Struggling with this problem....

[captain nemo1701]

I think it would weigh the same. Aren't we in danger of breaking one of the basic laws of thermodynamics here?.

I recall a similar question years ago: Imagine a truck that has birds in the rear. Would it weigh the same if they all took off & hovered?.

Wheres Brian Cox when you need him?....

 

[MartinQ]

I think it would weigh the same. Aren't we in danger of breaking one of the basic laws of thermodynamics here?.

I recall a similar question years ago: Imagine a truck that has birds in the rear. Would it weigh the same if they all took off & hovered?.

Wheres Brian Cox when you need him?....

The truck would weigh the same if the birds were not touching the truck. I guess if the truck was air tight, the air would act downwards and the truck would have to have a corresponding reaction. If the truck was not airtight, it would be different.

 

[Yellow Fang]

I think it will weigh slightly less. I think a bit of potential energy has changed to kinetic energy when the sand is falling. If the sand grains were as only as dense as air then there would be no change in weight, but then the sand would not fall. Also, you are weightless in orbit, and in orbit you are always falling. It's just you are falling the same rate at which the earth is receding from you because of sidewards motion. That's my guess.

 

[PeteXXX]

What if I had a digital timer?

Some of the numbers have less lines to display (ie 8 has 7 lines, whereas 1 has only 2) so would 58 minutes weigh less than 11 minutes?

 

[derrick]

Dears

This was on R4 the other morning and I have not heard the answer:

If you have an hour-glass/egg timer and you weigh it, then turn it over to allow the sand to run, does it weigh any less because some of the sand particles are in mid-air as they fall?
I think it will weigh very slightly less. @Hill Wimp thinks it will weigh the same.

Please respond with your thoughts UNLESS you heard the answer, then please keep it to yourself for a while OR hide it as a 'Spoiler'.

Cheers
FF

You must be really bored.

 

[wafter]

@Fab Foodie and @Hill Wimp

Physics has your answer:

https://www.europhysicsnews.org/articles/epn/pdf/2010/03/epn2010413p25.pdf

A special feature of an hourglass is that the mass flow m [kg/s] of sand through the narrow orifice is fairly independent of the sand level. This is caused by the granular nature of the sand. The flow m is present in the free-falling stream of sand but also in the slowly moving dense mass in the upper compartment. If the free-fall time of the sand is τ, the weight of the free-falling mass is g.(m. τ). When hitting the surface in the lower compartment, it exerts a momentum per second, i.e., a downward force (gτ) on the surface. This force exactly compensates the missing weight of the sand that is in free fall. This was the beautiful argument to conclude that the weight should stay the same.

However, in 1985 Shen and Scott showed that the centre of mass (c.o.m.) of the sand is not moving down at constant speed, but is decelerating during the steady operation of an hourglass. That causes a subtle gain of its weight on a balance. With an ingeniously constructed hourglass they even managed to confirm the presence of excess weight.

So........having said that, what has one said? .......it appears there is an answer but I'm not clever enough to understand it. I think we can confidently say that it might weigh the same, it might be heavier or it might be lighter.

I was about to say the same as the quote describes; although I think this also calls into question the definition of "weight" - which (IIRC) is defined as the force exerted by a mass being acted on under gravitational acceleration" or similar..

If it's assumed that this must be under static conditions then while the weight registered by the scale increases due to the scale reacting the force of the constant stream of sand impacting the bottom of the scale, the actual weight of the object cannot change as the moving components must be discounted.

It's like jumping up and down on the bathroom scales - while the weight value changes your intrinisic mass and gravitational acceleration remain the same; you're just skewing the value by introducing conditions to which the definition of "weight" do not apply.

 

[classic33]

The truck would weigh the same if the birds were not touching the truck. I guess if the truck was air tight, the air would act downwards and the truck would have to have a corresponding reaction. If the truck was not airtight, it would be different.

View: https://m.youtube.com/watch?v=lVeP6oqH-Qo


And
https://mythresults.com/episode77

 

[slowmotion]

The hour glass is a confined environment. Who cares what the sand grain particles are doing? They could be getting all jiggy and writhing about but the air around them and their non-jiggy pals still have the same mass. Their mid-air acrobatics make no difference.