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**Tip 1:**Problem related to universal law of gravitation:

- The universal law of gravitation includes only one formula i.e. $F=Gr_{2}m_{1}m_{2} $.
- If the force or the ratio of force between the two conditions is to be determined, then find out the quantities that remains constant in two situations.
- While taking the ratio, the constant quantities will be eliminated and the remaining change in the parameters can be addressed in terms of one another to make the solution simple and comprehensive.

Two lead spheres of same radius are in contact with each other. The gravitational force of attraction between them is F. If two lead spheres of double the previous radius are in contact with each other, the gravitational force of attraction between them will be:

The gravitational force of attraction between two bodies at a certain distance is $10N$. If the distance between them is doubled, the force of attraction: