Reading quiz. 1. What keeps a suction cup against the wall? a. sticky rubber, b. electrical attraction between oppositely charged particles, c. the force of air pressure, d. the buoyancy force, e. magic 2. You remove a partially filled sealed container of food from the refrigerator and let it warm up. You notice that the lid bows out as it warms up. This is because: a. as the food warms it produces gases that increase the pressure inside. b. the pressure on the outside of the container decreases as the container warms c. the air pressure in the room is higher than in the cold refrigerator d. as the gas inside the container warms up it increases the pressure. 3. A hot air balloon can lift more on a. a hot day, b. a cold day, c. there is no difference ans. 1 c, 2. d, 3. b.

PAYLOAD (~3 Large Paper Clips) #1 Air #2 He #3 AIR How does F buoyancy compare for each balloon? A. F buoyancy for #1 > #2 > #3 B. F buoyancy for #2 > #1 > #3 C. F buoyancy for #1 = #2; & both larger than #3 D. F buoyancy for #2 > #3 > #1 E. F buoyancy for #1 = #3; & both smaller than # 2. Correct answer is: C. F buoyancy for #1 = #2; & both larger than #3

#1 Air PAYLOAD (~3 Large Paper Clips) F buoyancy Weight Balloon + Air + string + paperclips How does F buoyancy compare for each balloon? 1. F buoyancy = upward force from pressure of air surrounding object 2.The air surrounding object cannot tell what it is pushing on … could be a balloon, a person, a table, or just air … hits it the same regardless of what it is hitting. 3.F buoyancy upwards equals the weight of the air displaced by the object. So depends on volume of object and density of air. ? F buoyancy

How does F buoyancy compare for each balloon? 1. F buoyancy = upward force from pressure of air surrounding object 2.The air surrounding object cannot tell what it is pushing on … could be a balloon, a person, a table, or just air … hits it the same regardless of what it is hitting. 3.F buoyancy upwards equals the weight of the air displaced by the object. So depends on volume of object and density of air. Pocket of air Why is #3 true? Because we know that if our object is just a pocket of air, then F buoyancy will exactly balance it’s weight and the pocket of air will feel no net force. F buoyancy = mass of air displaced by object*gravity Weight of air F buoyancy F buoyancy = (density of air * volume of object) * g

#1 Air PAYLOAD (~3 Large Paper Clips) #2 He #3 AIR F buoyancy Weight Balloon + He + string Weight Balloon + Air + string Weight Balloon + Air + string + paperclips Calculating F buoyancy for Balloon #1 : density of air at sea level = 1.25 kg/m 3 density of air in boulder = 1 kg/m 3 Volume = 4/3 pi r 3 = 4/3 pi (0.15 m) 3 = m 3 F buoyancy = Volume * density * g = (0.014 m 3 )(1 kg/m 3 )(9.8 m/s 2 ) = 0.14 N = 0.03 lbs

How to get Force net to be Up? What if we make a sealed light-weight Al foil balloon… remove air inside volume/balloon? Could we carry our selves? Nope… Squashed by atmosphere (just like Drum!) What could you do that could make this work? a. make out of really heavy metal to hold against air pressure b. fill with water so would not compress in c. fill with some other gas that is lighter than air. d. fill with air e. something else BIG VOLUME BIG F buoyancy

How to get Force net to be Up? What if we make a sealed light-weight Al foil balloon… remove air inside volume/balloon? Could we carry our selves? Nope… Squashed by atmosphere (just like Drum!) What could you do that could make this work? c. fill with some other gas that is lighter than air. BIG VOLUME BIG F buoyancy light gas atoms F buoyancy F gravity on He

Air vs. Helium Balloon AIR HELIUM How does the pressure in each balloon compare? a.Pressure in Air > Pressure in He b. Pressure in Air < Pressure in He c.Pressure in Air = Pressure in Helium c. Pressure in Air = Pressure in Helium…. Both are equal to air pressure in the room! Pressure pushing in = pressure pushing out or else bag will collapse. How do the number of He atoms compare to number of Air molecules in each balloon? a.# Air Molecules > # He atoms b. # Air Molecules < # He atoms c.# Air Molecules = # He atoms SAME VOLUMES Same volume so same F buoyancy = upwards net force from pressure of air surrounding balloon. F buoyancy Weight = mg

Review: Air vs. Helium Balloon AIR HELIUM How do the number of He atoms compare to number of Air molecules in each balloon? c. # Air Molecules = # He atoms…. Ideal Gas Law: P = k * (# molecules) * Temperature Volume Balloons have same volumes … same pressure … same temperature of gas … so have same # of gas particles. Air molecules heavier, but slower He atoms lighter, but faster Applet showed us that at same temperature

So we could use He Balloon to lift stuff! (Pressures balance) little helium atoms, only 2 protons and two neutrons each. Much less mass. fat air atoms, N- 14 neutrons & protons O- 16 neutrons and protons Same number of gas particles in each balloon. But weight of each He particle is less…

As the Helium balloon rises… a.the volume of the balloon increases b.the F buoyancy increases c.the pressure inside the balloon increases d.a and b e.a, b, and c

BALLOON AT 42 KM ALTITUDE! a.The volume of the balloon increases!!! Pressure of surrounding air decreases Balloon expands until pressure inside = pressure outside P = k * (# molecules) * Temperature Volume Same # He molecules inside, to decrease P, volume increases! F buoyancy does not increase. Volume is much bigger, but density of air is much less F buoyancy is weight of displaced air: = Volume of balloon x density of air around balloon x g Volume increases but air density decreases!

What will happen if we heat a beaker of air, and then remove the stopper? a. nothing, the air will just stay there but get hotter b. there will be fewer molecules inside because some will be pushed out into the room c.the molecules inside will become lighter because they are hotter d.there will be fewer molecules inside because heating destroys some of them. A look at hot air

Lower T, lower P outside Higher T, higher P inside Answer is b. Atoms push out into room until pressure inside and out is the same P= k (# molecules/Volume) T lower higher Hot air has fewer atoms in same volume, but same pressure.

If air inside balloon is heated so that it is 50 C hotter than before, how does the number of air molecules inside balloon change? What is ratio of number of air molecules of hotter balloon to number of air molecules of colder balloon? a.# air hot / # air cold = 70 C / 20 C b.# air hot / # air cold = 20 C / 70 C c.# air hot / # air cold = 293 K / 343 K d.# air cold / # air hot = 343 K / 293 K e.I do not really understand how to reason through this. 293 K 1.25 kg/m K 1.25 kg/m 3 Air at 20 Celcius Air at 70 Celcius P = k * (# molecules) * Temperature Volume

Answer is c: # air hot / # air cold = 293 K / 343 K 293 K 1.25 kg/m K 1.25 kg/m 3 Air at 20 Celcius Air at 70 Celcius P = k * (# molecules) * Temperature Volume When heating: NOT CHANGING: volume, pressure CHANGING: temperature, # of molecules P * V = (# molecules) * T k At 293 K: At 343 K: (# cold) * T_cold = (# hot) * T_hot (# hot)/(#cold) = T_cold/T_hot ALL CONSTANT DURING HEATING

weight = mg F buoy 293 K 1.25 kg/m 3 So how do the numbers work out for making a real hot air balloon (at + 50 degrees)? Net force upwards = F buoy – Weight of air inside F buoyancy = (density of air) (volume) (g) = (1.25 kg/m 3 ) * (5 m 3 ) (9.8 m/s 2 ) Density of hot air: # hot air molecules = # air molecules cold x (293K /343 K) VolumeVolume Weight of air inside = density of hot air x volume x g = (1.25 kg/m 3 * 293 K/343K) * 5 m 3 * 9.8 m/s 2 = 1 kg So to lift just 1 kg would need a volume of 5 m 3.

If I stick a balloon full of air into liquid nitrogen and wait for some time, then …. (Liquid Nitrogen temp = 77 K) a.number of molecules inside balloon will be less b.pressure inside the balloon will be lower c.volume will decrease d.b and c e.a and b

P= k  particle T T down, P down initially so gets squished in by air around it until (# air molecules / Volume) or density of particles (  particle ) so big that P inside balloon = P in room. With  particle (number density) is so large, mass density is large also: Mass = 1.25 kg/m 3 * (293 K / 77 K) ~ 3 times higher than air in room F buoy < < weight, drops like rock! F buoy Weight

A question from extra credit Bottle filled with hot liquid, sealed and then allowed to cool Why does this happen as it cools? a.Walls of bottle collapsed due to heat b.Pressure of the air outside is higher than pressure of air inside c.Liquid and air inside is pulling sides in from the inside b. Air inside cools.. Lowers pressure inside, so force of air pressure pushing out less than force of air pressure pushing in. Walls cannot sustain pressure difference, Volume decreases until pressure inside pushing out equals pressure outside, or until walls can sustain pressure difference.

Suction Cup Strength? A B The diameter of Suction Cup B is twice as big as the diameter of Suction Cup A. How much weight can each suction cup hold? a. A and B can support the same amount of weight b.A can support about 2 x more weight than B c.A can support about 4 x more weight than B d.B can support about 2 x more weight than A e.B can support about 4 x more weight than A

Suction Cup Strength? A B The diameter of Suction Cup B is twice as big as the diameter of Suction Cup A. How much weight can each suction cup hold? b.B can support about 4 x more weight than A Area of suction cup = pi * r 2 Area of B is 4 times Area of A Force upwards from air molecules colliding with suction cup = Pressure * Area Pushed many air molecules out of here Force up = Air pressure X Area Force down = Mass of weight x gravity

PAYLOAD (~3 Large Paper Clips) #1 He #2 He #3 AIR What is net force on each balloon? A. #1 and #2 up, #3 down B. #1 and #2 down, #3 up C. #1 down, #2 up, #3 down D. #1 can’t tell, #2 up, #3 down E. #1 and #2 can’t tell, #3 down D (know from experience) or E (uncertainty in string weight)

PAYLOAD (~3 Large Paper Clips) #1 He #2 He #3 AIR How do F buoyancy compare on each balloon? D. #1 = #2 = #3. They are all the same! All have same Volume! F buoyancy = upward force from pressure of air surrounding balloon F buoyancy Weight Balloon + He + string Weight Balloon + Air + string Weight Balloon + He + string + paperclips Calculate F buoyancy density of air at sea level = 1.25 kg/m 3 density of air in boulder = 1 kg/m 3 Volume = 4/3 pi r 3 = 4/3 pi (0.15 m) 3 = m 3 F buoyancy = Volume * density * g = (0.014 m 3 )(1 kg/m 3 )(9.8 m/s 2 ) = 0.14 N = 0.03 lbs density of air at sea level 1.25 kg/m 3, helium kg/m 3 Weight of He gas is small! = density of helium x volume x gravity