Buoyancy

What is buoyancy?

The ability to float.

BuoyancyWhich one is more likely to float?

https://www.youtube.com/watch?v=zm-AkvwPdZ4

Or this balloon?This bowling ball

Roughly the same size

Your students probably already understand some of the basics…

hh

w w

Buoyancy

Density = mass/volume

Even though they are the same size (dimensions),the balloon will float because it is less dense than the fluid around it!

Buoyancy

THIS GUY

Was Archimedes buoyant?!

“Any body wholly or partially

immersed in a fluid

experiences an upthrust equal

to, but opposite in sense to,

the weight of the fluid

displaced.”

“Any body wholly or partially

immersed in a fluid

experiences an upthrust

equal to, but opposite in

sense to, the weight of the

fluid displaced.”

BuoyancyArchimedes looked at forces

Buoyancy: upward force on an object

Gravity: downward force on an object

Pressure: force all around an objectThe ability

to float.

Gravity

Buoyancy

Pressure Pressure

Buoyancy

Archimedes principle simplified

A body immersed in a fluid experiences a buoyant force equal

to the weight of the fluid it displaces

Upward force (buoyant force)equal to the weightof the displaced fluid

Downward force equal to the weightof the object

BuoyancyTypes of Buoyancy

Positively Buoyant Neutrally Buoyant

Negatively Buoyant

BuoyancyRemember

these?

Density = mass/volume

When the density of the object equals the density of the water

around it, the object is neutrally buoyant

Buoyancy

Play with these concepts

Archimedes loves

neutrally buoyant

ROVs

http://phet.colorado.edu/sims/density-and-buoyancy/density_en.html

So how do we make our ROVsneutrally buoyant?

Buoyancy and Balance

Once you drill holes in your frame, your

ROV will be negatively buoyant

Buoyancy

BallastDynamic (soft): system that changes the amount of water displace inside the ballast chamber. (Think submarines)

Static: ballast chamber does not change. ROV is designed to be neutrally buoyant and motors move it up and down. The volume of air stays the same.

Buoyancy

What types of materials could you use to make your ROV neutrally buoyant?

AND LOTS MORE! What are some pros and cons to different

materials?

Think about how you place buoyant materials

Buoyancy and Balance

Build an ROV Interactive:

http://www.mysticaquarium.org/fun-and-learning/games/302-build-an-rov

What about balance?

Buoyancy – going further

*NOTE* Totally optional

If your students are grasping the concepts of buoyancy – you can push them further by

introducing some basic calculations.

Buoyancy – going furtherPVC Pipe

specifications1’’ PVC 1 ¼’’ PVC 1 1/3’’ PVC 2’’ PVC

Diameter (cm) 3.340 4.216 4.826 6.033

Weight in Air (g/cm)

2.532 6.357 7.174 10.000

Volume percm (cc)

8.762 13.960 18.292 28.581

Net Buoyancy (g/cm) [weight in air per cm –

Volume per cm]

-6.230 -7.603 -11.118 -18.581

Calculations provided by the MATE center

Essentially, this figure gives us the “float” value of each cm of PVC

Then, weigh your ROV!

Fnet (apparent weight or wet weight) of ROV = ____________g

Buoyancy – going further

If your ROV weighs 645 g, then you will need -645g of positive, buoyant force!

Buoyancy – going further

How many cm of 2’’ PVC pipe would you

need?

~34.712 cm

*You’d probably also want to add in the weight of the 2’’ PVC endcaps to the weight of your ROV before you did the calculation.

Buoyancy

Buoyancy and BalanceLet’s make these items

positively, neutrally, and negatively buoyant and

balanced!