## Density Lab B

Nba2302
Member
Posts: 38
Joined: November 7th, 2018, 2:28 pm
Contact:

### Re: Density Lab B

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.

knightmoves
Member
Posts: 54
Joined: April 26th, 2018, 6:40 pm
State: -
Contact:

### Re: Density Lab B

Nba2302 wrote: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.

Nba2302
Member
Posts: 38
Joined: November 7th, 2018, 2:28 pm
Contact:

### Re: Density Lab B

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/WindTunnel/Activities/ArchAnswer5.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

John Richardsim
WikiMod
Posts: 685
Joined: February 26th, 2014, 10:54 am
State: MI
Location: R12
Contact:

### Re: Density Lab B

Nba2302 wrote: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/WindTunnel/Activities/ArchAnswer5.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.

$Weight_{suspended}=Weight_{air,displaced}$

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.

$M_{suspended}*g=M_{air,displaced}*g$

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

$M_{suspended}=M_{air,displaced}$

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

$M_{suspended}=V_{air,displaced}*\rho_{air}$

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:

$M_{load}=(V_{air,displaced}*\rho_{air})-M_{balloon}-M_{helium}$

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.
"More like, 'Belongs in the Trash!'"

Si Quaeris Peninsulam Amoenam Circumspice

Nba2302
Member
Posts: 38
Joined: November 7th, 2018, 2:28 pm
Contact:

### Re: Density Lab B

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

knightmoves
Member
Posts: 54
Joined: April 26th, 2018, 6:40 pm
State: -
Contact:

### Re: Density Lab B

UTF-8 U+6211 U+662F wrote:#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.