Earth's Path: Why Ellipse? - How Come, Why Not Circular Motion?

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Why ellipse? How come? Why not circular motion? If the path is changing, then the speed is changing, if the speed is changing it means acceleration, if u have acceleration u need extra energy, but Earth doesn't get one..
Thanks
 

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Bassalisk said:
Why ellipse? How come? Why not circular motion? If the path is changing, then the speed is changing, if the speed is changing it means acceleration, if u have acceleration u need extra energy, but Earth doesn't get one..
Thanks

When the Earth is closer to the sun it is moving faster, and therefore has more kinetic energy than when it is further away, so the total energy stays the same, as kinetic energy gets transformed into gravitational potential energy and vice versa.
 
Vagn said:
When the Earth is closer to the sun it is moving faster, and therefore has more kinetic energy than when it is further away, so the total energy stays the same, as kinetic energy gets transformed into gravitational potential energy and vice versa.

But why does Earth do this? Isn't it more simple for "her" to just have the same energy over and over ?
 
Well other thing that crossed my mind, is inertia. A body wants to go in a straight line right? But sun pulling the Earth and makes orbit elliptical? Is this is it?
 
To have a perfectly circular orbit, then your body initially needs to have exactly the right speed, if this speed is more than the speed for circular motion, but is less than the larger body's escape speed, then the smaller body will undergo an elliptical orbit, if it is equal to the escape speed, then the body will move along a parabolic orbit. Finally if it is greater than the escape speed then it will follow a hyperbolic orbit. Both para and hyperbolic orbits would cause the smaller body to leave the system.

To have exactly the right speed for either parabolic or circular orbits have a very low probability. i.e. 0 chance of occurring.
 
Vagn said:
To have a perfectly circular orbit, then your body initially needs to have exactly the right speed, if this speed is more than the speed for circular motion, but is less than the larger body's escape speed, then the smaller body will undergo an elliptical orbit, if it is equal to the escape speed, then the body will move along a parabolic orbit. Finally if it is greater than the escape speed then it will follow a hyperbolic orbit. Both para and hyperbolic orbits would cause the smaller body to leave the system.

To have exactly the right speed for either parabolic or circular orbits have a very low probability. i.e. 0 chance of occurring.
I get it now, thanks!
 
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