1. Chapter 5.1 Announcements:
Homework 5.1: due Thursday, March 4, in class (Calli Nguyen) long homework  start early
Exercises: 1, 2, 3, 4, 6, 7, 8, 12, 13, 15, 16, 18
Problems: 1, 2, 3,4, 6, 7, 8, 9
- Remember: Homework 3.1 is due Thursday, Feb. 25, in class
- Remember: Homework 3.3 is due Tuesday, March 2, in class
All grades will continue to be posted at:
Listed by last four digits of student ID
We’ll now cover only parts of each chapter (tentative outline):
5.1 Balloons
7.1 Woodstoves
9.1 Clocks
9.2 Musical Instruments
10.3 Flashlights
Household Magnets & Electric Motor
11.2 Electric Power Distribution
15.1. Optics, cameras, lenses
16.1 Nuclear Weapons

2. (long Chapter section with many concepts
Chapter 5.1: Balloons
(long Chapter section with many concepts
Demos and Objects
Concepts
Buoyancy
Archimedes’ principle
air and air pressure
density
pressure
temperature
earth’s atmosphere
helium balloons
hot air balloons
ideal gas law
helium balloons
imploding can
Magdeburg spheres
expanding foam
particles in a box
submerging metal block
weighing things in water
diet coke and real coke

3. i-clicker-1
A helium balloon has mass, yet it rises up in the air. Is there a real force pushing up on the helium balloon?
Yes No
A definite maybe
Does it ever stop rising, and why?

4. Air and Air Pressure Air is a gas
What is air?
Air is a gas
Consists of individual atoms and molecules
Particles kept separate by their thermal energy
Particles bounce around in free fall
Air does have a mass
Air does have weight
1 kg of air can have many different shapes
Air is compressible

5. Air and Air Pressure Air has pressure Air particles bounce around
The higher the temperature, the more they bounce
Air particles exerts forces on container walls
Average force is proportional to surface area
Average force per unit area is called “pressure”
This formula is true for many situations.
Blaise Pascal (June 19, 1623, in Clermont-Ferrand, France – August 19, 1662, in Paris) was a French mathematician, physicist, and religious philosopher. He was a child prodigy who was educated by his father. Pascal's earliest work was in the natural and applied sciences where he made important contributions to the construction of mechanical calculators, the study of fluids, and clarified the concepts of pressure and vacuum by generalizing the work of Evangelista Torricelli
Unit of pressure:
1N/m2 = 1 Pascal = 1 Pa

6. Examples:
You weigh 800 N, you fell like wearing high heels (area 1 cm2 = m2) and you step on somebody’s foot. What pressure does that person feel?
A tank weighs 400,000 N and rolls around on a 10 m2 chain. What is the pressure on the ground?

7. Pressure Imbalances Balanced pressure exerts no overall force
Forces on opposite sides of object cancel
Unbalanced pressure exerts an overall force
Forces on opposites sides of object don’t cancel
Forces push object toward lower pressure

8. i-clicker-2 (Madgeburg spheres)
In 1654, Otto von Guericke gave the citizens of Magdeburg a remarkable lesson in the force of the atmospheric pressure. He machined two hollow hemispheres, twenty inches in diameter (0.5m) so they fit snuggly into a sealed sphere. He pumped the air out of it. Then he put sixteen horses, eight on each side, to the task of pulling the halves apart. The horses hard a very hard time pulling them apart. If the atmospheric pressure is 1.0·105 Pa, what force would be required to pull the spheres apart?
5000 N
10,000 N
20,000 N
50,000 N
100,000 N

9. You purchase a bottle of water in an airplane, drink half of it and then seal it again.
When you arrive at the airport, the bottle is all dented in.
Explain.

10. Air and Density Air has density Air particles have mass
What is density?
Air has density
Air particles have mass
Each volume of air has a certain amount of mass
Average mass per unit volume is called “density”
Units: kg/m3

11. Density:
The more packed the particles are the higher the density of the object.
i-clicker-3
1 kg of water has a volume of one liter (0.001 m3). What is it’s density?
Gold: 19,300 kg/m3
Iron: 7,860 kg/m3
Water: ______ kg/m3
Air: kg/m3
Helium: kg/m3
0.001 kg/m3
1 kg/m3
10 kg/m3
1000 kg/m3
10,000 kg/m3

12. Air Pressure and Density
Air pressure is proportional to:
Density (in this case particle density)
Denser particles hit surface more often
Denser air  more pressure A B
Which box has the higher pressure gas?
Which box has the higher density gas?

13. Air pressure in a box depends on: Particle density
(the denser the higher the pressure)
Temperature
(the hotter  the more bouncing  the higher the pressure.

14. Ideal Gas Law
Pressure = Boltzmann constant · Particle density · Absolute temperature
Only applies perfectly to independent particles
Real particles are not completely independent
kB … Boltzmann constant k = 1.381·10-23 Pa·m3/(particle·K)
rparticle … particle density (how many particles per m3)
T … temperature in Kelvin
See problem 1 on homework

15. An Aside About Temperature
Air has temperature
Air particles have thermal kinetic energy
Average thermal kinetic energy is proportional to absolute temperature
SI absolute temperature scale: kelvins or K
0 K is absolute zero—no thermal energy left
1 K is equivalent to 1 degree Celsius
room temperature ~ 293 K = 20°C = 68F
0 K = °C = F

16. Temperature Scales
Reliable and quantifiable way of measuring how “hot” an object is.
How to convert from one temperature scale to another:
Where TC, TF, T is the temperature in Celsius (centigrade), Fahrenheit and Kelvin
Kelvin is the proper SI unit

17. Homework, Problem 1: The particle density of standard atmospheric air at K (0°C, 32F) is 2.687·1025 particles/m3. Using the ideal gas law, calculate the pressure of this air.

18. The Atmosphere Air pressure pushes on the air itself
Air near the ground supports air overhead
Air pressure is highest near the ground
Air density is highest near the ground
Key observations:
Air pressure decreases with altitude
A balloon feels more force at bottom than at top
Force imbalance yields an upward buoyant force
Diameter of earth: 12,700 km = 12,700,000m

19. Pressure Imbalances Balanced pressure exerts no overall force
Forces on opposite sides of balloon cancel.
Unbalanced pressure exerts an overall force
Forces on top and bottom of balloon don’t cancel.
Forces push balloon toward lower pressure.

20. A submerged object seems to weigh less. Where do the forces come from?
Buoyant force (in liquids and in gases)
A submerged object seems to weigh less.
Where do the forces come from?

21. Archimedes’ Principle:
An object partially or wholly immersed in a fluid is acted upon by an upward buoyant force equal to the weight of the fluid it displaces.
An object that has a higher density than the fluid it is in, will sink (slower than free fall).
An object that has a lower density than the fluid it is in, will rise.

22. A iron block that has a volume of 1 m3 is immersed in water
A iron block that has a volume of 1 m3 is immersed in water. The density of iron is 7860 kg/m3 and the density of water is 1000 kg/m3.
What is the weight of the iron block?
What is the weight of the displaced water?
What is the buoyant force?
What is the “weight” of the immersed iron block? (What force is required to hold the block still in water?)
cA N
cB N
cC N
cD N
cE. 10,800 N
dA N
dB N
dC ,200 N
dD. 77,000 N
dE. 86,800 N
i-clicker-4
i-clicker-5

23. Floating and sinking in air

24. Hot air contains fewer molecules than cold air
(at the same pressure, or a box with hole, or an elastic box).
Thus, hot air has a smaller density than cold air!!!
Hot air will rise in cold air!!
P = k rpart. T
P = k rpart. T

25. Do Now
What is the volume of a aluminum cylinder that has a density of 5 g/cm3 and a mass of 20 g?
What is the mass of a object that has a density of 20 g/cm3 and a volume of 12 cm3?
What is the density of a boat that has a mass of grams and a volume of cm3? Will this rock float or sink in water?

26. Density is the measure of the “compactness” of a material
How close the atoms or molecules are to eachother
More than “heaviness” - density includes how much space an object takes up!!
All substances have density including liquids, solids, and gases
Bread slice and compacted bread in large flask of water

27. “Compactness”
Hot air balloon - Describe how a hot air balloon works. Defend your argument.

28. Why do things float?

29. Why do things float?

30. Why do things float?
Mass = 42,116,363 Kg

31. Why do things float? Mass = 42,116,363 Kg Volume = 44,070,909 L
It is bigger than it is heavy
So it will float

32. Hey…what’s all that stuff floating out there…it must have a low density…let’s not worry about it.?

33. Ouch!

34. Yikes! Mass = 42,999,363 Kg Volume = 44,070,909 L
Yikes…the mass is increasing due to water inflow
But the volume is staying the same!
D =42.9 / 44.0 =.97

35. Yikes! Mass = 43,256,363 Kg Volume = 44,070,909 L
Yikes…the mass is increasing due to water inflow
But the volume is staying the same!
D =43.2 / 44.0 =.98

36. Yikes! Mass = 43,943,363 Kg Volume = 44,070,909 L
Yikes…the mass is increasing due to water inflow
But the volume is staying the same!
D =43.9 / 44.0 =.99

37. Yikes! Mass = 44,060,363 Kg Volume = 44,070,909 L
Yikes…the mass is increasing due to water inflow
But the volume is staying the same!
D =44.0 / 44.0 =1.0

38. Density does not depend on the amount of the substance.
Mass= 32g
Volume=2cm3
The density of the block is 16 g/cm3

39. If it is cut in half, what happens to the density?
Mass= 16g
Volume= 1cm3
Mass= 16g
Volume= 1cm3
Density of block is 16 g/cm3
Density of block is 16 g/cm3

40. Density and Size
Density of a substance doesn’t change no matter how big or small
As long as temperature and pressure remain the same

41. TEMPERATURE AFFECTS DENSITY PHASE OF MATTER EFFECTS DENSITY
As the temperature of a gas ____________the density___________
Explain how a hot air balloon can float in the air. Make sure to use the terms temperature and density
Create a diagram showing the air inside the balloon and out.
PHASE OF MATTER EFFECTS DENSITY
What are the 3 phases of matter?
Put the phases of matter in order of increasing density
Create a drawing of a solid, liquid and a gas that shows the spacing of atoms(molecules) in each
increases
decreases
Gas, liquid, solid

42. Hot air has lower density than cold air.
Will a balloon rise for ever??

45. Phase of Matter Affects Density
3 phases of matter
Solid
Liquid
Gas
Put the three in order of increasing density
Gas, liquid, solid

46. HW #6 1. Fill in the empty boxes on the data table below. 2
HW #6 1. Fill in the empty boxes on the data table below. 2. Put the 4 liquids in the data table in order of increasing density.
LIQUID
Density
Mass
Volume A
5 g/cm3
15g B
100g
2cm3 C
8g/cm3
4cm3 D
1000g
500cm3

48. Hot-Air Balloon in Air A rubber, hot-air-filled balloon
contains fewer air particles than if it were cold
weighs less than the cold air it displaces
experiences an upward net force in cold air
floats in cold air
has an average density less than that of cold air

49. Helium vs. Air Replacing air particles with helium atoms
reduces the gas’s density
helium atoms have less mass than air particles

50. dA. 7,000 N dB. 40,000 N dC. 47,000 N dD. 54,000 N i-clicker-6
A helium balloon has a volume of 4000 m3.
The density of air is 1.2 kg/m3 and the density of helium is 0.18 kg/m3.
What is the weight of the displaced air?
What is the buoyant force on the balloon?
What is the weight of the helium in the balloon?
How much weight could the balloon carry up?
dA. 7,000 N
dB ,000 N
dC ,000 N
dD. 54,000 N
dE. None of the above
i-clicker-6
(question d)