Gravitational Force between two Rods of Mass M and Length L

[Harshwardhan]

Find the Force of Gravitation between two Uniform Rods of Mass M and Length L.
I heard my prof say that we have to do double integration to solve this.
Please explain the above step and step with Diagram

 

[PeroK]

Find the Force of Gravitation between two Uniform Rods of Mass M and Length L.
I heard my prof say that we have to do double integration to solve this.
Please explain the above step and step with Diagram

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