1. Mar 7, 2012

### Vorde

Why do objects fall in parabolas, but things orbit in either ellipses or hyperbolas?

2. Mar 7, 2012

### DaveC426913

They don't.

They move in an elliptical orbit about the centre of mass of the Earth; it just happens that the orbit intersects the surface of the Earth.

When the minor axis of an ellipse is on the order of few dozen yards, but the major axis is on the order of 4000 miles, an ellipse looks very much like a parabola i.e. the focus (of the ellipse) is so far away it might as well be at infinity (i.e. a parabola) for all the difference it makes.

If you tossed a ball in the air, and and that moment, the Earth suddenly collapsed to the size of a marble while keeping its mass, your ball would fall all the way to the centre, round it (with a perigee of just a few dozen yards) and come back out, following that elliptical orbit.

Last edited: Mar 7, 2012
3. Mar 7, 2012

### Vorde

So really whenever anyone tells us that things fall in parabolas, they are lying. Makes sense, but still....

4. Mar 7, 2012

### Integral

Staff Emeritus
I don't think that is a fair statement. We do not have instrumentation that can seperate the path of a thrown rock from a parabola. In fact, under the assumption that you are close to the earths surface, our basic models say that it IS a parabola.

5. Mar 7, 2012

### Vorde

I don't disagree, but I went through a whole Analytic Geometry class with the term 'things fall in parabolas' etched into my brain. I'm surprised this wasn't at least mentioned.

6. Mar 7, 2012

### DaveC426913

They are parabolas to an acceptable degree of accuracy. Two plumb bobs 10 yards apart, both pointing at the centre of the Earth, will be parallel to within one part in 700,000.

There are many factors affecting ballastic trajectories that are much greater than that.