Density Lab B

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Re: Density Lab B

Postby Nba2302 » November 30th, 2018, 2:48 am

I am really confused on hands on task, part 4. I know it uses Archimedes principle, but how are you supposed to know how deep the object will go. I couldn't find it anywhere.

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Re: Density Lab B

Postby knightmoves » November 30th, 2018, 11:19 am

I am really confused on hands on task, part 4. I know it uses Archimedes principle, but how are you supposed to know how deep the object will go. I couldn't find it anywhere.
You have a solid object (say a block of wood). When you place it in water, it floats partially submerged. What determines how much of the wood is submerged?

You know the buoyant force on the wood is equal to the weight of the water that you displace, which is the volume of wood underwater multiplied by the density of water.

You know that in the steady state, there's no net force on the wood.

So you can calculate where on the wood block the surface of the water ends up.

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Re: Density Lab B

Postby Nba2302 » November 30th, 2018, 2:36 pm

I am on hands on task, part 3 (how much mass a helium balloon can lift). I went to this website-https://www.grc.nasa.gov/www/k-12/WindT ... swer5.html. I am wondering what the formula for this is and what is the answer in the that nasa example, because i couldn't find how much that balloon in the example could lift

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Re: Density Lab B

Postby John Richardsim » December 1st, 2018, 7:08 pm

I am on hands on task, part 3 (how much mass a helium balloon can lift). I went to this website-https://www.grc.nasa.gov/www/k-12/WindT ... swer5.html. I am wondering what the formula for this is and what is the answer in the that nasa example, because i couldn't find how much that balloon in the example could lift
In this case, the maximum weight that can be suspended in the air without moving downwards is equal to the mass of air that is displaced.



Putting this into terms of forces, the force of gravity on the suspended mass equals the force of gravity that would act on the volume of displaced fluid.



The "g" (acceleration due to gravity) in the above equation may be eliminated, leaving:



Therefore, the amount of mass that may be suspended in the air is identical to the mass of the displaced air. Using the equation for mass density, we can rewrite this equation as



This is what was found in part 2 of that link.

Part 3 find the maximum load that may be carried by the balloon by accounting for the mass of the balloon itself and the helium:



Part 4 of that link then translates the mass of the load back into the gravitational force on the load by multiplying the mass by the acceleration due to gravity.
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Re: Density Lab B

Postby Nba2302 » December 1st, 2018, 7:21 pm

I just took the density lab test. It is extremely easy. I was shocked

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Re: Density Lab B

Postby knightmoves » December 9th, 2018, 8:30 pm

#3 would probably require the density of helium.
You're overthinking this - it's a hands-on task.

Weigh some object on the balance. Tie the balloon to the object and weigh it. Subtract.

Or (probably works with most electronic balances) hold the balance upside down, zero it, then place the balloon under the pan and record reading.

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Re: Density Lab B

Postby OrangeQuail9 » December 17th, 2018, 8:01 pm

Shouldn't Density Lab be a cheat sheet event? There are some formulas but not big complicated ones and it's mostly math so I don't really need that much information.

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Re: Density Lab B

Postby MattChina » December 18th, 2018, 5:17 am

Shouldn't Density Lab be a cheat sheet event? There are some formulas but not big complicated ones and it's mostly math so I don't really need that much information.
Well, I mean the point of most physics events is that you can put a lot of information, but you have to be able to apply the equations and figure it ouy for yourself. As for density lab, I agree there's not that much information you can put on there, and the formulas are really simple, but I guess it just wouldn't really make a difference. Also i think notesheets are for events more information-based so theres a lot of info to put in a short space and instead of a small amount in a small amount of space
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Re: Density Lab B

Postby chalker » December 19th, 2018, 7:22 pm

Shouldn't Density Lab be a cheat sheet event? There are some formulas but not big complicated ones and it's mostly math so I don't really need that much information.
Different committees and rules chairs have different philosophies regarding notes. In my committee (Physics), the general philosophy is that we'll let you bring as many notes as you want. The logic behind this is many-fold, including:

1. It reduces the burden on the event supervisor, who doesn't have to check / police the event for extra notes
2. It makes it more likely that event supervisors don't re-use tests from year to year
3. It ensures we don't unfairly treat competitors with poor eyesight who might not be able to read the microprint some competitors put on their note sheets
4. It helps incentivize competitors to research a much broader topic area (and bring corresponding notes to the event)

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Re: Density Lab B

Postby pikachu4919 » December 19th, 2018, 8:28 pm

Shouldn't Density Lab be a cheat sheet event? There are some formulas but not big complicated ones and it's mostly math so I don't really need that much information.
Different committees and rules chairs have different philosophies regarding notes. In my committee (Physics), the general philosophy is that we'll let you bring as many notes as you want. The logic behind this is many-fold, including:

1. It reduces the burden on the event supervisor, who doesn't have to check / police the event for extra notes
2. It makes it more likely that event supervisors don't re-use tests from year to year
3. It ensures we don't unfairly treat competitors with poor eyesight who might not be able to read the microprint some competitors put on their note sheets
4. It helps incentivize competitors to research a much broader topic area (and bring corresponding notes to the event)
To add to this, even though the committee sets the parameters for the limit of notes that competitors can bring, it is the competitor's ultimate decision to decide how many notes they want to bring to the event. If you feel like you really only need one sheet of info even though the event allows a binder, by all means, you can decide to only bring that one sheet, and the judges won't stop you from doing that. They'll only get on you if you exceed the limit, which you clearly wouldn't be doing.

Speaking as a former competitor, I can say that if you put in the right amount of preparation into the event beforehand, then there isn't really a need to go to absurdly small font sizes or margins to fit all you need to fit on there (for single sheets of notes, that is - different story for those 6-inch binders I've sometimes seen around). Even though I didn't really do physics events, I can say that for the events I competed in, I noticed how much in the latter years of my career that I was actually struggling to fill my cheat sheets with information rather than cramming it to the edges with information due to how well I felt I knew the material. And to some extent, that could actually be more helpful in the long run. During the competition, that's lessens the amount of time you spend scouring your notes for the information you may need but you either 1) don't necessarily know where it is within your notes or 2) might not even have it at all. That can free up more time you can spend on taking the test, which is arguably more important. It ultimately depends on how much you are certain you know like the back of your hand and how much you feel maybe a little or a lot more shaky about.

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TL;DR: If you feel like you don't need an entire binder's worth of info, that's perfectly fine and possibly even more beneficial to you, and no one will stop you from bringing less. They'll only care if you bring more than you're allowed.
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