the  quantity of

matter contained in it.

The weight of an object is the force with which it is

attracted towards the centre of the  earth.
2 The SI unit of mass is kilogram(Kg) The SI unit of weight is newton (N).
3 The mass of an object is constant.

The weight of an object is not constant. It changes with the 

change in acceleration due to gravity (g).
4 The mass of an object can never be zero.

The weight of an  object  can be zero.

for example, in the interplanetary space, where g = 0.

The weight of an object

becomes zero.

10. What is the ratio of SI units to CGS units of gravitational constant?

SI unit of  G is Nm2kg-2

CGS unit of G is dyne cm2g-2

The required ratio is

Thus, the ratio of SI unit to CGS unit of G is 103.

 


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