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Reading quiz. 1. What keeps a suction cup against the wall?

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

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Reading quiz. 1. What keeps a suction cup against the wall?

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  1. 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.

  2. #1 Air #2 He #3 AIR PAYLOAD (~3 Large Paper Clips) 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

  3. How does F buoyancy compare for each balloon? • 1. F buoyancy = upward force from pressure of air surrounding object • 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. • 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 F buoyancy ? #1 Air Weight Balloon + Air + string + paperclips PAYLOAD (~3 Large Paper Clips)

  4. How does F buoyancy compare for each balloon? • 1. F buoyancy = upward force from pressure of air surrounding object • 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. • F buoyancy upwards equals the weight of the air displaced by the object. So depends on volume of object and density of air. Why is #3 true? Because we know that if our object is just a pocket of air, then Fbuoyancy 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 Pocket of air F buoyancy Weight of air F buoyancy = (density of air * volume of object) * g

  5. F buoyancy F buoyancy F buoyancy #1 Air #2 He #3 AIR Weight Balloon + Air + string + paperclips Weight Balloon + He + string Weight Balloon + Air + string PAYLOAD (~3 Large Paper Clips) Calculating F buoyancy for Balloon #1 : density of air at sea level = 1.25 kg/m3 density of air in boulder = 1 kg/m3 Volume = 4/3 pi r3= 4/3 pi (0.15 m)3 = 0.014 m3 Fbuoyancy = Volume * density * g = (0.014 m3)(1 kg/m3)(9.8 m/s2) = 0.14 N = 0.03 lbs

  6. scrunch!! How to get Force net to be Up? BIG VOLUME BIG F buoyancy 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

  7. scrunch!! How to get Force net to be Up? BIG VOLUME BIG F buoyancy 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. light gas atoms F buoyancy F gravity on He

  8. Fbuoyancy Air vs. Helium Balloon AIR SAME VOLUMES HELIUM Weight = mg Same volume so same Fbuoyancy = upwards net force from pressure of air surrounding balloon. • How does the pressure in each balloon compare? • Pressure in Air > Pressure in He b. Pressure in Air < Pressure in He • 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? • # Air Molecules > # He atoms b. # Air Molecules < # He atoms • # Air Molecules = # He atoms

  9. 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. Applet showed us that at same temperature Air molecules heavier, but slower He atoms lighter, but faster

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

  11. As the Helium balloon rises… • the volume of the balloon increases • the Fbuoyancy increases • the pressure inside the balloon increases • a and b • a, b, and c

  12. BALLOON AT 42 KM ALTITUDE! • 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! Fbuoyancy does not increase. Volume is much bigger, but density of air is much less Fbuoyancy is weight of displaced air: = Volume of balloon x density of air around balloon x g Volume increases but air density decreases!

  13. A look at hot air 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 • the molecules inside will become lighter because they are hotter • there will be fewer molecules inside because heating destroys some of them.

  14. 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.

  15. 293 K 1.25 kg/m3 293 K 1.25 kg/m3 Air at 70 Celcius Air at 20 Celcius • 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? • # air hot / # air cold = 70 C / 20 C • # air hot / # air cold = 20 C / 70 C • # air hot / # air cold = 293 K / 343 K • # air cold / # air hot = 343 K / 293 K • I do not really understand how to reason through this. P = k * (# molecules) * Temperature Volume

  16. 293 K 1.25 kg/m3 293 K 1.25 kg/m3 Air at 70 Celcius Air at 20 Celcius Answer is c: # air hot / # air cold = 293 K / 343 K P = k * (# molecules) * Temperature Volume ALL CONSTANT DURING HEATING P *V = (# molecules) * T k At 293 K:At 343 K: (# cold) * T_cold = (# hot) * T_hot (# hot)/(#cold) = T_cold/T_hot When heating:NOT CHANGING: volume, pressure CHANGING: temperature, # of molecules

  17. 293 K 1.25 kg/m3 Fbuoy So how do the numbers work out for making a real hot air balloon (at + 50 degrees)? Net force upwards = Fbuoy – Weight of air inside Fbuoyancy= (density of air) (volume) (g) = (1.25 kg/m3) * (5 m3 ) (9.8 m/s2 ) Density of hot air: # hot air molecules = # air molecules cold x (293K /343 K) Volume Volume Weight of air inside = density of hot air x volume x g = (1.25 kg/m3 * 293 K/343K) * 5 m3 * 9.8 m/s2 = 1 kg So to lift just 1 kg would need a volume of 5 m3. weight = mg

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

  19. P= k particle T Fbuoy Weight 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/m3 * (293 K / 77 K) ~ 3 times higher than air in room Fbuoy < < weight, drops like rock!

  20. A question from extra credit Bottle filled with hot liquid, sealed and then allowed to cool Bottle filled with hot liquid, sealed and then allowed to cool • Why does this happen as it cools? • Walls of bottle collapsed due to heat • Pressure of the air outside is higher than pressure of air inside • 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.

  21. 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 • A can support about 2 x more weight than B • A can support about 4 x more weight than B • B can support about 2 x more weight than A • B can support about 4 x more weight than A

  22. Suction Cup Strength? Pushed many air molecules out of here A B Force up = Air pressure X Area Force down = Mass of weight x gravity • 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 can support about 4 x more weight than A • Area of suction cup = pi * r2 • Area of B is 4 times Area of A • Force upwards from air molecules colliding with suction cup = Pressure * Area

  23. #1 He #2 He #3 AIR PAYLOAD (~3 Large Paper Clips) 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)

  24. F buoyancy F buoyancy F buoyancy #1 He #2 He #3 AIR Weight Balloon + He + string + paperclips Weight Balloon + He + string Weight Balloon + Air + string PAYLOAD (~3 Large Paper Clips) 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 Calculate F buoyancy density of air at sea level = 1.25 kg/m3 density of air in boulder = 1 kg/m3 Volume = 4/3 pi r3= 4/3 pi (0.15 m)3 = 0.014 m3 Fbuoyancy = Volume * density * g = (0.014 m3)(1 kg/m3)(9.8 m/s2) = 0.14 N = 0.03 lbs density of air at sea level 1.25 kg/m3, helium 0.175 kg/m3 Weight of He gas is small! = density of helium x volume x gravity

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