New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

deepfake vids or it really happened

SCIENCE said:

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

deepfake vids or it really happened

No vids here.

It’s all in people’s heads.

The Rev Dodgson said:

SCIENCE said:

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

deepfake vids or it really happened

No vids here.

It’s all in people’s heads.

we imagine it would not noticeably move relative to lift observer but we haven’t done the experiment yet so as SCIENCE we defer our answer

SCIENCE said:

The Rev Dodgson said:

SCIENCE said:

deepfake vids or it really happened

No vids here.

It’s all in people’s heads.

we imagine it would not noticeably move relative to lift observer but we haven’t done the experiment yet so as SCIENCE we defer our answer

OK, here’s a vid then:

https://www.youtube.com/watch?v=jkKEMF38FIQ

He uses a horizontal lift, but same principle.

The Rev Dodgson said:

SCIENCE said:

The Rev Dodgson said:

No vids here.

It’s all in people’s heads.

we imagine it would not noticeably move relative to lift observer but we haven’t done the experiment yet so as SCIENCE we defer our answer

OK, here’s a vid then:

https://www.youtube.com/watch?v=jkKEMF38FIQ

He uses a horizontal lift, but same principle.

deepfaked

SCIENCE said:

The Rev Dodgson said:

SCIENCE said:

we imagine it would not noticeably move relative to lift observer but we haven’t done the experiment yet so as SCIENCE we defer our answer

OK, here’s a vid then:

https://www.youtube.com/watch?v=jkKEMF38FIQ

He uses a horizontal lift, but same principle.

deepfaked

And all the ones showing hydrogen balloons in cars moving in the direction of acceleration?

wait so are they saying the ballons move in the acceleration direction or in the opposite direction ¿

SCIENCE said:

wait so are they saying the ballons move in the acceleration direction or in the opposite direction ¿

So long as the lift/car/container is accelerating, the balloon moves in the direction of acceleration, relative to the container.

The Rev Dodgson said:

SCIENCE said:

wait so are they saying the ballons move in the acceleration direction or in the opposite direction ¿

So long as the lift/car/container is accelerating, the balloon moves in the direction of acceleration, relative to the container.

the reason being the air is rarefied, due to the acceleration, more in the direction of travel so the balloon moves in that direction.

ChrispenEvan said:

The Rev Dodgson said:

SCIENCE said:

wait so are they saying the ballons move in the acceleration direction or in the opposite direction ¿

So long as the lift/car/container is accelerating, the balloon moves in the direction of acceleration, relative to the container.

the reason being the air is rarefied, due to the acceleration, more in the direction of travel so the balloon moves in that direction.

I prefer to think of it being more compressed at the back :)

The Rev Dodgson said:

ChrispenEvan said:

The Rev Dodgson said:

So long as the lift/car/container is accelerating, the balloon moves in the direction of acceleration, relative to the container.

the reason being the air is rarefied, due to the acceleration, more in the direction of travel so the balloon moves in that direction.

I prefer to think of it being more compressed at the back :)

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

Smarter Every Day, helium balloon in car.
A pretty darn good YT channel.

SCIENCE said:

The Rev Dodgson said:

ChrispenEvan said:

the reason being the air is rarefied, due to the acceleration, more in the direction of travel so the balloon moves in that direction.

I prefer to think of it being more compressed at the back :)

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

OK, but I didn’t mention they said a “slack string”. Depends how slack I suppose.

The Rev Dodgson said:

SCIENCE said:

The Rev Dodgson said:

I prefer to think of it being more compressed at the back :)

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

OK, but I didn’t mention they said a “slack string”. Depends how slack I suppose.

plus nothing is totally rigid and non-elongateable.

The Rev Dodgson said:

SCIENCE said:

The Rev Dodgson said:

I prefer to think of it being more compressed at the back :)

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

OK, but I didn’t mention they said a “slack string”. Depends how slack I suppose.

if it were an elastic string

ol’ Julius had something about a cork attached by spring to the bottom of a bucket of water, dropped

SCIENCE said:

The Rev Dodgson said:

SCIENCE said:

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

OK, but I didn’t mention they said a “slack string”. Depends how slack I suppose.

if it were an elastic string

ol’ Julius had something about a cork attached by spring to the bottom of a bucket of water, dropped

I’ll have to think about that one.

The ice cube in the cup in the lift, I thought would go lower in the water, because it would need to displace more water to transfer the acceleration force, but their answer is that the effective weight of the water is increased too, so it still displaces the same volume, which I now think is correct.

ChrispenEvan said:

The Rev Dodgson said:

SCIENCE said:

¿¡
yes but we thought being on a string that constrains it from moving in that direction would constrain it from moving in that direction

OK, but I didn’t mention they said a “slack string”. Depends how slack I suppose.

plus nothing is totally rigid and non-elongateable.

yes hence “not noticeably move relative to lift observer”, though in the other lift, we’d‘v’ also expected an unnoticeable relative movement opposite to the direction of acceleration as the process is not quasistatic

as before, haven’t done the experiment and need a high speed camera

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

If the hydrogen balloon was not on a string, when the lift accelerates upwards we would expect the balloon to accelerate upwards faster, ie to rise with respect to the elevator compartment.

However, taking the problem as written, it can’t do that because of the string. Oh, there might be a few microns due to stretching I suppose.

SCIENCE said:

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

Ditto

PS. I take it you’ve all taken a hydrogen balloon in a car. As the car accelerated forwards, the hydrogen balloon accelerates forwards faster than the car.

Lift is full of water, as for air, because hydrogen is less dense than water.

mollwollfumble said:

SCIENCE said:

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

Ditto

PS. I take it you’ve all taken a hydrogen balloon in a car. As the car accelerated forwards, the hydrogen balloon accelerates forwards faster than the car.

Lift is full of water, as for air, because hydrogen is less dense than water.

I have to admit I’ve never had a hydrogen balloon …

dv said:

mollwollfumble said:

SCIENCE said:

what if the lift is full of water

Ditto

PS. I take it you’ve all taken a hydrogen balloon in a car. As the car accelerated forwards, the hydrogen balloon accelerates forwards faster than the car.

Lift is full of water, as for air, because hydrogen is less dense than water.

I have to admit I’ve never had a hydrogen balloon …

All ur blow up dolls are using air?

roughbarked said:

dv said:

mollwollfumble said:

Ditto

PS. I take it you’ve all taken a hydrogen balloon in a car. As the car accelerated forwards, the hydrogen balloon accelerates forwards faster than the car.

Lift is full of water, as for air, because hydrogen is less dense than water.

I have to admit I’ve never had a hydrogen balloon …

All ur blow up dolls are using air?

semtex

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

The string wasn’t taut to start with.

dv said:

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

If the hydrogen balloon was not on a string, when the lift accelerates upwards we would expect the balloon to accelerate upwards faster, ie to rise with respect to the elevator compartment.

However, taking the problem as written, it can’t do that because of the string. Oh, there might be a few microns due to stretching I suppose.

The string wasn’t taut to start with.

SCIENCE said:

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

Good question.

Let’s suppose the balloon is supporting something heavy, so it can still be on a slack string.

My first thought was that it now becomes equivalent to the ice cube in the cup, but it isn’t because the increased pressure of the water causes the balloon to shrink, so it displaces less water, so it moves downwards, relative to the lift.

The important difference between the floating body in water and the floating balloon in air is that air is compressible, so when the lift accelerates upwards you get a pressure gradient in the air, and this is what makes the balloon accelerate upwards faster, until the string becomes taught.

mollwollfumble said:

SCIENCE said:

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

Ditto

PS. I take it you’ve all taken a hydrogen balloon in a car. As the car accelerated forwards, the hydrogen balloon accelerates forwards faster than the car.

Lift is full of water, as for air, because hydrogen is less dense than water.

From yesterday:

OK, here’s a vid then:

https://www.youtube.com/watch?v=jkKEMF38FIQ

He uses a horizontal lift, but same principle.

The Rev Dodgson said:

SCIENCE said:

mollwollfumble said:

I suspect New Scientist is right.

The hydrogen balloon experiences a net downward force due to the upwards acceleration of the lift. But this downward force never exceeds the upwards force of the balloon on the string, so the string stays taut.

what if the lift is full of water

Good question.

Let’s suppose the balloon is supporting something heavy, so it can still be on a slack string.

My first thought was that it now becomes equivalent to the ice cube in the cup, but it isn’t because the increased pressure of the water causes the balloon to shrink, so it displaces less water, so it moves downwards, relative to the lift.

The important difference between the floating body in water and the floating balloon in air is that air is compressible, so when the lift accelerates upwards you get a pressure gradient in the air, and this is what makes the balloon accelerate upwards faster, until the string becomes taught.

There are pressure gradients in water, which will be higher under upwards acceleration.

P = P ₀ + (h ₀ – h) * rho * (g + a)

dP/dh = – rho * ( g + a)

OTOH the density gradients will be negligible, at least over the scale of an elevator compartment.

The Rev Dodgson said:

dv said:

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

If the hydrogen balloon was not on a string, when the lift accelerates upwards we would expect the balloon to accelerate upwards faster, ie to rise with respect to the elevator compartment.

However, taking the problem as written, it can’t do that because of the string. Oh, there might be a few microns due to stretching I suppose.

The string wasn’t taut to start with.

Then yeah the balloon will accelerate upward faster than the lift compartment: it will rise, relative to the compartment. They should probably use a helium balloon for safety reasons but that’s by the by.

for repeatability maybe use 99 lift balloons.

ChrispenEvan said:

for repeatability maybe use 99 lift balloons.

I hear the joke police sirens … Nina Nina Nina Nina

dv said:

The Rev Dodgson said:

SCIENCE said:

what if the lift is full of water

Good question.

Let’s suppose the balloon is supporting something heavy, so it can still be on a slack string.

My first thought was that it now becomes equivalent to the ice cube in the cup, but it isn’t because the increased pressure of the water causes the balloon to shrink, so it displaces less water, so it moves downwards, relative to the lift.

The important difference between the floating body in water and the floating balloon in air is that air is compressible, so when the lift accelerates upwards you get a pressure gradient in the air, and this is what makes the balloon accelerate upwards faster, until the string becomes taught.

There are pressure gradients in water, which will be higher under upwards acceleration.

P = P ₀ + (h ₀ – h) * rho * (g + a)

dP/dh = – rho * ( g + a)

OTOH the density gradients will be negligible, at least over the scale of an elevator compartment.

Yes, density gradients are what I meant.

The Rev Dodgson said:

dv said:

The Rev Dodgson said:

Good question.

Let’s suppose the balloon is supporting something heavy, so it can still be on a slack string.

My first thought was that it now becomes equivalent to the ice cube in the cup, but it isn’t because the increased pressure of the water causes the balloon to shrink, so it displaces less water, so it moves downwards, relative to the lift.

The important difference between the floating body in water and the floating balloon in air is that air is compressible, so when the lift accelerates upwards you get a pressure gradient in the air, and this is what makes the balloon accelerate upwards faster, until the string becomes taught.

There are pressure gradients in water, which will be higher under upwards acceleration.

P = P ₀ + (h ₀ – h) * rho * (g + a)

dP/dh = – rho * ( g + a)

OTOH the density gradients will be negligible, at least over the scale of an elevator compartment.

Yes, density gradients are what I meant.

And just to labour the point, the increased pressure gradient is what provides the additional force to accelerate the ice cube upwards at the same rate as the lift. The balloon also has a density gradient in the surrounding air, and this results in an additional force and additional upward acceleration.

so my joke about pressure gradient is now being discussed seriously?

ChrispenEvan said:

so my joke about pressure gradient is now being discussed seriously?

The joke was a little rarefied for my tastes.

(I actually thought it was a good statement of what happens)

The Rev Dodgson said:

dv said:

The Rev Dodgson said:

New Scientist have a weekly puzzle. Last week it asked if you hold a hydrogen balloon on a string in a lift, what happens when the lift accelerates upwards?

This week it says the balloon doesn’t move relative to you (or the lift). You all move up together.

I’m pretty sure that is wrong.

What do you think?

(They also asked about an ice cube floating in a cup and a spider hanging on a thread. You can answer those as well if you like)

If the hydrogen balloon was not on a string, when the lift accelerates upwards we would expect the balloon to accelerate upwards faster, ie to rise with respect to the elevator compartment.

However, taking the problem as written, it can’t do that because of the string. Oh, there might be a few microns due to stretching I suppose.

The string wasn’t taut to start with.

implying the average density of balloon is actually greater than or equal to ambient, in which case it still remains stationary in frame, on the floor

Hang on I’m not believing Rev about the balloon in water. Basic action considerations make this seem wrong.

SCIENCE said:

The Rev Dodgson said:

dv said:

If the hydrogen balloon was not on a string, when the lift accelerates upwards we would expect the balloon to accelerate upwards faster, ie to rise with respect to the elevator compartment.

However, taking the problem as written, it can’t do that because of the string. Oh, there might be a few microns due to stretching I suppose.

The string wasn’t taut to start with.

implying the average density of balloon is actually greater than or equal to ambient, in which case it still remains stationary in frame, on the floor

I’d say it implies the balloon is floating, but with some slack in the string, so it will accelerate upwards.

dv said:

Hang on I’m not believing Rev about the balloon in water. Basic action considerations make this seem wrong.

Can you expand on that?

A helium balloon in the middle of a water filled compartment is already going to be rising rapidly because of gravitational acceleration. Increasing the acceleration should increase the forces making it rise, and hence make it rise faster.

dv said:

A helium balloon in the middle of a water filled compartment is already going to be rising rapidly because of gravitational acceleration. Increasing the acceleration should increase the forces making it rise, and hence make it rise faster.

You missed the bit about adding a weight to the balloon to make the average density the same as the water, so it starts off similar to an ice block (except it’s fully under water), but it is also compressible, so will behave differently to the ice block.

The Rev Dodgson said:

dv said:

A helium balloon in the middle of a water filled compartment is already going to be rising rapidly because of gravitational acceleration. Increasing the acceleration should increase the forces making it rise, and hence make it rise faster.

You missed the bit about adding a weight to the balloon to make the average density the same as the water, so it starts off similar to an ice block (except it’s fully under water), but it is also compressible, so will behave differently to the ice block.

Oh right. Yes I did miss that.