Question

what would happen if the earth is Flat and not round

Answer 1

It's very difficult to imagine the earth being flat.

But yes initially it was believed that earth is flat
But I don't think if earth would have been flat , it would have made much difference

But then there would be no rotation and revolution possible

Answer 2

There are many concepts of Physics that work only with the assumption that the Earth is spherical, not flat. We do not know what will be the effect on these concepts and laws of Physics if the Earth is considered to be flat.

Take the example of Gravity. It works perfectly for the spherical model of the Earth. If the Earth is considered to be flat than hoe the Gravity will act on the objects, we don't know. Some flat-earthers believe that in flat Earth model the Gravity will act towards the center of the large Earthian disk. But as the distance from the center will increase the Gravity will more likely to act horizontally and this will be very strange. 

In Spherical Earth model, there is a molten magma at the very center of the Earth. This molten magma rotates continuously to create a magnetic field around the earth. If the Earth is considered to be flat than this rotating molten Magma will not be there and in the absence of magnetic field the solar radiations will burn the entire planet down.

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