Fluids and Buoyant Force

Matter solid gas liquid Definite: mass volume shape Definite: mass

will take the shape of the container within its own definite volume Fluid – state of matter in which molecules and atoms are free to move past each other. liquid gas will take the shape of the container within its own definite volume will take the shape and fill the entire volume of the container (no definite volume)

volume – [m3] - V mass – [kg] - m Density – the concentration of matter of an object. density [kg/m3] – ρ

Example The body of a man whose weight is 690 N contains about 5.2·10-3m3 of blood. a)Find the weight of blood, and b)express it as a percentage of the body weight. The density of blood is 1060 kg/m3.

Buoyant Force The upward force exerted on a body immersed or floating in the fluid (liquid or gas)

The volume of the displaced fluid = the volume of the object (part of the object immersed in it)

Critical thinking Two solid spheres of the same radius are placed in two identical beakers with water. One sphere is made of gold (ρ = 19600 kg/m3) and the other of iron (7860 kg/m3). Which sphere will displace more water? Explain and illustrate your answer.

Critical thinking Two solid spheres of the same mass are placed in two identical beakers with water. One sphere is made of gold (ρ = 19600 kg/m3) and the other of iron (7860 kg/m3). Which sphere will displace more water? Explain and illustrate your answer.

Archimedes’s Principle Any object completely or partially submerged in a fluid experiences an upward buoyant force equal in magnitude to the weight of the fluid displaced by the object.

Sink or Swim? If the object will stay afloat or partially immersed in the fluid If the object will sink

Density and Buoyant Force weight of displaced fluid (in the volume of the object) An object will not sink in a fluid, if

A bargain hunter purchases a “gold” crown at a flea market A bargain hunter purchases a “gold” crown at a flea market. After she gets home, she hangs the crown from a scale and finds its weight to be 7.84 N. She then weighs the crown while it is immersed in water, and the scale reads 6.86 N. Is the crown made of pure gold? Explain.

The crown is either hollow or it is made of something other than gold.

Astronauts sometimes train underwater to simulate conditions in space Astronauts sometimes train underwater to simulate conditions in space. Explain why this is so. A student claims that if the strength of Earth’s gravity doubled, people would no be able to float on water. Is that true? Why? Explain why balloonists use helium in balloons.

Pressure and Pascal’s Principle

PRESSURE!

F O R C E

Pressure – force per unit area

Quick lab. Find the pressure each of your feet exerts on the floor. How would the pressure change if you stood on one foot? How would the pressure change if your weight doubled?

Pascal’s Principle Any change in pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls

Compare work??? Compare distance traveled by each piston? Download Pascal worksheet from the website

Practical use: A bottle of juice is sealed under partial vacuum, with a lid on which the red dot or “button” is painted. Around the button the following phrase is printed: “Button pops up when seal is broken”. Explain why the button remains pushed down when the seal is intact.

Download gauge pressure worksheet from the website

Gauge Pressure Pressure in a fluid at the depth h Absolute pressure (including atmospheric)

What is the sub-atmospheric pressure in Pa and atm if the height of the Hg column in the barometer is 690 mm? If the tube were doubled in diameter, what would happen to the height of the column? How tall should a barometer be to measure normal atmospheric pressure if instead of Hg it is filled with water?

Find the density of oil

Fluid in Motion

Ideal Fluid Ideal fluid is incompressible

Ideal Fluid Ideal fluid has no viscosity

Ideal Fluid Ideal fluid has a laminar (nonturbulent) flow

Continuity of Flow

As the air passes across the tube of perfume, the air pressure above it becomes lower, and the fluid (perfume) is pushed up the tube

Flying