Using the

1
Using the Clicker

• If you have a clicker now, and did not do this
  last time, please enter your ID in your clicker.
• First, turn on your clicker by sliding the power
  switch, on the left, up. Next, store your student
  number in the clicker. You only have to do this
  once.
• Press the button to enter the setup menu.
• Press the up arrow button to get to ID
• Press the big green arrow key
• Press the T button, then the up arrow to get a U
• Enter the rest of your BU ID.
• Press the big green arrow key.

2
Archimedes Principle

• This is true - the buoyant force acting on an
  object is proportional to the volume of fluid
  displaced by that object.
• But, we can say more than that. The buoyant force
  acting on an object is equal to the weight of
  fluid displaced by that object. This is
  Archimedes Principle.

3
A Floating Object

• When an object floats in a fluid, the downward
  force of gravity acting on the object is balanced
  by the upward buoyant force.
• Looking at the fraction of the object submerged
  in the fluid tells us how the density of the
  object compares to that of the fluid.

4
Beaker on a Balance

• A beaker of water sits on a scale. If you dip
  your little finger into the water, what happens
  to the scale reading? Assume that no water spills
  from the beaker in this process.
• 1. The scale reading goes up
• 2. The scale reading goes down
• 3. The scale reading stays the same

5
Three Blocks

• We have three cubes of identical volume but
  different density. We also have a container of
  fluid. The density of Cube A is less than the
  density of the fluid the density of Cube B is
  exactly equal to the density of the fluid and
  the density of Cube C is greater than the density
  of the fluid. When these objects are all
  completely submerged in the fluid, as shown,
  which cube displaces the largest volume of fluid?
  
• 1. Cube A
• 2. Cube B
• 3. Cube C
• 4. The cubes all displace equal volumes of fluid

6
Three Blocks

• Each cube displaces a volume of fluid equal to
  its own volume, and the cube volumes are equal so
  the volumes of fluid displaced are all equal.

7
Three Blocks

• Which object has the largest buoyant force acting
  on it?
• 1. Cube A
• 2. Cube B
• 3. Cube C
• 4. The cubes have equal buoyant forces

8
Weight in a Boat

• A boat containing a heavy anchor floats in a
  reservoir. If the anchor is thrown overboard and
  is completely submerged, what happens to the
  water level in the reservoir?
• 1. The water level falls
• The water level rises
• The water level stays the same

9
Weight in a Boat

• When the anchor is in the water, it displaces a
  volume of water that ______________________.
• When the anchor is in the boat, it is responsible
  for displacing a volume of water that
  ______________________.
• Which of these is larger?

10
Weight in a Boat

• When the anchor is in the water, it displaces a
  volume of water that is equal to the volume of
  the anchor.
• When the anchor is in the boat, it is responsible
  for displacing a volume of water that
  ______________________.
• Which of these is larger?

11
Weight in a Boat

• When the anchor is in the water, it displaces a
  volume of water that is equal to the volume of
  the anchor.
• When the anchor is in the boat, it is responsible
  for displacing a volume of water that has a
  weight equal to the weight of the anchor.
• Which of these is larger?

12
Weight in a Boat

• When the anchor is in the water, it displaces a
  volume of water that is equal to the volume of
  the anchor.
• When the anchor is in the boat, it is responsible
  for displacing a volume of water that has a
  weight equal to the weight of the anchor.
• Which of these is larger?
• The anchor, because it is more dense than water,
  displaces more water when it is in the boat.
  Thus, the water level drops when the anchor is
  thrown overboard, displacing less water.

13
Density (Mass density, that is)

• Mass density is mass per unit volume
• If an object has a lower density than a
  surrounding fluid, the object floats in the
  fluid if the object has a higher density it
  sinks.
• A materials specific gravity is the ratio of the
  density of the material to the density of water
  at 4C. What is special about water at 4C?
• Aluminum has a specific gravity of 2.7 it is
  2.7 times more denser than water at 4C.

14
Density (Mass density, that is)

• Mass density is mass per unit volume
• If an object has a lower density than a
  surrounding fluid, the object floats in the
  fluid if the object has a higher density it
  sinks.
• A materials specific gravity is the ratio of the
  density of the material to the density of water
  at 4C. What is special about water at 4C? Water
  is most dense at that temperature.
• Aluminum has a specific gravity of 2.7 it is
  2.7 times more denser than water at 4C.

15
A table of densities
Material Density (kg/m3)
Interstellar space 10-20
Air (20C, 1 atm.) 1.21
Water (4C, 1 atm.) 1000
Sun (average) 1400
Earth (the planet) 5500
Iron 8700
Mercury (the metal) 13600
Black hole 1019
16
Pressure

• Pressure is force per unit area
• The force exerted on an object by a fluid is
  toward the object and perpendicular to its
  surface. At a microscopic level, the force is
  associated with the atoms and molecules in the
  fluid bouncing elastically from the object.
• The SI unit for pressure is the pascal.
• 1 Pa 1 N/m2

17
Atmospheric pressure

• 1 atm 1.01 x 105 Pa 14.7 psi 760 torr
• (psi pounds / square inch)
• At atmospheric pressure, every square meter has a
  force of 100,000 N exerted on it, coming from air
  molecules bouncing off it!
• Why dont we, and other things, collapse because
  of this pressure?

18
Atmospheric pressure

• 1 atm 1.01 x 105 Pa 14.7 psi 760 torr
• (psi pounds / square inch)
• At atmospheric pressure, every square meter has a
  force of 100,000 N exerted on it, coming from air
  molecules bouncing off it!
• Why dont we, and other things, collapse because
  of this pressure?
• We have an internal pressure of 1 atmosphere.
  Objects like tables do not collapse because
  forces on top surfaces are balanced by forces on
  bottom surfaces, etc.

19
Unbalancing the forces

• If we remove the balance between forces, we can
  produce some interesting effects. Demonstrations
  of this include
• The Magdeburg hemispheres (see below)
• Crushing a can
• The atmosphere cannon

20
Rank by pressure

• A container, closed on the right side but open to
  the atmosphere on the left, is almost completely
  filled with water, as shown. Three points are
  marked in the container. Rank these according to
  the pressure at the points, from highest pressure
  to lowest.
• A B gt C
• B gt A gt C
• B gt A C
• C gt B gt A
• C gt A B
• some other order

21
Pressure in a static fluid

• A static fluid is a fluid at rest. In a static
  fluid
• Pressure increases with depth.
• Two points at the same vertical position
  experience the same pressure, no matter what the
  shape of the container.
• If point 2 is a vertical distance h below point
  1, and the pressure at point 1 is P1, the
  pressure at point 2 is
• Point 2 does not have to be directly below
• point 1 - what matters is the vertical distance.
• Simulation

22
Measuring pressure

• The relationship between pressure and depth is
  exploited in manometers (or barometers) that
  measure pressure.
• A standard barometer is a tube with one end
  sealed. The sealed end is close to zero pressure,
  while the other end is open to the atmosphere.
  The pressure difference between the two ends of
  the tube can maintain a column of fluid in the
  tube, with the height of the column being
  proportional to the pressure difference.

23
Whiteboard