Investigating the Weight of an Hourglass with Sand

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In summary, the conversation discusses the difference in weight of an hourglass filled with sand when it is tipped over and the sand is falling down versus when the sand is on the bottom and not moving. The discussion includes the calculation of normal force and the concept of weight versus mass. The conclusion is that the weight of the hourglass will appear to be less when the sand is falling, due to the acceleration of the center of mass of the closed system.
  • #1
bjon-07
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My proffesor assigned the question to the class.

Will an hourglass filled with sand weigh any different if you tip over (i.e. the sand is on the top falling down vs. the sand is on the bottom not moving)


If the sand is on the top. a few grains of sand will be in free fall (these would be weightless) however when those grains of sand it the bottom of the glass they will have KE which should in theory increase the doward force the hour glass itself is pushing on the scale.

In class we calculated the normal force a failing chaing of height L to be N=3mgs/l.

I am having a really hard time with this problem. Can some one please help me. I will be forever greatfull.


I need to mathmatically prove what will happen in this experiment.


btw I did this in this experiment on a scale in the chem lab of my school. The two weighted the same. (not that the scale I used was not very accurate)
 
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  • #2
bjon-07 said:
Will an hourglass filled with sand weigh any different if you tip over (i.e. the sand is on the top falling down vs. the sand is on the bottom not moving)
An hourglass is a closed system, so it can't gain or lose mass. The individual grains of sand that fall gain relative kinetic energy, but the overall system doesn't. Can't help with the math. Sorry.
 
  • #3
Danger said:
An hourglass is a closed system, so it can't gain or lose mass. The individual grains of sand that fall gain relative kinetic energy, but the overall system doesn't.

This was already discussed in the homework section, but remember that there's a difference between weight and mass. Weight has units of force, so it depends on the internal motion as well as the mass. To see that something weighs a different amount while its content are in free fall, just imagine a large sealed bag of groceries. What happens when you turn it over? Does the weight change when the groceries hit the bottom?
 
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  • #4
SpaceTiger said:
Weight has units of force, so it depends on the internal motion as well as the mass. To see that something weighs a different amount while its content are in free fall, just imagine a large sealed bag of groceries. What happens when you turn it over? Does the weight change when the groceries hit the bottom?
I see your point, but disagree. Perhaps it's because I'm using improper definitions. Bear in mind that I have no formal education. To me, weight has always meant the measured quantity of mass within a gravitational or accelerating environment, and therefore doesn't have units of energy attached. What you describe would be impact energy in my understanding. If that's the case, then the weight would change every picosecond as new grains of sand intermittently hit bottom. You wouldn't be able to get an accurate measurement. If this is an incorrect assessment, please explain further. I hate having my facts wrong; it's worse than not having any at all. :redface:

edit: If that is also explained in the homework section, can you tell me where or provide a link? I have a lot of trouble trying to find things around here.
 
  • #5
There seems to be some confusion about the meaning of "weight". There are two useful meanings:
(1) "true" weight: this is a measure of the gravitational force exerted on an object. The weight of an object of mass m at the Earth's surface is mg.
(2) "apparent" weight: this is a measure of the supporting force an object experiences.​
It is apparent weight that changes when an object accelerates; it is apparent weight equalling zero that is called "weightlessness".

In this problem, the hour glass is presumed to rest on a scale; that scale reads the apparent weight of the hour glass. (Its true weight will not change.)


https://www.physicsforums.com/showthread.php?t=70670
 
  • #6
The hourglass with the falling sand will register a smaller reading on a scale.
This is because the center of mass of the closed system of the glass and the sand is accelerating downward due to the downward acceleration of the falling sand.
Therefor the upward normal force on the system (which is what the scale interprets as "weight") will be less than Mg, where M is the mass of the sand plus glass.
To give a numerical answer, you would have to know the mass of the sand in the air, acelerating down.
 
  • #7
Meir Achuz said:
This is because the center of mass of the closed system of the glass and the sand is accelerating downward due to the downward acceleration of the falling sand.

The center of mass is moving downward, but it is not necessarily accelerating. You should envision the falling sand as a standing vertical column, and not as a falling piece of mass.
 
  • #8
Doc Al said:
There seems to be some confusion about the meaning of "weight".
Thanks a million, Doc. The link only confused the hell out of me, but I like your explanation preceding it. It makes sense now. I've never heard of 'apparent weight' as an actual entity. Obviously, Space Tiger and I were referring to different things, and his explanation is the one that would be appropriate under the given circumstances. My apologies to Bjon-07 for misleading you, and to Tiger for contradicting you. :redface:
I'm going back to GD; I only have to be funny there, not smart. :biggrin:
 
  • #9
Danger said:
My apologies to Bjon-07 for misleading you, and to Tiger for contradicting you.

You don't need to apologize. If you ask me, people shouldn't be afraid to speak up just because they think they might be wrong. That's part of how we learn. I didn't know that there were two kinds of "weight", so I learned something as well.
 
  • #10
SpaceTiger said:
You don't need to apologize. If you ask me, people shouldn't be afraid to speak up just because they think they might be wrong. That's part of how we learn. I didn't know that there were two kinds of "weight", so I learned something as well.
I think that this might be the beginning of a beautiful friendship, if you don't mind me quoting the Bogieman. I wasn't kidding about the 'no formal education' thing (as mentioned elsewhere, I never graduated high school). In instances like this where HS or university learning is in advance of my own, I will always defer to the more educated combatent. (Unless, of course, he says something really stupid... but that seems to be restricted to GD.)
 

1. What is the purpose of investigating the weight of an hourglass with sand?

The purpose of this investigation is to understand the relationship between the amount of sand in an hourglass and its weight. This can help us determine the accuracy of hourglasses for timekeeping and also provide insight into the properties of sand.

2. How do you measure the weight of an hourglass with sand?

To measure the weight of an hourglass with sand, you will need a scale. Place the hourglass on the scale and record the weight. Then, carefully turn the hourglass over and allow all the sand to flow to the other side. Record the new weight. The difference between the two weights is the weight of the sand in the hourglass.

3. How does the amount of sand affect the weight of an hourglass?

The amount of sand in an hourglass directly affects its weight. As more sand is added, the weight of the hourglass will increase. This is because the sand has mass and therefore adds to the overall weight of the hourglass.

4. What factors can affect the weight of an hourglass with sand?

The weight of an hourglass with sand can be affected by several factors, such as the type and size of sand used, the shape and material of the hourglass, and any external forces acting on the hourglass (e.g. air resistance).

5. How can the findings from this investigation be applied in real-life scenarios?

The findings from this investigation can be applied in various real-life scenarios, such as in the manufacturing of hourglasses for accurate timekeeping or in the study of the properties of sand. It can also provide insight into the principles of mass and weight, which are important concepts in physics and engineering.

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