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enter image description here

I just start learning Physics.

The picture shows the ISS orbiting Earth.

Suppose now we (or rather, God) put a baseball behind the ISS. The ball is not moving with the ISS, we just put it on the track of the ISS and let go of it.

Will this ball fall directly to Earth while things in the ISS are floating?

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    $\begingroup$ Yes. If the ball has no orbital velocity then the ball will "fall" to earth. The only reason that an object stays in orbit is due to the orbital velocity which makes it miss the earth when it "falls" towards it. $\endgroup$
    – ludz
    Commented Sep 15, 2023 at 8:33
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    $\begingroup$ It depends on the velocity of the ball. Is the ball moving with the ISS, or is it completely still (with respect to the earth)? $\endgroup$
    – Gabriel Ybarra Marcaida
    Commented Sep 15, 2023 at 8:37
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    $\begingroup$ If we drop the ball from the ISS, because of inertia (I imagine you already know Newton's first law), the speed of the ball won't change. So the ball would be moving with the ISS. This is different from the case were you throw something out of the windows of your car, because in the case of the car there's air, which slows the ball until it stops. If you want the ball to fall to earth, you will hsve to throw it backwards (with a ver high speed). $\endgroup$
    – Gabriel Ybarra Marcaida
    Commented Sep 15, 2023 at 8:50
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    $\begingroup$ I said backwards so that the ball would stop with respect to the earth. In that case, the ball would fall. If you throw the ball very hard downwards, the ball could fall to the earth. The could is important here, because the ball will also have a horizontal velocity (because of inertia). If this horizontal velocity is high enough, the ball might miss the earth. $\endgroup$
    – Gabriel Ybarra Marcaida
    Commented Sep 15, 2023 at 9:05
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    $\begingroup$ "The ball is not moving with ISS, we just put on the track of ISS and let go of it" Why would the ISS or anything else be relevant to interfering with the ball just falling down, like from your hand? $\endgroup$
    – philipxy
    Commented Sep 16, 2023 at 5:19

2 Answers 2

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It is all about the sideways speed of the object.

  • The ISS and also your baseball is falling towards Earth constantly. If you just let go of the ball up there, then it will fall down and crash on the ground underneath.

  • Push it a bit sideways while letting go and it still crashes but a bit more to the side.

  • Push it even more, and it still crashes but this time far to the side.

Now push so hard that it reaches such a great sideways speed that although it still falls, it misses! It misses Earth entirely! It falls past Earth and is now flying away from earth on the other side. Earth's gravity slows it down until it eventually comes back again - and it will due to symmetry miss again, this time from the other side. It will repeat this forever. Your baseball is now tracing out an elliptic orbit.

Give it an even greater sideways speed, and the ellipsis widens. At some specific speed, the ellipsis becomes just as wide as it is tall - you now have circular orbit. The distance to the ground is now constant, the same at any moment.

Adjust your altitude, and you will be able to find a place where the sideways speed necessary for circular orbits perfectly matches the sideways speed of Earth's surface due to its rotation. Meaning, a place where the orbital period is 24 hours. Then you have what is called a geosynchronous orbit.

This might be the most peculiar of all orbits, because now your ball looks stationary when we look at it from the ground. It looks as though it is just hanging there in an invisible thread, just floating weightlessly. This is typically the positioning of satellites that must cover the globe in precise and sowewhat fixed patterns.

But in reality it is still falling. Every satellite and space station and astronaut and baseball in geosynchronous orbit, is still falling. Your rotational speed while standing on Earth's surface just happens to match this motion, so that the relative speed* seems to be zero. Only an object in deep outer space away from any gravitational field can truly be called weightless. Anywhere else it would be constantly falling.

Whenever you see astronauts in orbit free-floating, then they are actually not stationary. They just move (fall) at exactly the same speed* as the cameraman.

* To be precise, it is the rotational or angular relative speed that is zero, since it is the angular speeds of you and of the geosynchonous space object that are equal (you both sweep through the same angle per second, hence both covering 360 degrees per 24 hours). Your linear or translational speeds are still very different, since the space object farther away has a longer orbit to move through during those same 24 hours than you standing on Earth's surface farther down (closer to the circle centre).

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    $\begingroup$ Minor nitpick, an orbital period matching the earth's rotational period implies geosynchronous orbit, but it's only geostationary if the 24-hour orbit is also on the plane of the equator. $\endgroup$
    – Nuclear Hoagie
    Commented Sep 15, 2023 at 17:50
  • $\begingroup$ @NuclearHoagie Thanks, good remark. I have changed the term. $\endgroup$
    – Steeven
    Commented Sep 21, 2023 at 12:09
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If the ball is simply kept in the orbit without any orbital velocity provided to it the ball will simply be attracted by the earth as it is within its gravitational field.

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