Copernicus postulated the earth moves around the sun rather than the other way around but he was troubled about the idea what concepts of mechanics was he missing that would have eased his doubts?

Answer 1

One of the things he was bothered by was the long-held belief of Aristotle that

objects, including the Earth, naturally come to rest unless they are forced to move by

an external force(the motion of the other planets was considered special- they had the

property of quintessence so that their natural state was to move in circles). One thing

that bothered Copernicus is that nobody could imagine a force large enough to keep the

Earth moving around the Sun. Galileo and Newton understood that objects had inertia-

that they tended to keep moving in a straight line at constant speed (or sit still) unless

acted on by an external force- in the cases of objects 'naturally' coming to rest, friction

was identified as the responsible force. So for the Earth, there doesn't need to be an

external force to keep it moving, just so long as it isn't being affected by friction. As we

will learn in subsequent chapters, gravity is what keeps the Earth moving in an ellipse

around the Sun rather than just moving through space in a straight line.

There was another prevailing argument against the moving Earth. It went something

like this: if the Earth were to move around the Sun, it would have to be traveling at

speeds of around 30 km/s to make it all the way around in one year (that bit's pretty

much right). People argued that if a bird were to drop from a tree to snatch up a worm,

and it took one second for it land, the Earth would have therefore moved some 30 km

out from under the bird, leaving it mighty confused in a neighboring county just before

it met with a presumably unpleasant and messy landing. Since birds had been observed

to drop from trees to eat worms, with confusion and harm coming only to the worm,

people argued that there was no way the Earth could be moving around the Sun. What

these people did not understand is the concept of relative motion: the bird and the air

were moving around the Sun, with the Earth at 30 km/s, so the velocity of both the bird

and air, relative to the Earth, were basically zero (not counting a gentle breeze or the

bird's downward velocity as it fell).

Answer 2

Copernicus was missing the concept of inertia, as explained later by Galileo. If he had known about this, it would have eased his doubts because inertia could explain why objects move in a straight line unless acted upon by a force, thus supporting the idea of Earth’s motion around the Sun.

Answer 3

yes he did