Circular motion is caused by a force which always acts towards the centre of the circle (the centripetal force).
At the position of the brown dot, the centripetal force is acting horizontally. Gravity acts vertically downwards and has no horizontal component; so gravity is perpendicular to the centripetal force and makes no contribution to the the centripetal force in this position.
At the position of the brown dot, the centripetal force is acting horizontally. Gravity acts vertically downwards and has no horizontal component; so gravity is perpendicular to the centripetal force and makes no contribution to the the centripetal force in this position.
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Which direction is gravity in this picture? What do you mean by "gravity has no effect on the circular motion"?
I think you're misinterpreting something you heard about the analysis of this situation.
I think you're misinterpreting something you heard about the analysis of this situation.
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I'm not entirely sure that I'm answering the question you're asking because it's worded in kind of a weird way. If I'm interpreting the question correctly, though, the answer is fairly simple Newtonian physics:
The object's inertia is constantly acting against the gravity of the body it's in orbit of. The orbiting object is in motion, and 'wants' to travel in a straight line, but gravity is trying to pull the two objects together. The only way the object can move is in a direction in which the two opposing forces are in equilibrium: a circular (well, elliptical, but let's not nitpick) path.
The object's inertia is constantly acting against the gravity of the body it's in orbit of. The orbiting object is in motion, and 'wants' to travel in a straight line, but gravity is trying to pull the two objects together. The only way the object can move is in a direction in which the two opposing forces are in equilibrium: a circular (well, elliptical, but let's not nitpick) path.