Water Bucket: Newsletter Challenge (04/25/06)

All you need is an elevator that accelerates fast enough and the acceleration forces only affects the wood and not the water!

Try SunBlock 2000 on the kids head.

The acceleration on the water is counteracted by the bucket & lift (elevator) floor, and since water is pretty incompressible, its volume will remain pretty constant. The acceleration on the wood is counteracted by the water itself...

Since F=ma, the water will experience a heavier object floating in it...and we all know that heavier objects "float" deeper than light ones of the same displacement (to a limit).

Whether the lift's acceleration will be enough to produce a noticeable lowering of the wood's position (or that of a scribed line on it), I'm not sure.

Actually, it is the denser object that floats deeper than the less dense ones.

Density = mass / volume

Even under acceleration, the mass and volume of an object never change.

However, if you drop a log into a pond it will sink into the pond and then surface. The same thing happens to you when you dive into a pool (assuming you don't exhale under water!).

Okay, so a falling object accelerates as it falls into the water. Its inertia carries it down, the acceleration stops, and the object will float back up if the density of the object is lower than the medium it is suspended in.

Now, in the case of the elevator, instead of the object falling into the water, now the water falls into the object!

The water is contained, as you observed, but the wood floats on the water. As the system (elevator and its contents) accelerates, the inertia of the wood will force the wood deeper into the bucket for as long as the acceleration continues. The amount that the wood sinks into the water is a function of its mass, its volume, the magnitude of the acceleration of the system, and the density of the liquid the wood floats on.

Yes, theoretically you could submerge the wood in the bucket if the acceleration was high enough. However, it is unlikely that an ordinary elevator would provide enough acceleration (unless the wood was water logged) to submerge the wood.

Very well articulated point. There is one thing missing from the original question that should be addressed. What percentage of the wood's volume is submerged under the original conditions? If 99% is submerged, the specific gravity of the wood is only slightly lower than that of water (SG = 1) SG(wood)=.99 In these conditions, the block will sink more easily since it is only a little less dense than the water. But if the volume block is 99% above the surface of the water (very unlikely) then the elevator would have very little effect on it.

I assume that that there is a steady acceleration, so the water and wood are experiencing steady state conditions. With acceleration of the elevator upwards, all that increases is the weights of both the water and the wood, Weight = acceleration X mass. Since their mass to volume ratios do not change, the relative densities of the wood and water does not change. Therefore the wood will continue to float because by achimedes principle, floatation is achieved when the wood displaces its own weight of water. Samsook

I have not run the math, but I think Anonymous hit it on the head from the start. The entire system is accelerating for some period of time, so all you are doing is changing the effective value of "g". Any motion would only be due to the secondary fluid effects. If you were to use a centrifuge where you could maintain the altered "g", the system would settle into exactly the same relationship it had under 1g.

I think that you are close, but changing the value of "g" will increase the density of the water, but not the wood. The bouyancy of the wood is proportional to the difference of the densities of the wood and the water, so the wood should actually float higher in the water while the system is being accelerated upward. If you want the wood to sink, you need to hit the "down" button.

No, no, no! Mass is always a constant. Weight is not and is another matter!

Density = mass / volume

The density of an object does not change when accelerated.

It is easy to confuse the two (weight and mass), but they are not the same thing. A particle's mass does not change unless you are under the infuence of Special Relativity (I think the legal limit is somewhere above 99.99%).

The key here is inertia.

Ummm, I think you might have misunderstood my post. While the elevator accelerates the wood will tend to displace more water because the wood is floating on a bath of water and experiencing a change in its inertia. The acceleration does not really impact the container of water.

Given enough acceleration the wood would (no pun intended)submerge. Although the ride for the rest of the passengers would be rather uncomfortable!

If you consider the fact that the pressure differential (usually caused by gravity at 1G) will increase, then the system will not change because the increased reactive force to keep the block at the same point above the water line will be generated by the increased acceleration on the water itself.
The down button will do approximately the same, but if the elevator was so quick to induce a net force near zero, then the surface tension of the water will draw the block within it, causing the block to 'sink'.
If the elevator accelerated faster than gravity downward, it would make the block sink, causing the same floating situation only inverted. However this does all change based on what you would call up and down in an elevator causing you to be pushed against the cealing.

Guess this one was one of the easy ones. Looks like the kid has already been out in the sun as the questions are getting easier... Just to add to the many correct answers there. Whether the wood sinks or not would depend on whether the hotel has a regular elevator like the rest of us do, or whether it has a slingshot to take you to your room on the moon. The acceleration is the only period during which this will be noticed. So even if the acceleration was substantial it would cause a bobbbing of the wood rather than a permanent "sink".

I believe the child has been in the sun too long. For the wood to accelerate upwards, it must have a force acting on it. Since the wood is initially floating, the substance directly pushing the wood up, as the elevator rises, is the water. Why wouldn't the wood continue to float as it is lifted by the water? Think of a small boat on the ocean. No matter how fast a wave rises up from underneath, it will always lift the boat along with it, not somehow envelop and sink the boat. Of course, the trip down from a wave crest is a different matter. Could you ride the elevator down fast enough and come to a sudden enough stop to sink the wood?

"Could you ride the elevator down fast enough and come to a sudden enough stop to sink the wood?"

YES!

That is what happens to you when you dive into a pool. Your inertia carries you below the surface of the water, and then your buoyancy causes you to rise to the surface. Try an apple in a bucket of water and see what happens!!!

Actually, the draft of a boat should increase when a swell in the ocean or lake takes place. However, the rate of acceleration isn't enough to make a significant change unless the ship's waterline is already up to the gunnels.

In this case the forces of inertia will be common to both the wood and the water unlike when you dive into a pool

NO - In the 'boat` case, the deceleration caused by shape of the wave is not 'felt` by the water, it's weight does not increase as the boat does. In the decelerating elevator, the deceleration acts on both equally maintaining the relationship that floats the wood.

The block would ride lower in the water because atmospheric pressure decreases faster with elevation than gravitational pull. You might need a sophisticated device to measure the difference.

My Kid did a similar expriment for her science class... by the by... she's in first grade.
Close all the windows in the car.
hold a helium baloon.
accelerate from a stop.
Watch what happens to the ballon.
I would think the same thing would happen to the wood... it would float higher because the higher density of the water... but there's the force of gravity and the acceleration of the wood... but the effect on the bouyancy would be greater than the downward force of acceleration, I bet... without doing the math.

I'd tell you that the balloon would move toward the front and not the back like you'd expect, but that would ruin the whole surprise.

Interesting. So the balloon would actually be traveling slightly faster than the car?

Where is the energy coming from to do that?

I would expect it to follow the same principle that a marble would on the floor of the car. As you accelerate the marble would roll backwards. When you stop the car it rolls forward under your feet where it prevents you from operating the gas or brake pedal.

Newton's First Law stated that "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Blah, blah, blah.

Back to the balloon. You are stopped and the balloon is resting at the center of the roof. You hit the gas and go. What pushes the balloon forward? My guess would be friction between the headliner of the roof and the rubber balloon's skin. There is some force of the air in the car, too. However, the balloon is at rest, so it initially will appear to move backwards relative to the car's motion.

Now to the wood floating on the water in a bucket. The wood floats on the water exerting a force equal to its mass on the water. The same laws of Newton will apply when the bucket is accelerated upwards, the wood will exert a force downward on the water that is the sum of its mass plus the F=ma. The reason it is the sum is because the vector force of gravity and the acceleration are in diametrically opposite directions so that the vector addition is purely the sum of the two vectors. The equation is the same when at rest. The net force on the water by the sum of the wood's mass plus the equation F=ma. At rest a = 0, so it simplifies to the mass of the wood.

Inertia is the tendency of an object to resist changes in their state of motion. For the bucket and its water, vertical acceleration does not change its shape or properties (water remains a liquid at its current temperature and density and the bucket is unyielding in shape and size). For the wood it is suspended on a cushion of water (like me at night on my water bed). When the wood and the bucket undergo vertical acceleration, the wood's inertia tends to want to keep the wood at its present position; however, the bucket and water are rising upwards. Before the acceleration started the wood exerted a force on the water equal to its mass and the water exerted an equal force upon the wood. At this state of equilibrium the wood floats with a percentage submerged below the water line. When the bucket accelerates upward the equilibrium experienced at rest is upset. There is a tendency for the wood to resist the force acting on it by the water because the wood wants to stay where it was. As the argument develops between the wood and the water below it a compromise is reached. The water allows the wood to accelerate at a slower rate than the water for short period of time and catch up a little later. The net result is the wood sinks into the water a little more than normal and remains there as long as the acceleration continues.

Let's do the math. Water's density is 1.0, which is 1 Kg per liter (remember density = m/volume). If we take a block of wood that weights 1 Kg and has a volume of 2 liters, then the wood will be submerged so that 1 liter is below the surface of the water. We have 1 Kg of force exerted upon the water and 1 Kg of force exerted on the wood by the water. Hit the up switch on our super elevator so that it accelerates at 1 g. The force exerted by the wood is now its mass of 1 Kg plus the force of its mass times the acceleration, which is 1 Kg * 9.8 m/(s*s), or 9.8 Kgm/(s*s). I chose 1 g because that is the force of gravity and it simplifies the process a little. One kilogram-force = 9.8 kgf. So the total force that the wood exerts on the accelerating bucket of water is the original force of the wood at rest of 9.8 kgf plus the 9.8 kgf induced by the acceleration or 19.6 kgf. 19.6 kgf is equal to 2 kg!

If the wood now has a force of 2 kg acting upon the water and the woods volume is still 2 liters, the wood must submerge 100% into the water in order to reach equilibrium with the water.

Actually the ballon moves forward, that is if it floats in the first place. The tendency to remain suspended is a result of higher density air collecting "under" the balloon and pushing it up, when the car accelerates, this "heavy" air shifts towards the rear pushing the lighter balloon forward. A bulk of air is moving slightly slower than the car in order to make the balloon move slightly faster. I believe this was a globalspec question at some point.

First the Balloon...
http://www.pbs.org/wgbh/nova/lasalle/buoycar.html
Yep, it goes forward as you accelerate...

2nd, your math is nice... but wrong, Anarchimide.
Buoyancy has gravity all over it.
To simplify the thought process, just think about moving the bucket to a bigger planet. Say Kat's planet which is 2x as attractive or twice the gravity of earth which is the same as accelerating your frame of reference in an elevator at 9.8m/ss. Buoyancy is wholy dependent on Gravity since it is a force and not just a mass. Buoyancy = Mass * Gravity.
Lets say that piece of wood has 1/2 the density of water. Call the density of water 1 and earth's gravity = 1 also.
The upward force of Buoyancy would be the difference in mass (1 - 1/2) * gravity or 1/2 dyne and half that sucker would be out of the water. Now double the gravity... the mass difference is still 1/2 but gravity is 2 so 1/2 * 2 = 1. Our wood's weight also increased by 2 so it also equals 1. 2 times the "up force" and 2 times the "down force" Same as it was before. No sinking deeper, no rising farther, just the same as on the beach where I'd like to be right now.

"Say Kat's planet which is 2x as attractive or twice the gravity of earth which is the same as accelerating your frame of reference in an elevator at 9.8m/ss."

I disagree with that. First, the initial state for the bucket, water, and wood is at 1 G gravity. The second state is to accelerate vertically to some rate, let's say 1 G. The act of changing from one state, a state of rest, to another, a state of acceleration, must involve Newton's first law. This is where the inertia of the wood tends to want to stay at rest, but is forced into acceleration by the water that suspends it. Since the water is fluid, unless it is frozen, it will "give" a little and the wood will sink down.

I am not quite sure I follow your explanation that follows. Maybe I am hung up on the fact that whatever force the wood has on the water must be equal to the force the water has on the wood.

It's Robhendr, just can't remember my PW and little time... d:o)

Picture yourself in the elevator not moving. Then start increasing gravity... slowly or quickly, makes no difference. you have no idea you're moving upward, just that you're getting heavier. Wood still floats exactly the same.
Then look in the bucket... (I'll hit the stop button).

Anyone recall the photo of an astronaught blowing a bubble inside of a drop of water?

No, I think Newton shall not be denied. ;-)

This is where I am struggling with the idea of simply beaming the experiment to another planet with higher gravity,

Beaming to another planet is one thing (if it was even possible). The experiment is still at rest.

However, an elevator is another situation. The artificial gravity is induced by a change in the objects position or more accurately, its velocity is changing. That is the definition of acceleration.

As you ride up the elevator you feel heavier because your inertia generates a downward force on the elevator floor in conjunction with Earth's gravity. Your physical position is changing at a rate of some meters per second per second, which is the first derivative of velocity.

Once the velocity of the elevator stops changing, then I agree with the fact that the wood will return to the same level it was floating at prior to starting the elevator. At that point your acceleration is zero.

I think that is the confusion. You have two stages to the experiment. Actually, three stages or phases. One, is where the experiment is at rest. Two, is the experiment under acceleration as the elevator goes from zero vertical velocity to its cruising velocity during ascent. Three, is the steady state velocity where the elevator's vertical velocity is constant.

During phase 1 the wood block floats at its normal level in the bucket's water.

Phase 2 the wood is under acceleration and the wood's inertia will force the wood down further into the water.

Phase 3 is when the acceleration stops and the wood floats at the same level in the bucket as it did in Phase 1.

I propose that the level that the wood floats at can only change in Phase 2 where Newton's First Law applies.

At the start and end of Phase 2 the wood will bob a little because of the elastic properties of water like a bobbin on a fishing line when cast into the water.

Here is proof!!!!

I just did this and it works! Fill a jar up with water to the brim. Place an ice cube in the jar and screw the lid on so there is no air in the jar. This prevents splashing.

Turn the jar upside down and the ice floats to the top, which is the jar's bottom.

Hold the jar down below your waist and as rapidly as possible raise the jar vertically. Be careful not to hit anything because it is glass!!

Watch the ice cube as you reach the top of your swing. The ice cube momentarily sinks into the water and then raises back up. I was able to repeatably do this so the ice cube dropped almost 1" into the water.

The ice cube is demonstrating exactly what I said would happen due to the ice cube's inertia.

QED!

Ice cubes in a jar at the top of your swing?!? I thought you were bedridden. It's probably the water moving inside as you accelerate it in the horizontal direction also. Try dropping the jar on the floor and see what happens when it inpacts the floor.... Does the ice sink? That's if the jar doesn't break. Now there's a good example of acceleration...

A force is being applied to the elevator to accelerate it's mass vertically. That force is also applied to the water itself. The water wants to stay put also. That force is applied to the water making it heavier. Heavier water makes for more bouyancy... See the great salt lake... kind of sort of the same... higher density or mass per unit volume would be the same as adding additional gravity or force in the buoyancy equation. As the wood feels the force it's apparent weight also increases the same amount. net effect, no change.

Newton is happy, Archimide is happy.

No, you move the jar vertically upward, not horizontally.

I'll let you drop the jar. ;-)

I think this was a "Challenge" question once . . . I may be wrong about the claim about it being a "Challenge" question, and such may or may not have anything to do with water and wood.

If the elevator accelerated fast enough the water might give way to the wood. The water is a fluid and would not necessarily hold its shape when subjected to pressure from another object. However, after the intial effect, I would expect that the displaced water would try to return to its previous state causing the wood to return to the surface. I guess it is possible some intermediate stage of equilibrium could be reached where the wood might remain somewhat more submerged than what it was prior to the movement of the elevator.

In the car example that Robhendr mentioned, the force that causes the balloon to move forward is the heavier air rushing to the rear of the car pushing the lighter balloon forward much the same as it "floats" in the opposite direction of g, it "floats" also in the opposite direction of a acting upon the the contents of the car. This is much like the bucket question. In order to make the helium balloon in an elevator go down, make the elevator go down. Theoretically, when you accelerate downward, the air (with it's mass) is concentrated at the top of the elevator and the absolute pressure decreases at the bottom surface. As well, the pressure (rho*a*h) at all heights within the bucket decreases. It is this water pressure that ultimately supports the block. Another way to look at it is the definition of bouyancy is F (b) = (displaced fluid density)x(gravity acceleration)x(displaced volume. If the gravity acceleration decreases, then the bouyant force also decreases. Also, the density of the displaced volume decreases slightly when accelerating downward, which further decreases this bouyant force. Actually, unless the block is waterlogged and barely floats, I don't belive the elevator would have enough acceleration to affect the block...But, yes, your son has had too much sun!

The sacific weight of both will change , depending on the brand name of the elavator. however because the water cannot be compressed, the wood will remain floating. Get the kid out of the sun it will cause cancer in 20 years or so.

Though the elevator goes up,it doesn't affect the position or the condition of thr block.this is because the greater acceleration as a result of this will also increase the buoyont force which will balance the increased weight. As a result the wood will not sink.

I think the force of gravity will pull the piece of wood to the bottom.

Any alteration of the bouyance of the wood block suspended in the bucket of water would be momentary. The elevator does not accelerate constantly. It accelerates to a specific velocity, then maintains that velocity until approaching the floor where it should stop, and then finally decelerates to zero velocity. The wood block under the additional acceleration of the elevator would first sink lower (how much lower is dependant on the acceleration rate of the elevator), then would resume its normal floatation, and finally float higher than normal as the elevator slows to a stop. For best results of such an experiment try doing it in the elevator in the CN Tower in Toronto. It may just sink to the bottom. No, the kid has not been out in the sun too long. He may, however, have a career in research ahead of him.

You are 100% right except when it decelerates at the top flloor both the water and wood will raise because there is nothing to stop the water from going up!. The water is contained only at the sides and bottom of the bucket.

Actually there are some things that prevent the water from going up. Elevators do not stop quickly enough to overcome atmospheric pressure, water tension and the fact that nature abhors a vacuum. Also, the weight of the water will not change enough to make it go flying out of the bucket. Otherwise, you probably would hit your head on the ceiling every time the elevator stopped.

First, there is no effect at constant lift upward speed. There's no experiment conducted inside the lift that could detect constant motion of the lift.

When the lift is accelerating, water and wood feel same effect, so provided both are incompressible submergence does not change. Incidentally, by Einstein's Principle of Equivalence, there's no way of distinguishing (from inside the lift) between forces caused by gravity and by acceleration.

But wood and water are both subject to increased force, and in principle, if the wood is more compressible than the water its density increases more so it sinks deeper. But with real wood and water, and the sort of acceleration the lift could provide and the occupants could stand, the effect would be quite negligible. This doesn't contradict previous para as it's same for upward acceleration and moving to a planet of greater mass.

Separately from lift motion, increased air pressure causes the wood to float higher. Higher pressure air has higher density so the part of the wood above the surface displaces greater mass of air, and it moves up so total mass of fluids (air + water) displaced still = mass of wood. Air is much more compressible than water or wood. I calculate that with a wood block 1m x 1m x 2m floating straight up (I know it wouldn't conveniently float that way in practice, it's a thought experiment) with vacuum above, 1m is out of the water. If pressure increased to 10 bara, density of air ~ 12 kg/m3, it floats 12mm higher. Easily measurable, if anybody wanted to bother.

Should have added to last bit of this comment - water density 1000 kg/m3 and assuming density of wood 500 kg/m3.

The wood will sink in the water due to the acceleration in the vertical direction. When the acceleration stops the wood will return to its previous level.

Here is the proof!!!!

I just did this and it works! Fill a glass jar up with water to the brim. Place an ice cube in the jar and screw the lid on so there is no air in the jar. This prevents splashing.

Turn the jar upside down and the ice floats to the top, which is the jar's bottom. It is easy to see the ice.

Hold the jar down below your waist and as rapidly as possible raise the jar vertically. Be careful not to hit anything because it is glass!!

Watch the ice cube as you reach the top of your swing. The ice cube momentarily sinks into the water and then raises back up when the vertical acceleration stops. I was able to repeatably do this so the ice cube dropped almost 1" into the water every time.

The ice cube is demonstrating exactly what I said would happen due to the ice cube's inertia. Newton rules!

QED!

Replying to Proof - Try this yourself by Anonymous Coward on Wednesday April 26, @11:49AM (#4502)

Interesting experiment, but I think what you're doing is producing a downward acceleration greater than g - either a reducing upward velocity or an increasing downward velocity. If this is say 1.5g, there is a net force 0.5g towards the top of the room. So the system behaves as it would with the jar sitting right way up on a table. It sees gravity only 0.5g instead of 1g, but that's a detail - the ice-cube of course rises to the top. In your experiment it's similar, but the conditions don't last long enough for it to reach the top.

You'll see the same effect, probably more so, if you start with the jar above your head and pull it down rapidly (faster than it would fall).

No. The cube will drop into the water because the cube wants to remain at rest when the jar is accelerated upward.

Try it and you will see that it works as advertised.

The jar is only turned upside down so that you can better see the ice cube. Let the ice float to the top, when it has settled you then thrust the jar upward and the cube will drop into the water during the vertical acceleration phase of the experiment and float back up when the acceleration stops.

Try the experiment yourself.

Replying to Anonymous Coward on Wednesday April 26, @01:39PM (#4512)

I'll try it this evening (UK time), but I remain sceptical. Accelerating the jar upwards is equivalent to a downward g-force, so total g is above normal. A force is needed to fully submerge an ice cube (which is trying to float) and this force is proportional to the total g, so the cube presses harder upwards against the bottom of the (upturned) jar than normal.

Of course it can't rise during the upward acceleration but you clearly have to reverse the acceleration at some point to bring the jar to a stop, and I believe this is causing the observed fall, for reason given in my previous posting.

Cheers

Okay, so you presented a theory; why not test it?

I am confident that if you try it for your self you will see the same results I have observed.

Replying to Anonymous Coward, Thursday April 27, @08:32AM (#4559

As indicated in previous posting, I did the experiment last night and results were as I predicted. The cube moved downwards for the last few inches of upward movement, when the jar was being decelerated to a stop. It also moved downwards when the jar was pulled rapidly downwards.

Anybody else out there like to try it? be interested in your observations. It doesn't need to be an ice cube, a cork works better, because the effect depends on the difference of densities between water and cube or cork, and cork is a lot lighter than ice.

Another experiment would be to use a longish jar held horizontally in an accelerating car - I predict the cork moves forward, analogous to the helium balloon in car situation. With an ordinary cork friction with the glass might stop it moving, better to use a spherical shape. Need to make sure the road's flat and the jar stays horizontal to avoid spurious movements, but no major difficulty here.

To reply to your comment from earlier posting - "The cube will drop into the water because the cube wants to remain at rest when the jar is accelerated upward" - the water also wants to remain at rest, and the whole system is subject to same total downward g, due to gravity plus upward acceleration. This causes a higher upthrust on the submerged cube than the static (gravity-only) case.

I tried a different idea and laid the jar over horizontally and spun it around me in a horizontal plane. Just like the balloon, the wooden plug (I used oak) moved forward in the same direction as the jar.

However, I am not sure that the same forces that propel the wood and the balloon forward under horizontal acceleration would be the same for vertical acceleration with the wood floating on the water's surface. They might be, but I don't understand why.

Replying to Anonymous Coward, Sunday April 30, @10:54AM (#4612)

Sounds like you held the jar with axis along circumference of circle. Forward movement of the plug suggests forward acceleration (as with car/balloon), but circular motion irrelevant, it might as well have been acceleration in a straight line.

Better to have the axis of the jar radial, so the centrifugal force acts like g away from the centre of the circle, and along the axis of the jar. The plug would move towards the centre of the circle, i.e. opposite direction from the g. Spinning the jar in a circle is a good idea as it can be maintained as long as you like, unlike raising or lowering it.

These experiments are just like holding a cork under water in a (stationary) bucket and releasing it, except that in that case you have to hold/release it because you can't turn Earth's g on and off. With the spinning jar you can put the cork part way along, then start the acceleration.

Incidentally, if the circle is 1m radius (~ arm's length) need to rotate at ~ 30 rpm to give centrifugal force = g.

Submerged corks move, or press harder against surface, depending on direction of acceleration, but a floating block is in equilibrium with the water and altering the effective g by acceleration or moving to Jupiter affects block and water equally so causes no change (as the setter's answer says).

The different densities of the wood and water are what cause the wood float. The elevator adding to grav's force will be equal on the wood, and the water, and the block will not change position. On the other hand, if the elvator cable snaps, and all masses begin freefalling, the wood block may be able to sink since the mass of water will exert no directed force at all, and will simply flow around the block........for a few seconds anyway ;-)

In order for the wood to sink the acceleration of the elevator would have to be so great you would not see it anyway because the blood in your brain would be in your feet.

Depending on variables such as size & type of wood,degree of saturation,barametric pressure,air temp,water temp,shape & sidewall flex ratio + bucket material comp; along with water purity & oxygen content assuming the elevator is indoors with no 300+ mph down drafts & whether or not it's salt water the wood would sink to varying degrees as long as the boy put it (the bucket) down real fast and hard .Who said anything about the elevator moving? Just drop a jar of water & a toothpick-with or without a lid.Next question?

Ref#4537-I was A.Caward in this instance,couldn't log in then.Ain't skaared.

General Relativity says that there is no way that the 'bucket/water/floating wood' system accelerating at a constant acceleration "a" could "know" the difference between being accelerated in an elevator or being placed in a gravitational field such as a planet more massive than Earth with a G equal to g(earth)+a(elevator). The level at which an object floats in a liquid is dependent upon the ratios of the densities of the floating object and the liquid in which it is floating and is independent of "g". Therefore, whether the block of wood is floating in a bucket of water on the Earth, being constantly accelerated in an elevator, sitting on the moon, or on Jupitor (assuming the water dosent freeze), the wood will float at the same level.

But the elevator transfers from a 1 g stationary environment to a higher G environment. The transition from steady state to one of upward acceleration will force the wood to drop downward for the duration of the acceleration (see my ice experiment above).

Next, the acceleration phase stops when the elevator gets up to its normal speed. At that point the block returns to its initial relative level in the bucket.

I am only Anonymous Coward (II or more) because the system won't let me log on (despite recognising me enough to allow me to change my password) I think the first Anonymous Coward is right, and the analogy of chucking a log into water (and changing the frame of reference) is apposite. The reason why the explanation in the newletter is misleading is that it tries to pick itself up by its own bootlaces. It says that it is the same g on both sides of the equation. But one needs to assume that the log does not sink to say that. If the log does sink, then g is different for the log compared to the bucket. I think rapidrobot is wrong about the small boat being uplifted by the wave - I think the initial effect is to make the level of water rise on the hull. He or she admits that the reverse happens when the wave subsides - which is the giveaway. Alan

Reply to Xeni (1120) Thursday April 27, @03:51PM (#4573)

Agreed, I said same thing in different words in posting on Wednesday.

Elevator won't do it! Have your son get his friend to push down on the block of wood. After it is submerged your son can say "See, I never touched it!"