Example Lesson:
Buoyancy
gravity
buoyancy
Why some objects float
Preliminary Concepts
Density = mass / volume
h
w
-or-
For example a rock has
more mass and thus
weighs more than a
dinner roll of equal size.
ρ=m/V
h
w
Which object do you think would be most likely
to float in water, the stone or the dinner roll?
This is because the dinner roll is less dense.
Archimedes
According to legend…
King Hiero II gave a goldsmith an amount of
pure gold to construct a crown. After the
crown was delivered the king became
suspicious that the craftsman had stolen
some of the gold and mixed the rest with
silver. He asked Archimedes to determine if
this was true
without melting the crown.
Archimedes
While taking a bath, Archimedes noticed that the level
of the water in the tub rose as he got in, and realized
that this effect could be used to determine the volume
of the crown. The submerged crown would displace an
amount of water equal to its own volume. By dividing
the mass of the crown by the volume of water
displaced, the density of the crown could be obtained.
This density would be lower than that of gold if
cheaper and less dense metals had been added.
Archimedes
Archimedes then took to the streets naked,
so excited by his discovery that he had
forgotten to dress, crying "Eureka!"
(meaning "I have found it!"). The test was
conducted successfully, proving that silver
had indeed been mixed in.
Archimedes Principle
A body immersed in a fluid experiences a
buoyant force equal to the weight of the fluid it
displaces
Downward force
equal to the weight
of the object
Upward force
equal to the weight
of the displaced fluid
=
Buoyancy
Positively Buoyant
+ An object that is less dense than water will float
Neutrally Buoyant
= An object that is just as dense as water will float
Negatively Buoyant
- An object that is less dense than water will float
Buoyancy and ROVs
• Typically ROVs used by companies and
universities are made to be slightly
positively buoyant
– Why might this be?
• This is so the ROV will float to the surface
if it is damaged or communication is
broken
Buoyancy and Your ROV
• Should you make your own ROV
positively, negatively, or neutrally buoyant?
• What are the advantages and
disadvantages of each?
• In this setting it is typically best to make an
ROV neutrally buoyant
How to Change Buoyancy
+ Floats
To increase buoyancy add
something that is very
positively buoyant like foam
- Ballast
To decrease buoyancy add
something that is very
negatively buoyant like metal
Why Might You Want Both?
How to Actively Change
Buoyancy
Rises
Adding air to a balloon on
the ROV makes it rise
Sinks
Adding water to a tank on
the ROV makes it sink
Buoyancy Activity
• How could we determine the mass of an
object without using a scale?
• Hints:
– The density of water is 1g / mL
– This will not work if the object sinks in water
– (Archimedes Principle applies to floating
objects)
Buoyancy Activity
• See how much water the object displaces
• Use the density equation to solve for mass
– ρ = m / V (remember ρ stands for density)
– (1g / mL) = (mass) / ( volume measured in mL)
– (1 g / mL) * ( # mL) = the object’s mass
• Check your answer using a scale