Physics puzzle

Huh?

That wasn't a great explanation reading back. I presume that Tbourner is asking about wind resistance on the bucket?

I meant that it has no bearing, as it only matters that the bucket is free falling and therefore accelerating at it's maximum.

but it will only accelerate up to a point - and that point, in air, will be a slower speed that it could achieve in a vacuum, wouldn't it?

So, if the air was slowing it then it wouldn't quite be in relative zero G (this is what I think Trev was getting at anyway)

but it will only accelerate up to a point - and that point, in air, will be a slower speed that it could achieve in a vacuum, wouldn't it?

Aye, it will reach terminol velocity at some point (from a very very high place), and it would be slower than in air, so I quess air resistance would have an effect on the experiment. I wonder how much though.

but it will only accelerate up to a point - and that point, in air, will be a slower speed that it could achieve in a vacuum, wouldn't it?

 

So, if the air was slowing it then it wouldn't quite be in relative zero G (this is what I think Trev was getting at anyway)

Yep.

I think we've agreed that with no wind resistance the bucket (and water) would fall at it's terminal velocity, so the water wouldn't be able to displace the cork and it would stay at the bottom - but if there WAS wind resistance the buckets velocity would be slightly less, so the water would be trying to fall slightly quicker and the cork would rise up very slowly.

That's what my head sees but I was asking if it's true cos I don't actually know.

Yep.

 

I think we've agreed that with no wind resistance the bucket (and water) would fall at it's terminal velocity, so the water wouldn't be able to displace the cork and it would stay at the bottom - but if there WAS wind resistance the buckets velocity would be slightly less, so the water would be trying to fall slightly quicker and the cork would rise up very slowly.

That's what my head sees but I was asking if it's true cos I don't actually know.

Yeah I'd go with that

Because lots of other people in your demographic(s) have. Simples.

yeah what he said....

I really should have kept the Actuary thing to myself shouldnt i!..

I work in commercial lines and not personal lines (which is the car insurance type of stuff).. if that stops people swearing at me at renewal time!

Surely with the air resistance the cork would fall? The resistance would hold the bucket back, the bucket would hold the water back so the cork would fall at 9.81m/s/s?

Surely with the air resistance the cork would fall? The resistance would hold the bucket back, the bucket would hold the water back so the cork would fall at 9.81m/s/s?

What do you mean 'the cork will fall'?

Aye, it will reach terminol velocity at some point (from a very very high place), and it would be slower than in air, so I quess air resistance would have an effect on the experiment. I wonder how much though.

I think we've agreed that with no wind resistance the bucket (and water) would fall at it's terminal velocity,

Eh?? In a vacuum it won't have a terminal velocity!

Freefalling objects in air reach a terminal velocity when the drag produced by the air exerts a decerative force equal and opposite to the accelerative force produced by gravity. Therefore in a vacuum, the falling body will continue to accelerate @ 9.81m/s/s ad infinitum because there is no decelrative force acting against gravity.

In such a situation, the bucket, the water and the cork will all stay static relative to each other - whether one is contained within the other or they are falling side by side. The fact that the cork is in the water which is in the bucket is a red herring that's just meant to confuse you.

This is a difficult one to get your head round. It was for me anyway.

A cork will normally float to the top of a stationary bucket of water as the relative density of the cork is less than the relative density of the water. Density is simply a mass per unit volume. The cork is therefore buoyant, and the force of this buoyancy will cause the cork to rise to the top of the water.

When stationary, the bucket, water and cork are all subjected to gravity in the same way, and their relative densities will cause the cork to rise.

When dropped from the tall building, the bucket, water and cork are effectively under freefall conditions, and therefore weightless for the duration of the fall to the ground.

As the water and the cork are weightless, their relative densities play no part in the problem, and therefore the cork has no buoyancy force within the water and cannot rise.

Buoyancy is dependent on displacing a volume of water than weights more than your own weight. Cork is a lighter material than water in terms of volume to weight, so it floats. Even in free-fall, whether at or less than terminal velocity both the water and the cork are experiencing the same gravitational force, so the cork should still float up. I think the air-resistance comment is a red herring as it only matters to the bucket, not the water OR the cork - think of the experiments on the moon of dropping a hammer and

a feather (or whatever it was) to prove that rates of falling are dependent on air resistance.

Oh, and removing air resistance doesn't create free fall conditions - the absence of measurable gravity. It is only possible to SIMULATE free fall by creating an artificial environment where the floor falls away at an increasing rate compared to your own fall - eg flying a plane in an inverted parabolic arc.

BTW, scuba tanks do float once some of the air has been used, reducing their weight below that of the water it displaces.

Edit: too slow, thanks Ewen! In physics though - weight is mass multiplied by gravitational force. If the bucket is falling, it must be because it is in a gravitational field, therefore it all has a weight. So surely the principles of buoyancy still apply...

Buoyancy is dependent on displacing a volume of water than weights more than your own weight. Cork is a lighter material than water in terms of volume to weight, so it floats. Even in free-fall, whether at or less than terminal velocity both the water and the cork are experiencing the same gravitational force, so the cork should still float up. I think the air-resistance comment is a red herring as it only matters to the bucket, not the water OR the cork - think of the experiments on the moon of dropping a hammer and

a feather (or whatever it was) to prove that rates of falling are dependent on air resistance.

 

Oh, and removing air resistance doesn't create free fall conditions - the absence of measurable gravity. It is only possible to SIMULATE free fall by creating an artificial environment where the floor falls away at an increasing rate compared to your own fall - eg flying a plane in an inverted parabolic arc.

 

BTW, scuba tanks do float once some of the air has been used, reducing their weight below that of the water it displaces.

When falling weight is meaningless though. If you take away air resistance, ie a vacuum, you will fall at 9.81m/s/s as long as you are within 1000m of sea level iirc. With no air resistance there is no terminal velocity.

Eh?? In a vacuum it won't have a terminal velocity!

I know, Tbourner just asked the question about what the score is with wind resistance though.

I know, Tbourner just asked the question about what the score is with wind resistance though.

I'm sure air resistance is ignored for this problem.

I'm sure air resistance is ignored for this problem.

I know!! But we went off topic

I know!! But we went off topic

:)Ah, I'll shut up then.

So, what do I win?

(and don't say a cork and a bucket )

ok,

I've just carried out this experimentation myself and the conclusion is,....

..... anyone know how to get a bucket shaped dent out of my Supra's roof.

ok,

 

I've just carried out this experimentation myself and the conclusion is,....

 

 

..... anyone know how to get a bucket shaped dent out of my Supra's roof.

At least you can console yourself with that freshly opened bottle of wine

At least you can console yourself with that freshly opened bottle of wine

... well played.

Can we have the answer please?

Can we have the answer please?

The answer is the cork will stay at the bottom.

http://www.mkivsupra.net/vbb/showpost.php?p=2593566&postcount=60

The site with the best explanation / answer comes up with a Malware warning so I wont link it.