[Harri (OP)]

Just watched TV and saw a bowling ball and feather dropped from a height and of course the ball lands first. The same experiment is repeated but in a vacuum chamber. The ball and feather land at the same time! Shouldn't the feather and ball just hang suspended side by side in a vacuum until a force acts on them to either move them of pull them to the ground?

[Bored chemist]

Gravity is acting on them, pulling them to the ground.

[Halc]

Shouldn't the feather and ball just hang suspended side by side in a vacuum until a force acts on them to either move them of pull them to the ground?

But gravity does act on them, not the air. The absence of air just eliminates the friction, that friction being the only thing that retards the progress of the feather more than it does the ball.

Expressed in the terms of GR, there is no force of gravity, but the ground accelerates upward to the stationary ball and feather, and there's no way it can get to one sooner than the other if they're both stationary.  The acceleration of the ground can be measured with any accelerometer.

[Harri (OP)]

So what am I actually seeing? The force of gravity acting on the ball and feather pulling them down or the ground accelerating up to the stationary ball and feather?

[Petrochemicals]

There is no falling, only attraction. Accelleration.

[Kryptid]

There is no falling, only attraction. Accelleration.

That's what falling is: acceleration due to gravitational attraction.

[alancalverd]

But gravity does act on them, not the air. The absence of air just eliminates the friction, that friction being the only thing that retards the progress of the feather more than it does the ball.

Expressed in the terms of GR, there is no force of gravity, but the ground accelerates upward to the stationary ball and feather, and there's no way it can get to one sooner than the other if they're both stationary.  The acceleration of the ground can be measured with any accelerometer.

Er, no. The model sort of works in the vacuum case, but if there is air in the tube, why does the ground accelerate faster towards the ball than to the feather? and if our chum Evan does the same experiment at the same time in Australia, why doesn't the earth split open as the ground rushes simultaneously towards both balls?

An important strength of GR is that it gives the same result as newtonian mechanics for mesoscopic objects and low speeds.

[Halc]

The model sort of works in The model sort of works in the vacuum case, but if there is air in the tube, why does the ground accelerate faster towards the ball than to the feather?

Because the ground accelerates the air up with it, and that wind pushes the feather away much in the same way it doesn't put the ball.

Consider a centrifuge in deep space (away from gravity). You have two rooms spinning around a central axis, one with air, the other in a vacuum. You drop the feather and rock in each room, and in the vacuum case the two hit the floor at the same time, but in the air case, the rock hits first because the floor is accelerating the air which in turn exerts an accelerating force on both objects towards the axis (up).  The feather, massing much less, accelerates away from the floor far more than does the more massive rock, per Newton's f=ma. No GR involved.

and if our chum Evan does the same experiment at the same time in Australia, why doesn't the earth split open as the ground rushes simultaneously towards both balls?

You're confusing coordinate acceleration with proper acceleration here.

Same example as above.  Both rooms have a proper acceleration towards each other, but their separation remains fixed, so using the rotating frame of reference as we do in labs here on Earth, the coordinate acceleration of the floor is zero and it is the feather and rock that accelerates (not quite straight) to the floor.

[Eternal Student]

Hi.  I think I've got to support Halc on this one.

Er, no. The model sort of works in the vacuum case, but if there is air in the tube, why does the ground accelerate faster towards the ball than to the feather?

   Good question.   It's because both the ground and the air is rushing toward those objects.  The air is getting some support from the ground (or from the bottom of the tube).   The air can deform and move past the objects and at that point, the objects are experiencing a force (friction).  The feather experiences a greater force (per unit mass).  Neither the feather or the ball remain stationary but the acceleration is greater for the feather.

and if our chum Evan does the same experiment at the same time in Australia, why doesn't the earth split open as the ground rushes simultaneously towards both balls?

    Another good question.  The gravitational field is not uniform as you cross planet earth (it completely reverses direction).  Your local inertial frame is different to Evan's as far as GR is concerned.  Locally,  Evan's ground is rushing upward and so is your own.  If you consider Evan's ground in your own local inertial frame then the whole planet is rushing toward your objects in the tube.

[alancalverd]

Because the ground accelerates the air up with it,

So why does it accelerate nitrogen molecules upwards, but not feather and ball molecules?

[alancalverd]

If you consider Evan's ground in your own local inertial frame then the whole planet is rushing toward your objects in the tube.

and therefore away from the objects in Evan's tube, which is pointing in the opposite direction to mine.

[alancalverd]

The acceleration of the ground can be measured with any accelerometer.

So I'm sitting in my airplane, which has an accelerometer, and you drop a bowling ball or a feather on the runway, and the plane takes off, or at least registers positive g. Saves a lot of fuel. 

[Halc]

So why does it accelerate nitrogen molecules upwards, but not feather and ball molecules?

Relative to a local inertial frame, all of those accelerate (proper) upward. If you put an accelerometer on any of them, it shows acceleration upward. There's no force pushing in the down direction.

So I'm sitting in my airplane, which has an accelerometer

It does?  I mean, you can put one in there, but I was unaware of it being equipped with a device that measures proper acceleration. I can assume such a device. Heck, cell phones have them.

and you drop a bowling ball or a feather on the runway, and the plane takes off, or at least registers positive g. Saves a lot of fuel.

Not following you. The plane parked should register positive g. Taking off, it should register some component forward as well as the force from the engines are greater than the wind pressure accelerating it backwards. No idea what you're trying to illustrate with that and with the objects you left back there on the runway.

[Harri (OP)]

I think it's obvious from my questions and responses that I read popular science books and I am in no way a scientist. I live in, and I am a product of my logical world where apparently illogical things occur. Illogical to my logical upbringing that is. The writer attempts to explain the nature of these illogical things without the math and depth of scientific fact and sometimes it just doesn't translate into understanding.

Could I ask, what if we return to the vacuum chamber where a bowling ball and feather are suspended from the ceiling. We then create a vacuum by withdrawing the air in the chamber. This time, midway between the objects and the floor we pass a light beam across their path. On release of the ball and feather what would reach the beam first, the ground or the objects? Or both simultaneously?

[alancalverd]

It does?  I mean, you can put one in there, but I was unaware of it being equipped with a device that measures proper acceleration. I can assume such a device. Heck, cell phones have them.

And you'd be mad to undertake aerobatics without one.

Not following you.

Nor I you, skipper.

Expressed in the terms of GR, there is no force of gravity, but the ground accelerates upward to the stationary ball and feather, and there's no way it can get to one sooner than the other if they're both stationary.  The acceleration of the ground can be measured with any accelerometer.

Now the mutual acceleration between the bowling ball and the ground must be g m/s^2 because that's what we can measure by plotting separation  vs time. So if the ball stays stationary and the ground suddenly starts hurtling upwards at g, the accelerometer in my plane will move from +1g to +2g because it is sitting on the surface of a planet that is now moving (it certainly works on the deck of a carrier that's plunging up and down). Let's drop the ball (or accelerate the planet) from 10 meters.  Now the planet hits the ball and stops moving, but the plane has some upward speed (196 m/s!) so it lifts off at several times its normal climb rate, without troubling the engine at all!

Of course there is a boring newtonian explanation, but that is so 17th century.

[alancalverd]

This time, midway between the objects and the floor we pass a light beam across their path. On release of the ball and feather what would reach the beam first, the ground or the objects? Or both simultaneously?

Whilst I'm teasing the experts, I'll let you into a newtonian secret. The planet and the ball and the feather all accelerate towards their mutual center of mass*, and would reach it at the same time if there were nothing in the way. But fortunately for you terrestrial beings, the planet is so much more massive than any artefact that the barycenter is pretty close to the middle of the planet and the time difference between the ball and the feather hitting the ground whether released separately or together is negligible.   


*the big question is why?

[Halc]

Could I ask, what if we return to the vacuum chamber where a bowling ball and feather are suspended from the ceiling. We then create a vacuum by withdrawing the air in the chamber. This time, midway between the objects and the floor we pass a light beam across their path. On release of the ball and feather what would reach the beam first, the ground or the objects? Or both simultaneously?

Depends on the frame in which the light beam is stationary. If it's mounted to something fixed relative to the ground, obviously the falling objects will pass through it. But if it is stationary in the local inertial frame of the falling objects (i.e. falling with them), the the beam will intersect the accelerating ground and the stationary objects will never reach it.

Now the mutual acceleration between the bowling ball and the ground must be g m/s^2 because that's what we can measure by plotting separation  vs time. So if the ball stays stationary and the ground suddenly starts hurtling upwards at g, the accelerometer in my plane will move from +1g to +2g because it is sitting on the surface of a planet that is now moving

Sorry. Your accelerometer on the plane should read zero if there's no force (the ground) pushing upward on it, just as it would in deep space.  So place it on the 1g accelerating ground and it should read 1g do to that one force acting on its wheels. You know this, but apparently you consider yourself 'teasing me'.

Let's drop the ball (or accelerate the planet) from 10 meters.  Now the planet hits the ball and stops moving

As you said, you're teasing. You know this is nonsense.

but the plane has some upward speed (196 m/s!) so it lifts off at several times its normal climb rate, without troubling the engine at all!

In the accelerating frame of the ground on which the plane is parked, it gains no speed at all. But yes, in the local inertial frame of a ball dropped in a vacuum from at least 2 km up, the plane very much acquires a speed of 196 m/sec after 20 seconds, and yes, all without help from the engine (useless in the vacuum I put it in). I imagine the fast moving plane will damage itself when it collides with the sufficiently large (up until the moment of impact) stationary ball. At that moment, the planet surface continues to accelerate and the ball is no longer stationary in that frame.

[Colin2B]

*the big question is why?

Too big 

This time, midway between the objects and the floor we pass a light beam across their path. On release of the ball and feather what would reach the beam first, the ground or the objects? Or both simultaneously?

Think. What have you fastened the light beam equipment to?

Edit: whoops, clashed with @Halc reply

[alancalverd]

Your accelerometer on the plane should read zero if there's no force (the ground) pushing upward on it, just as it would in deep space. 

Wrong. It must read 1g on the runway because the force acting on the wing strut is 1 g x the mass of the wing and the force on the undercarriage is 1 g x the mass of the entire aircraft.  When flying straight and level the lift provided by the wings equals the total mass of the aircraft x 1 g by definition of "straight and level". At the top of a gliding loop it will read 0 because the entire structure is in free fall.

Attached photo is the nearest one I have to straight and level showing a g meter. The artificial horizon (top right) and turn indicator (center bottom) show pretty close to S&L, and the g meter (bottom left) is close to 1. Interesting aircraft, incidentally, a Victa Airtourer, manufactured by a company that made lawnmowers.

[Janus]

Could I ask, what if we return to the vacuum chamber where a bowling ball and feather are suspended from the ceiling. We then create a vacuum by withdrawing the air in the chamber. This time, midway between the objects and the floor we pass a light beam across their path. On release of the ball and feather what would reach the beam first, the ground or the objects? Or both simultaneously?

Okay, we will assume that the source of the light is fixed relative to the ground.   The light is turned on.  The light, after leaving the source, will curve downward due to gravity. However since light travels so fast, it would take extremely accurate measurements to even notice that it hits the far wall a bit lower. 
But what happens to the beam if we let it continue on past the chamber wall?   If we assume that the ground extends as a flat plane, and that gravity acts perpendicular to that plane, the light will, at some distant point, hit the ground. The time it would take to do this would be equal to the time it would take for the ball and feather to fall to the height of the source (at least to close approximation and if we assume that gravity strength does not differ over height).  But, the beam will have dropped a tint bit by the time it reaches the middle of the chamber, so they will not quite have reached the light beam yet. 
So, under these conditions, the light will hit the ground just slightly ahead of the objects reaching the beam.

If we take into account the fact that gravity drops off with height above the surface, this means that the objects fall at a slightly less rate of acceleration than the light beam.  If the chamber is tall enough, this can lead to a significant difference.

However, we also assumed that the ground was an flat plane that gravity acted perpendicular to.
The ground however, is the surface of a sphere, and gravity acts toward the center of that sphere.
The amount that gravity can curve the light beam is much, much less than the curvature of the Earth. So the light beam ends up getting further and further from the Earth's surface as it travels,  and ends up heading out into space, never hitting the ground*.
So the question relies a great deal on your base assumptions.

* This doesn't just apply to light. Any object that is traveling fast enough, even if it starts off moving parallel to the ground, will end up never falling to the ground.  Look up Newton's cannon for further explanation.