Hello dear students in this session we are going to study the most important part from lesson i.e. Universal law of gravitation. This law was stated by Sir Isaac Newton. In this law he discussed about the force due to gravity on object and the factors on which this force depends. He performed series of experiments to study force due to gravity and comes to conclusion that the force due to gravity depends upon 1) mass of objects 2) distance between the objects
According to Newton’s law of gravitation, “Every particle of matter attracts every other particle of a matter with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between them”.
Consider two particles of masses m1 and m2 are separated by distance r. According to Newton’s law of gravitation, the gravitation force of attraction is
Where ‘G’ is constant known as universal gravitational constant.
The value of G in SI System is 6.67 x 10-11 Nm2/Kg 2 (discovered by Hinny Cavendish).
In CGS System, value of G is 6.67 x 10-8 dyne cm2/g2
Ratio of SI Unit of G & CGS Unit of G is 1000,
The dimensions of G = [L3M-1T2]
Characteristics of gravitational forces:
- It is attractive force.
- It is central force.
- It is conservative force.
- It obeys inverse square law.
- It forms action-reaction pair.
- It is long range force.
- The value of ‘g increases as we move from the equator to the pole and decreases as we move from pole to equator.
Consider the following example to understand the nature of gravitational force acting on the object.
Eg.1) Two objects each of masses 100 kg and 200 kg are placed at distance of 50 m from each other on road. Find the gravitational force between them, hence conclude that why these objects didn’t get collides on each other due to gravitational force.
Ans: m1= 100 kg, m2= 200 kg, r= 50 m.
By Newton’s law of gravitation the gravitational force between them can be given as,
∴ W1 = 1000 N
Similarly weight of second object will be
∴ W2 = 2000 N
From above calculations we can conclude that the magnitude of gravitational force is very small as compared to the weights of the object, hence they do not collide on each other due to gravitational attraction.