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.

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.

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.

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

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

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. 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

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 2. The water level rises 3. The water level stays the same

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? 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? 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? 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. Weight in a Boat

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 material’s specific gravity is the ratio of the density of the material to the density of water at 4°C. What is special about water at 4°C? Aluminum has a specific gravity of 2.7 – it is 2.7 times more denser than water at 4°C. 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 material’s specific gravity is the ratio of the density of the material to the density of water at 4°C. What is special about water at 4°C? 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 4°C. Density (Mass density, that is)

A table of densities MaterialDensity (kg/m 3 ) Interstellar space Air (20°C, 1 atm.) 1.21 Water (4°C, 1 atm.) 1000 Sun (average) 1400 Earth (the planet) 5500 Iron 8700 Mercury (the metal) Black hole 10 19

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/m 2

Atmospheric pressure 1 atm = 1.01 x 10 5 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 don’t we, and other things, collapse because of this pressure?

Atmospheric pressure 1 atm = 1.01 x 10 5 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 don’t 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.

Unbalancing the forces If we remove the balance between forces, we can produce some interesting effects. Demonstrations of this include: 1. The Magdeburg hemispheres (see below) 2. Crushing a can 3. The atmosphere cannon

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. 1. A = B > C 2. B > A > C 3. B > A = C 4. C > B > A 5. C > A = B 6. some other order

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 P 1, the pressure at point 2 is: Point 2 does not have to be directly below point 1 - what matters is the vertical distance. Simulation

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.

Whiteboard