### Lecture 3 Slides

```Labs
• In S415 ESC
•Can be done anytime during week
•Turn into boxes outside classroom.
•No make-up--equipment is taken down Sat.
A cube under water
Buoyant Force:
The force caused by the
difference in pressure
between the top and the
bottom of an object when
the object is in a fluid
(liquid or gas).
The buoyant force on an object totally
submerged in a fluid depends on
a) The
mass of the object
b) The volume of the object, or
c) Both.
A cube floating on water
What if it’s not a cube?
Eureka!
Archimedes principle: The buoyant force is
always the weight of the fluid that the
object is displacing at the moment.
Consider a rowboat floating on a lake.
The weight of the rowboat, including
the people sitting in it, is 500 pounds.
The buoyant force on the rowboat is
a)
b)
c)
Greater than 500 pounds,
Less than 500 pounds, or
Equal to 500 pounds.
Three cubes of the same size are
completely submerged under water:
lead, steel and wood. The bouyant
force is greatest on the _______ cube
2. Steel
3. Wood
4. None… it is the same
1.
Three Cases
1) Penny— More dense than the liquid
2)Ping Pong Ball— Less dense than the liquid
3) Submarine, fish – Neutral buoyancy
Submarines and fish can adjust their
density!
A cannon is placed in a boat. The boat
sinks a little to displace more water. The
amount of new water displaced is
1.
2.
3.
A volume of water
that weighs more
than the cannon.
A volume of water
that weighs as much
as the cannon.
A volume of water
that weighs less
than the cannon.
If the cannon now falls from the boat into
the water and sits on the bottom of the lake
the amount of water displaced by the
cannon is
1.
2.
3.
A volume of water
that weighs more
than the cannon.
A volume of water
that weighs as much
as the cannon.
A volume of water
that weighs less
than the cannon.
Discussion question: A glass is filled to
the top with ice and water. When the
ice melts, where is the water level?
Above the rim so it spills over.
2. At the rim—same place as before.
3. Below the rim
1.
A container of water is sitting on a scale. I dip my
finger into the water without touching the sides or
the bottom of the container. The normal force
exerted on the scale will
a)
b)
c)
Increase,
Decrease, or
Remain the same
An object is hanging on the end of a
string and is submerged in water. The
tension in the string is
a)
b)
c)
Less than,
Greater than, or
Equal to
the weight of the object.
Discussion Question: An Alka-Seltzer tablet
dissolves in water and the CO2 gas goes into
a balloon. When it dissolves the scale